Biology A2 (Unit 5)

Unit 5 AQA Biology A Level Revision Cards

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  • Created by: Mia
  • Created on: 01-04-12 15:17


  • Animals and Plants increase chances of survival by responding to changes in the internal or external environment, to ensure they avoid harmful environments and maintaining optimal conditions for metabolism.
  • Any change in the internal or external environment is called a stimulus.
  • Stimuli are detected by receptors, different receptors detect different stimuli.
  • Then effectors(e.g. muscle cells) respond to a stimulus to produce an effect.
  • Receptors communicate with effectors using the nervous or hormonal systems, or sometimes both.
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The Nervous system

  • The nervous system send information as electrical impulses
  • It is made up of a complex network of cells called neurones.
  • There are 3 main types of neurone:
    -Sensory Neurones send electrical impulses from receptors to the CNS
    -Motor Neurones send electrical impulses from the CNS to effectors
    -Relay Neurones send electrical impulses between sensory neurones and motor neurones.
  • Stimulus > Receptor Cells > Sensory Neurone > CNS > Motor Neurone > Effectors > Response.
  •  System Communication is localised, short-lived and rapid.
  • When an electrical impulse reaches the end of a neurotransmitters are secreted directly onto cells, so it is specific.
  • They are broken down soon after, so they are short-lived.
  • Electrical impulses are really fast, so the response is rapid.
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The Hormonal System

  • The hormonal system is made up of glands and hormones.
  • A gland is a group of cells which are specialised to secrete a useful substance.
  • Hormones are "chemical Messengers", many are proteins or peptides.
  • Hormones are secreted when a gland in stimulated, they can be stimulated by a change is substance levels or by an electrical impulse.
  • Hormones diffuse into the blood and diffuse out all around the body.
  • Hormones trigger a response in target cells.
  • Stimulus > Receptors > Hormone > Effectors > Response.
  • The hormonal system communication is slower, longer-lasting and widespread.
  • Hormones aren't released directly onto their target cells, they must travel in the blood, which is slow.
  • They aren't broken down as quickly as neurotransmitters, so they are long lasting.
  • They are widespread, as the hormones are transported all over the body.
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  • Receptors are specific to one type of stimulus.
  • There are many types of receptors for many different stimuli.
  • There are different types of receptors, some are cells, and some are proteins on cell membranes.
  • How Receptor cells communicate:
    When the nervous system receptor is resting (not being stimulated) there is a difference in charge inside and outside the cell. The difference in voltage is called the potential difference. The potential difference when the cell is at rest is called the resting potential. When a stimulus is detected the membrane becomes more permeable, so ions move in and out of the cell, altering the potential difference, this change in potential difference is called generator potential. The larger the stimulus the larger the generator potential. If the generator potential is large enough it will trigger an action potential, this is an electrical impulse along a neurone. This only happens if the generator potential reaches a certain level called the threshold level.
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Pacinian Corpscles

  • Pacinian Corpsules are Mechanoreceptors - they detect mechanical stimuli, eg. pressure and vibrations.
  • They are found in your skin.
  • They contain the end of a sensory neurons, called a sensory nerve ending.
  • The nerve ending is wrapped in layers of conective tissue, called lamellae.
  • When they are stimulated, the lamellae are deformed and press on the sensory nerve ending.
  • This causes deformation of stretch mediated sodium channels in the sensory neurones cell membrane.
  • The sodium channels in the sensory neurones open and sodium diffuses into the cell, creating a generator potential.
  • If the generator potential reaches threshold, it triggers action potential.
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  • Photoreceptors are light receptors found in the retina of the eye.
  • The amount of light that enters the eye is controlled by the muscles of the iris.
  • Light is focused by the lens onto the retina, where the photoreceptors are. The area with the most photoreceptors is called the fovea.
  • Nerve impulses from photoreceptors are taken to the brain by the optic nerve, which is a bundle of nerves. Where the optic nerve leaves the eye is called a blind spot, as there are no photoreceptors.
  • Light enters the eye and is absorbed by photosynthetic pigments in photoreceptors. Light bleaches the pigments, altering the membrane permeability to sodium, a generator potential is created, and if threshold potential is reached an impulse is sent along a bipolar neurone to the optic nerve to the brain.
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Rods and Cones

  • These are the two types of photoreceptor.
  • Rods are more sensitive.
  • Cones see more detail.
  • Rods are sensitive to light, as many robs join to one neurone, so combined generator potentials may reach threshold and trigger an action potential.
  • Cones are less sensitive than rods, as one cone joins to one neurone, so it takes more light to trigger an action potential.
  • Rods give low visual accuracy, as many rods join the same neurone.
  • Cones give high visual accuracy, as cones are close together, but joined only to a single neurone. This means the brain can separate information.
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Neurone Cell Membranes are polarised at rest

  • In a neurones resting state the outside of the membrane is positively charged compared to the inside, this is because there are more positive ions outside the cell than inside. So the membrane is polarised.
  • The difference in voltage across a membrane is called the resting potential (around -70mV)
  • The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone membrane. The sodium-potassium pumps move sodium ions out of the neurone, but they can’t diffuse back in. This creates a sodium ion electrochemical gradient, as there are more positive ions outside the cell than inside. The sodium-potassium pumps also move potassium ions into the neurone, but they are able to diffuse back out of the cell.
  • This makes the outside of the cell positively charged compared to the outside.
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Neurone Cell Membranes Become Depolarised when the

  • The neurone is excited by a stimuli, this causes sodium channels to open, so sodium diffuses INTO the neurone. This makes the inside of the neurone less negative.
  • If the potential difference reaches threshold (around -55mV) more sodium channels open, so more sodium diffuses into the neurone.
  • At a potential difference of around +30mV the sodium channels close and potassium channels open, and potassium ions diffuse OUT of the neurone. This starts to get the neurone back to resting potential.
  • Potassium ions are too slow to close though, so too much potassium leaves the cell, so the potential difference is lower then the resting potential. Then the potassium diffuses back into the cell.
  • The ion channels are then reset, the sodium-potassium pump returns the membrane to its resting potential.
  • The membrane can't be excited straight away after this. This is because the channels are recovering and won't open. This is called the refractory period.
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Action Potential and the Refractory period

  • When an action potential occurs some of the sodium ions that enter the neurones diffuse sideways. This causes sodium ion channels in the next region to open so sodium can diffuse into that part.
  • This causes a wave of depolarisation to travel along the neurone. The wave moves away from parts of the membrane in the refractory period because these parts can't fire an action potential.
  • During the refractory period, ion channels are recovering and can't be opened, so this period acts as a time delay in between action potentials. This ensures action potentials don't overlap, and remain separate. This ensures action potentials are unidirectional (they only travel in one direction).
  • When threshold value is reached the action potential always fires with the same voltage, no matter the size of the stimulus.
  • If the threshold isn't reached the action potential won't fire.
  • A larger stimulus doesn't cause a larger action potential, it just causes more frequent action potentials.
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Factors effecting the speed of conduction

  • Myelination
    Some neurone have a myelin sheath, made of Schwann cells. This acts as an electrical insulator. Between schwann cells are exposed patches of membrane, called nodes of Ranvier, this is when sodium ions get through the membrane. The neurones cytoplasm conducts enough charge to depolarise the next node, so the impulse "jumps" from node to node. This is called saltatory conduction, it's really fast. In a non-myelinated neurone the impulse must travel as a wave down the whole axon membrane, this is slower than saltatory conduction, but still pretty fast.
  • Axon Diameter
    Action potentials are conducted faster along axons with large diameters, as there is less resistance to the flow of ions, so depolarisation reaches other parts of the neurone cell membrane quicker.
  • Temperature
    A higher temperature results in faster conduction, up to 40 degrees, after this proteins begin to denature.
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  • A synapse is a junction between a neurone and the next cell.
  • The tiny gap netween cells at a synapse is called the synaptic cleft.
  • The presynaptic neurone has a swelling called a synaptic knob, which contains vesicles filled with neurotransmitters.
  • When an action potential teaches the synapse it causes neurotransmitters to be released into the synaptic cleft, they diffuse across to the postsynaptic membrane and bind to receptors.
  • This might trigger an action potential, cause muscle contraction or cause a hormone to be secreted.
  • Because the receptors are only on the postsynaptic membrane the impulses are unidirectional, can only travel in one direction.
  • Neurotransmitters are removed from the synaptic cleft so the response doesn't keep happening. They may be reabsorbed, or broken down by enzymes.
  • There are many different neurotransmitters.
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  • This binds to cholinergic receptors.
  • Synapses that use acetylcholine are called cholinergic synapses.
  • First an action potential arrives at the presynaptic knob, this stimulates voltage gated calcium ion channels to open, calcium ions diffuse into the knob.
  • The calcium causes the synaptic vesicles in the synaptic knob to fuse with the presynaptic membrane.
  • Then ACh is released into the synaptic cleft, in a process called exocytocis, and binds with cholinergic receptors on the post synaptic membrane.
  • This causes sodium channels in the postsynaptic neurone to open.  The influx of sodium ions on the postsynaptic membrane causes an action potential, if threshold is reached.
  • ACh is then removed from the synaptic cleft, to prevent the response from happening. It's broken down by an enzyme called AChE and the products are reabsorbed by the presynaptic neurone to make more ACh.
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Neuromuscular junctions

  • A neuromuscular junction is a synapse between a motor neurone and a muscle cell.
  • They use ACh, which bonds to nicotinic cholinergic receptors.
  • They work in the same way as a cholinergic synapse, but they are a few differences:
    - The postsynaptic membrane has lots of folds which form clefts, which store AChE the enzyme which breaks down ACh.
    -The postsynaptic membrane has more receptors than other synapses.
    -When a motor neurone fires an action potential it always triggers a response in a muscle cells, this is not always the case between two neurones.
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Neurotransmitters are Excitatory or Inhibitory

  • Excitory neurotransmitters depolarise the postsynaptiic membrane, so it fires an action potential if threshold is reached.
  • Inhibitory neurotransmitters hyperpolarise the postsynaptic membrane, preventing it from firing an action potential.
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Summation of Synapses

  • If the stimulus is small, little neurotransmitter will be released, so it will not be sufficient to trigger action potential.
  • Summation is where the effect of neurotransmitter released from many neurones is added together.
  • There are 2 types of summation:
  • Spatial summation, sometimes many neurotransmitters connect to one neurone, the amount of neurotransmitters from each neurone add together to reach threshold value and trigger an action potential.
  • If some neurotransmitters release an inhibitory neurotransmitter then the total effect may be no action potential.
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How Drugs affect the action of Neurotransmitters

  • Some drugs affect synaptic transmission.
  • Some drugs are similar shapes to neurotransmitters, so they mimic their effects at receptors. These are called agonists, and cause more receptors to be activated.
  • Some drugs block receptors, which stops them being activated, these are called antagonists.
  • Some drugs inhibit the enzyme that breaks down neurotransmitters. This means neurotransmitters are in the system for longer.
  • Some drugs stimulate the release of neurotransmitters, so more receptors are activated.
  • Some drugs inhibit the release of neurotransmitters, so fewer receptors are activated.
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Skeletal muscle

  • Skeletal muscle (also called striated, striped or voluntary muscle)
  • This is made up of long muscle fibre.
  • It is made of bundles of long muscle cells.
  • The cell membrane of muscle fibre cells is called the sarcolemma.
  •  Bits of the sarcelemma fold inwards across the muscle fibre , and stick into the muscles cytoplasm, also called the sarcoplasm.
  • These folds are called transverse tubules, and they help to spread electrical impulses throughout the sarcoplasm, so they reach all parts of the muscle fibre.
  •  A network of internal membranes called the sarcoplasmic reticulum runs throught the sarcoplasm. This stores and releases calcium ions that are needed for muscle contraction.
  • Muscle fibres have lots of mitochondria to provide ATP thats needed for muscle contraction.
  • They are multinucleate (have many nuclei)
  • They have lots of long cylindrical organelles called myofibrics, which are highly specialised for contraction.
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Myosin and Actin

  • Myofibrils contain thick filaments made of protein called myosin and thin filaments made of protein called actin.
  •  If you look at a mycrofibril under an electron microscope you can see alternating light and dark bands.
  • Dark patches contain myosin filaments and some overlapping thin actin fillaments, these are called A bands.
  • Light bands contain only actin, these are called I bands.
  • A myofibril is made up of many short units called sarcomeres.
  • The ends of each sarcomere are marked by the Z-line.
  • The middle of each sarcomere is marked by the M line, this is the middle of the myosin fillaments.
  • Around the M line is the H zone, which contains only myosin filaments.
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Myosin heads and Tropomyosin

  • Myosin filaments have globular heads that are hinged so they can move back and forth.
  • Binding sites allow myosin to bind to actin and ATP.
  • Actin filaments have binding sites for myosin heads, called actin-myosin binding sites.
  • There are two other proteins called troponin and tropomyosin, which are found between actin fillaments. These two proteins are attached to one another and help myofilaments move past each other.
  • In resting muscle the actin-myosin binding sites are blocked by tropomyosin, which is held in place by toponin.
  • So myofilaments can't slide past eaxh other because the myosin heads can't bind to the actin filaments.
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ATP and Phosphocreatine

Aerobic respiration:

  • Most ATP is generated via oxidative phosphorylation in the cells mitochondria.
  • Aerobic respiration works in oxygen, its good for long periods of low-intensity exercise.

Anaerobic respiration:

  • ATP is made rapidly by glycolysis, this produces pyruvate, which is converted to lactate by lactate fermentation.
  • Lactate builds up in muscles, so its only good for short hard exercise.

ATP-Phosphocreatine (PCr) System:

  • ATP is made by phosphorylating ADP and adding phosphate from PCr.
  • PCr is stored in cells and the ATP-PCr system generates ATP quickly.
  • PCr runs out quickly, and the PCr-ATP system is anaerobic, and doesn't produce lactate.
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Slow and Fast Muscle Fibres

Slow twitch muscle fibres

  • They contract slowly
  • They are mainly used for posture (eg. in the back)
  • Good for endurance activities (eg. running)
  • can work for a long time without getting tired.
  • Have lots of mitochondria, as energy is released slowly by aerobic respiration, they also have lots of blood vessels to maintain oxygen supply.
  • They are reddish in colour, as they are rich in myoglobin (a red protein which stores oxygen)

Fast twitch muscle fibres

  • Contract quickly
  • Are used for fast movement (eg. in the legs)
  • Are good for short bursts of speed and power (eg, eye movement)
  • Get tired Quickly
  • There are few mitochondria or blood vessels.
  • Are whitish in colour, as they have little myoglobin.
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Control of Heart Rate

  • The sinoatrical node (SAN) sends electric impulses that cause muscles to contract.That rate the SAN fires is unconsciously controlled by the medulla.
  • Animals alter heart rate in response to internal stimuli. These stimuli are detected by pressure or chemical receptors.
  • There are pressure receptors called baroreceptors in the aorta and vena cava, which detect blood pressure.
  • There are chemoreceptos which detect oxygen levels, CO2 levels and pH.
  • High/low blood pressure stimulates baroreceptors, which causes impulses to be sent to the medulla, which send impulses along parasympathetic/sympathetic neurones. These secrete acetylcholine/norafrenaline which binds to the receptors of the SAN, causing heart rate to slow/ speed up.
  • (High O2, low CO2, high pH)/(Low O2, high CO2, low pH) is detected by chemoreceptors, impulses are sent to the medulla,which send impulses along parasympathetic/sympathetic neurones. These secrete acetylcholine/norafrenaline which binds to the receptors of the SAN, causing heart rate to slow/ speed up
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  • A reflex is when the body responds to a stimulus without a conscious decision.
  • As no decision is required, it is fast.
  • They help to avoid damage to the body.
  • The pathway of neurones linking receptors to effectors in a reflex is called a reflex arc.
  • A reflex arc uses 3 neurones, a sensory, a relay and a motor neurone.
  • If there is a relay neurone involved the reflex can be overridden by the brain.
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Taxes and Kineses


  • The organisms move towards or away from a directional stimulus eg. Light.


  • The organisms movement is affected by a non directional stimulus eg. Humidity.
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Chemical Mediators

  • A chemical mediator is a chemical messenger that acts locally.
  • Chemical messenger communication is similar to hormone communication, but they are different in several ways:
    They are secreted all over the body, not just from glands, Their target cells are right next to where they are produced, They only have to travel a short distance, so they have a faster response than hormones.


  • Histamine is a chemical mediator, stored in mast cells and basophils (immune system cells). It is released in response to injury or infection. It increases permiability of capillaries nearby, to allow more immune cells to move out of the blood, to the injured area.


  • These are produced by most body cells, and are involved in inflammation, fever, bloodpressure and blood clotting.
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Survival Responses in Plants

  • Plants increase chances of survival by responding to stimuli, eg, they grow towards light to maximise photosynthesis.
  • A tropism is the plants response to a directional stimulus.
  • Plants respond to stimuli by growth.
  • A postive tropism is towards the stimulus, a negative is away from it.
  • Phototropism is the response to light.
  • Geotropism is the response to gravity.
  • Responses are brought about growth factors.
  • Plants have no nervous system, so they use growth factors, which are chemicals that speed up or slow down plant growth.
  • They are produced in growing regions, and move to where they are needed.
  • Gibberellin stimulates flowering and seed germination.
  • Auxins stimulate shoot growth by cell elongation.
  • High auxin levels inhibit root growth though.
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Indoleacetic Acid (IAA)

  • Indoleacetic Acid (IAA) is an important auxin, thats produced in the tips of shoots in plants.
  • It is moved around the plant, by active transport and diffusion, to control tropisms.
  • This results in different parts of the plant having different levels of IAA, this uneven distribution results in uneven plant growth.
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  • Homeostasis involved control systems to maintain a constant internal environment
  • Maintaining internal conditions is essential for cells to function normally, and not be damaged.
  • Blood pH  and temperature are especially important as they affect enzymes.
  • Optimum temperature is around 37 degrees, and optimum pH is around 7.
  • It is also important to maintain blood glucose, as if it is too high, water potential is reduced, so water diffuses out of cells into the blood, which can cause cells to shrivel up and die. However, if glucose levels are too low there will not be enough energy for respiration to occur.
  • Homeostasis involves receptors which detect when levels are too high or low, then effectors bring the level back to normal using negative feedback.
  • Fore negative feedback to work, the change must be within a certain level.
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Negative Feedback

  • Negative feedback brings levels back to normal.
  • Homeostasis uses multiple negative feedback mechanisms to give more control.
  • This means you can actively increase or decrease a level to return it to normal from either direction.
  • If you only had one negative feedback mechanism, you could only turn it on or off, you could only actively change a level in one direction
  • Multiple negative feedback mechanisms mean a faster response and more control.
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Positive Feedback and Hypothermia

  • Positive feedback amplifies change from the normal level.
  • It is useful to rapidly activate something, for example, a blood clot after an injury.
  • It also happens when a homeostatic system breaks down, eg, if you're too cold for too long.
  • Hypothermia:
    Hypothermia is low body temperature (Below 35degrees)
    It happens when heat is lost faster then it can be produced.
    As body temperature falls the brain doesn't work properly, and shivering stops, so temperature falls even more. Positive feedback takes temperature even further away from normal level.
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Endotherms and Ectotherms


  • Can't control their body tempreture.
  • Include reptiles and fish.
  • The internal tempreture depends on external conditions. This also affects their levels of activity.
  • They have a variable metabolic rate and can generate very little heat themselves.


  • Control their body tempreture, through their constant high metabolic rate.
  • Their internal tempreture is less affected by the external tempreture.
  • Body tempreture is independant of the external temperature.
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Tempreture Control Mechanisms in Mammals

Heat loss:

  • Sweating- sweat is secreted from glands, and as it evaporates, it cools the skin
  • Hair lies flat- removes trapped air, air is a poor conductor of heat.
  • Vasodilation-blood is diverted so it flowsclose the the surface, and heat is lost by radiation.

Heat production:

  • Shivering, muscle contraction causes heat production.
  • Hormones, adrenaline and thyroxine increase metabolism, producing heat.

Heat conservation:

  • Less sweat
    hairs stand up, insulting the skin.
  • Vasoconstriction- less blood flows near the skin, so less heat is lost.
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The Hypothalamus

  • Body temperature in mammals is controlled by a part of the brain called the hypothalamus.
  • The hypothalamus recieves information abouth both internal and external temperature.
  • This information comes from thermoreceptors in the skin, and in the hypothalamus itself.
  • Thermoreceptors send impulses along sensory neurones to the hypothalamus, which sounds impulses along motor neurones to effectors (muscles and glands)
  • The neurones are part of the autonomic nervous system, so its all done unconsciously.
  • The effectors return body tempreture to normal.
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Control of Blood Glucose Concentration

  • All cells need glucose for energy, blood glucose needs to be around 90mg per 100cm^3 of blood, this is monitered by the pancreas.

Insulin lowers blood glucose:

  • It binds to receptors on the liver an muscle cells, increasing permeability, the cells absorb more glucose.
  • It activates enzymes which convert glucose to glycogen, this is stored in cells as an energy source. The creation of glycogen is called glycogenesis.
  • Insulin also increases the rate of respiration, lowering glucose levels.

Glucagon raises blood glucose:

  • It binds to receptors on liver cells, and activates enzymes that break down glycogen into glucose, this process is called glycogenesis.
  • It alsopromotes the formation of glucose from fatty acids and amino acids, the forming of glucose from non-carbohydrates is called glucognesis.
  • Glucagon decreases the rate of respiration in cells.
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Negative Feedback in Maintaining Blood Glucose Lev

Reducing blood glucose concentration:

  • The pancreas detects high glucose levels.
  • Beta cells secrete insulin, Alpha cells stop producing glucagon.
  • Insulin binds to receptors on liver and muscles.
  • Cells absorb more glucose, as they respire more, glycogenesis is activated.
  • This lowers blood glucose levels.

Increasing blood glucose concentration:

  • The pancreas detects low glucose levels.
  • Alpha cells secrete glucagon, Beta cells stop producing insulin.
  • Glucagon binds to receptors on the liver.
  • Glycogenesis is activated, glucogenesis is activated, cells respire less.
  • Cells release glucose into the blood.
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Adrenaline Increases Blood Glucose Concentration

  • Adrenaline is a hormone secreted from adrenal glands.
  • It issecreted when there is low blood glucose concentration, when you're stressed or when you're exercising.
  • Adrenaline binds to receptor, on liver cells.
  • It then activates glycogenoysis (glycogen > glucose)
  • It also inhibits glycogenesis (glucose> glycogen)
  • It activates glucagon secretion and inhibits insulin secretion.
  • It gets the body ready for action, by making more glucose available to the muscles.
  • Both adrenaline and glucagon can activate glycogenesis inside a cell, even through they bind to receptors outside the cell.
  • They do this by binding to their specific receptors, which activate an enzyme called adenylate cyclase, this converts ATP into a chemical signal, called a second messenger, cyclic AMP. cAMP activates a series of reactions which break down glycogen into glucose (glycogenolysis)
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  • Diabetes occurs where blood glucose concentration is not controlled.
  • Type 1
    This occurs when the beta cells don't produce any insulin.
    After eating blood glucose rises and stays high, this is called hyperglycaemia, and can be fatal. The kidneys can't reabsorb all this glucose so is is excreted in urine. It can be treated by regular doses of insulin, but this must be carefully controlled, to prevent an extreme drop in blood glucose called hypoglycaemia.
  • Type 2
    This is aquired in later life, and is often linked with obesity. It occurs when beta cells don't produce enough insulin, or when the body doesn't respond to insulin, as the receptors don't work properly, so cells don't absorb glucose. This means blood glucose concentration is high. It can be controlled by controlling carbohydrate intake and loosing weight, and if these don't work, then glucose-lowering tablets can be used.
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The Human Menstrual Cycle is Controlled by Hormone

  • The human menstrual cycle, also known as the oestrous cycle, lasts 28 days.
  • It involves many things:
  • A follice (egg and protective cells) developing in the ovary.
  • Ovulation - an egg being released.
  • The uterus lining becoming thicker, so a fertilised egg can implant.
  • A structure called a corpus leuteum developing from the remains of the follicle.
  • If there's no fertilisation the uterus lining breaks down and leaves the body through the vagina, this is menstruation and marks the end of a cycle.
  • This cycle is controlled by 4 hormones:
  • FSH, stimulates the follicle to develop.
  • LH stimulates ovulation and the corpus leuteum to develop.
  • Oestrogen stimulates the uterus lining to develop.
  • Progesterone, maintains the thick uterus lining.
  • FSH and LH are produced by the anterior pituitary gland.
  • Oestrogen and progesterone are secreted by the ovaries.
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Hormone concentrations change during the cycle

  • High FSH concentration:
    This stimulates follicles development, the folicle then releases oestrogen, FSH stimulates the ovaries to release oestrogen.
  • Rising Oestrogen concentration:
    This stimulates the womb lining to thicken, it also inhibits FSH being produced by the pituitary glands.
  • Oestrogen concentration peak:
    High oestrogen stimulate LH and FSH production, by the pituitary gland.
  • LH surge:
    Ovulation is stimulated by LH, the follicle ruptures and the egg is released, it also stimulates the ruptured follicle to turn into corpus luteum, which produces progesterone.
  • Rising progesterone:
    This inhibits FSH and LH, this maintains the uterus lining, if no embryo implants the corpus luteum breaks down, so progesterone production stops.
  • Falling progesterone:
    FSH and LH are no longer inhibited, so they are produced, and the womb lining breaks down.
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Positive and Negative Feedback control hormone lev

  • Negative feedback
    FSH stimulates the ovary to produce Oestrogen, then Oestrogen inhibits FSH production.
  • Negative Feedback
    LH stimulates the corpus leuteum to develop, which produces progesterone, then progesterone inhibits LH production.
  • Positive Feedback
    Oestrogen stimulated the production of LH, this stimulates the ovaries to release more oestrogen, which stimulates LH production. This cycle allows ovulation to occur.
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The Structure of DNA

  • DNA is a polynucleotide, it is made of multiple nucleotides.
  • Each nucleotide is made of a pentose deoxyribose sugar (with 5 carbon atoms), a phosphate group and a nitrogenous base.
  • The nucleotides join together from the phosphate group of one and the sugar of the next. This creates a sugar-phosphate backbone.
  • The two strands join together by the bases being joined by hydrogen bonds.
  • Genes are sections of DNA, they are found on chromosomes.
  • Genes code for proteins.
  • DNA is copied into RNA for protein synthesis:
    DNA is found in the nucleus, but is too large to move out of it to get to ribosomes, where protein synthesis occurs. So the section of DNA is copied onto RNA. This process is called transcription. The RNA leaves the nucleus and joins with a ribosome in the cytoplasm, where it can be used to synthasise a protein. This process is called transcription.
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DNA, mRNA and tRNA


  • Is double stranded, twisted into a double helix.
  • Has deoxyribose sugar and bases ATCG, which are arranged into codons.


  • Is single stranded.
  • Has ribose sugar and bases AUCG, which are arranged into codons.


  • Is single stranded, folded into a clover leaf shape, held by hydrogen bonds.
  • Has ribose sugar and bases AUCG.
  • Each tRNA molecule has a specific sequence of 3 bases, called an anticodon.
  • Has an amino acid binding site.
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  • Transcription starts when  RNA polymerase attaches to the beginning of a gene.
  • The hydrogen bonds between strands break, and the strands seperate.
  • One of the strands is then used as a template for an mRNA copy.
  • The RNA polymerase lines up free RNA nucleotides alongside the template DNA strand. Specific base pairing means the RNA strand ends up being complementary to the DNA strand, except there are U bases instead of T.
  • Once the RNA neucleotides have paired up with their specific bases on the DNA strand they're joined together, forming an mRNA molecule.
  • The RNA polymerase moves along the DNA, seperating the strands, and assembling the mRNA strand.
  • The DNA strands rejoin as hydrogen bonds are reformed.
  • When RNA polymerase reaches a set base sequence called a stop signal, it stops making mRNA and detatches from the DNA.
  • The mRNA moves out of the nucleus, through a nuclear pore and attatches to a ribosome, where translation occurs.
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  • This occurs at the ribosomes, in the cytoplasm.
  • During translation amino acids are joined together to make a polypeptide chain.
  • The mRNA attatched the a ribosome, and tRNA molecules carry amino acids to the ribosome.
  • A tRNA with an anticodon thats complementary to the first codon on the mRNA attaches to the mRNA by specific base pairing.
  • Then a 2nd tRNA attaches itself to the next codon, in the same way.
  • The two amino acids attatched the the tRNA are joined by a peptide bond.
  • Then the first tRNA molecule moves away, but leaves its amino acid behind.
  • This process continues, producing a chain of linked amino acids, untill there is a stop signal on the mRNA molecule.
  • The protein moves away from the ribosome and translation is complete.
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mRNA is edited in Eukaryotic cells

  • Eukaryotic cells contain introns which don't code for genes.
  • When DNA is converted the pre-mRNA the introns are left in, but the whole sequence is transcribed, with T being replaced by U.
  • Then the pre-mRNA is converted to mRNA by being spliced, the introns are removed, but it is not transcribed futher, so the sequence is the same.
  • This all takes place in the nucleus, after this is complete the mRNA molecule leaves the nucleus.
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The Genetic Code

  • The genetic code is non-overlapping, degenerate and universal.
  • The genetic code is the sequence of base triplets in mRNA which code for specific amino acids.
  • Each triplet is read in sequence, seperatley, and as triplets don't share bases, it is non-overlapping.
  • It is also degenerate, there are more combinations of triplets, than there are amino acids, so some amino acids are coded for by more than one base triplet.
  • Some triplets are start or stop signals.
  • The genetic code is universal, the same triplets code for the same amino acids in all living things.
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Transcription Factors

  • All the cells in an organism carry the same genes, but the structure and function of the cell varies, due to the genes being expressed.
  • The genes expressed result in different proteins being made, which alter cell structure and function.
  • The transcription of genes is affected by proteins called transcription factors.
  • These transcription factors move from the cytoplasm to the nucleus.
  • In the nucleus they bind to DNA sites near the start of target genes, that they control expression of.
  • They control expression by controlling the rate of transcription.
  • They can be either activators or repressors, and they work by either helping or stopping RNA polymerase bind to the start of the target gene.
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Oestrogen affects the Transcription of Target Gene

  • Oestrogen can affect transcription by binding to a transcription factor called an oestrogen receptor, this forms an oestrogen-oestrogen receptor complex.
  • This complex moves from the cytoplasm of the cell to the nucleus. In the nucleus it binds to specific DNA sites, near the start of the target gene.
  • This complex can act as either an activator or a repressor, assisting or blocking RNA polymerase.
  • Whether or not it acts as an activator or repress or depends on the type of cell and the target gene.
  • So the level of oestrogen in a particular cell affects the rate of transcription of target genes.
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siRNA Interferes with Gene Expression

  • Small interfereing RNA or siRNA, are short double stranded RNA molecules, that interfere with the expression of different genes.
  • Their bases are complimentary to those of target genes, and the mRNA derived from it.
  • siRNA can affect both transcription and translation.
  • It affects translateion by RNA interference.
    In the cytoplasm siRNA and assosciated proteins bind to target mRNA, the proteins then cut the mRNA into sections, so it can't be translated, so it prevents the expression of a specific gene,as the protein can't be made in translation.
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DNA Mutations

  • Any change to the base sequence of DNA is called a mutation.
  • This can be caused by error during DNA replication.
  • They can also be caused by mutagenic agents.
  • The types of error that can occur include,
  • The order of DNA bases in a gene determines the order of amino acids in a particular protein, so mutations could result in inactive proteins.
  • As amino acids can be coded for by more than one triplet, not all mutations need to change the amino acid sequence.
  • So substitution won't always lead to changes in amino acid sequence, but deletions will, as this will cause a shift in all the base triplets after it.
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Mutagenic Agents

  • Mutations occur spontaniously, but some things can cause an increase in the rate of mutations, these are called mutagenic agents.
  • Ultraviolet radiation, ionising radiation and some chemicals and some viruses are mutagenic agents.
  • They increase mutations by:
  • Acting as a base: chemicals called base analogs can substitute for a base during DNA replication, changing the base sequence of DNA, this causes a substitution mutation.
  • Altering bases, some chemicals alter or delete bases.
  • Changing the structure of DNA, some types of radiation of DNA, which causes problems during DNA replication.
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Genetic Disorders caused by Mutations

  • Some mutations cause genetic disorders, disorders caused by abnormal genes
  • These include cystic fibrosis and increased chances of some cancers.
  • If a mutant gamete is fertilised, the mutation will be present in the fetus.
  • Aquired genes can cause cancer:
  • Tumour supressor genes may be inactivated by a mutation, this results in supressor proteins not being produced, and the cells divide uncontrollably, resulting in a tumour.
  • The effect of a proto-oncogene can be increased by a mutation, a mutated proto-oncagene is called an oncagene. proto-oncagenes stimulate cell division, by producing proteins which make cells divide. However oncagenes overstimulate cell division, resulting in a tumour.
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Cancer Caused by Aquired Mutations

  • Aquired mutations are caused by mutatgenic agents, the effects of which can be minimised using Protective clothing, sunscreen and vaccinations.
  • Cancer is usually diagnosed after symptoms appear, however individuals at high risk may be regularly screened for cancer.
  • If a specific mutation is known then more sensitive tests can be developed, leading to earlier diagnosis and more effective treatment..
  • The treatment of cancer differs depending on the mutation.
  • The aggressiveness of the treatment depends on the mutation, as different mutations result in different types of cancer. eg, more aggressive cancers may be treated with higher doses of radiotherapy.
  • If the specific mutation is known then gene therapy may be used to treat it.
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Cancer Caused by Hereditary Mutations

  • Most cancers are caused by multiple mutated genes, so cancer does run in families. People with a family history of cancer should avoid mutagenic agents.
  • Increased/earlier screening in families with a history of cancer can lead to early detection and increased chances of recovery. Screening means analysing DNA.
  • If the gene causes a high risk preventative surgery may be carried out, eg, removing the likely organ, before the cancer develops.
  • Treatment depends on the particular mutation, as with Cancer caused by aquired mutations. But as inherited cancer is often detected earlier, this affects which treatments are used.
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Genetic Disorders Caused by Hereditary Mutations

  • If there is a family history of a genetic disorder, DNA can be analysed to see if they have the mutation or are a carrier, this means treatment can begin earlier than waiting to see if symptoms develop, and can see if children are at risk.
  • Gene therapy may treat some genetic disorders, this is inserting a normal copy of the mutated gene. The treatment is different for different mutations. Early diagnosis affects treatment options.
  • Carriers or sufferers of genetic disorders can undergo preimplantation genetic diagnosis during in vitro fertilisation to prevent offspring having the disease. Embryos are produced by IVF and screened for the mutation. Only embryos are implanted in the womb.
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Stem Cells are Totipotent

  • Multicellular organisms are made up of many different types of cells, all of which are specialised for their functions eg. muscle cells, white blood cells.
  • All specialised cells come fromstem cells.
  • Stem cells are unspecialised cells which then become specialised.
  • All multicellular organisms have some form of stem cells.
  • They are found in the embryo and in some adult tissue.
  • Stem cells are totipotet, they can develop into any type of cell.
  • Totipotent cells are only present in early life.
  • After this stem cells become multipotent, they can only develop into a few types of cell.
  • Mature plants also have stem cells, they are found in areas where the plant is growing.
  • All stem cells in plants are totipotent.
  • This means they can be used to grow plant organs, or whole new plants (in Vitro)
  • Growing plant tissue artificially is called tissue culture.
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How Stem Cells Become Specialised

  • Stem cells become specialised, as through development only certain genes are transcribed and translated.
  • Stem cells all contain the same genes, however the ones expressed depend on the conditions of the cell.
  • mRNA is only transcribed from specific genes, this mRNA is then translated into proteins.
  • These proteins affect cell structure and processes.
  • These changes mean the cell has become specialised.
  • Eg. Red blood cells:
    Red blood cells come from stem cells, they contains no nucleus and lots of haemoglobin. The stem cell produces a new cell in which lots of haemoglobin is produced. Other genes such as those involved in removing the nucleus are expressed too. This results in a specialised cell.
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Tissue Culture

  • Tissue culture can be used to grow plants from a single totipotent cell.
  • A single totipotent cell is taken from a growing area.
  • The cell is placed in some growth medium (eg. agar) that contains nutrients and growth factors. The growth medium is sterile, so there is no bacteria to compete with the plant cells.
  • The plant cell grows and divides into a mass of unspecialised cells. If the conditions are right the cells develop into specialised cells.
  • The cells grow and specialise to form a plant organ or an entire plant, depending on the growth factors used.
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Existing Stem Cell Therapies

  • Stem cells can be used to replace cells damaged by illness or injury.
  • Bone marrow contains stem cells which can become any type of blood cell.
  • Bone marrow transplants can replace faulty bone marrow in patients with blood disorders, to produce healthy blood cells.
  • This technique is successfully used to treat leukaemia and lymphoma, and some genetic disorders such as sickly cell anemia and sever combined immnodeficiency.
  • Eg Severe combined immunodeficiency (SCID)
    This is a genetic disorder which wekens the immune system, as white blood cells are defective.Treatment with bone marrow boosts immunity.
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Possible Stem Cell Therapies

  • As stem cells divide they could be used to replace damaged tissues.
  • This could be useful in treating:
  • Spinal injuries- they could replace damaged nerve tissue.
  • Heart disease and heart attack damage- they could replace damaged heart tissue.
  • Bladder condition- they could be used to grow whole new bladders to replace diseased ones.
  • Respiratory diseases- donated windpipes are stripped down to their collagen structure, then coated with stem cells. This can then be transplanted into patients.
  • Organ transplants- organs could be grown to provide organs for people on the donor waiting lists.
  • It may even be possible to engineer stem cells identical to the patients own, so they will not be rejected by the body.
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Human Stem Cells and Ethical Issues

  • There are two sources of stem cells:
  • Adult stem cells:
    These are obtained from adult tissues, they can be removed by a simple opperation, however it is uncomfortable. However they are mulitpotent, rather than totipotent, although scientists are trying to make them totipotent.
  • Embryonic stem cells:
    These are obtained from embryos, which are created using IVF in a lab. Once the embryos are around 4/5 days old, stem cells are removed and the embryo is destroyed. These are totipotent.
  • Ethical issues:
  • Destruction of embryo.
  • Only adult stem cells should be used.
  • Decisions about this should be made taking everyone's view into account.
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Gene Technology

  • Gene technology is basically all the techniques that can be used to study genes and their function.
  • This includes:
  • PCR (polymerase chain reaction) produces lots of identical copies of a gene.
  • In Vivo gene cloning, this also produces lots of Identical copies of a gene.
  • DNA probes, this is used to identify specific genes.
  • These techniques are used for genetic engineering, DNA fingerprinting, diagnosing diseases and treating genetic disorders.
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Obtaining DNA fragments using Reverse Transcriptas

  • Many cells have 2 copies of each gene. This makes it hard to obtain a target gene.
  • However they have alot of mRNA, which are complementary to the target gene, so this is easier to obtain.
  • The mRNA can be used as templates to produce more DNA. The enzyme reverse transcriptase makes DNA from an RNA template. The DNA produced in complementary, so its called cDNA.
  • To do this mRNA is isolated from cells and mixed with free DNA nucleotides and reverse transcriptase.
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Obtaining DNA fragments using Restriction Endonucl

  • Some sections of DNA have palindromic sequences of nucleotides. This means the sequences contain anti-parrallel base pairs (base pairs that read the same, but in opposite directions)
  • Restiction endonuclease are enzymes that recognise specific sequences and cut the DNA at these places.
  • Different restriction endonucleases cut at different sequences, because the shape of the sequence is complementary to the active site of the enzyme.
  • If recognition sequences are on both sides of the DNA fragment you can use restriction enzymes to seperate it from the rest of the DNA.
  • The DNA is incubated with specific enzymes, which cuts the fragement out using a hydrolysis reaction.
  • Sometimes the cut leaves sticky ends, this allows the DNA to bind to another DNA fragment, if it has complementary sticky ends.
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Obtaining DNA fragments using Polymerase Chain Rea

  • A reaction mixture is set up that contains the DNA sample, free nucleotides, primers and DNA polymerase.
  • Primers are short DNA sections, that are complementary to the start of the fragment you want.
  • DNA polymerase is an enzyme, that creates new DNA strands.
  • The DNA mix is heated to 95 degrees to break hydrogen bonds.
  • The mix is then cooled to around 50-65 degrees so the primers can bind to the strands.
  • The mix is heated to 72 degrees so DNA polymerase can work.
  • The DNA polymerase lines up free nucleotides along the DNA fragments, specific base pairing means complementary strands are formed.
  • Two new copies of the DNA are formed, and 1 cycle of PCR is complete.
  • The cycle starts again, and this time all 4 strands and used as templates.
  • Each PCR cycle doubles the amount of DNA.
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Gene Cloning can be In Vitro or In Vivo

  • Can be done In Vitro (In Glass)
  • Or In Vivo (In Life) inside a living organism.
  • Gene Cloning is making Identical Copies of a gene.
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In Vivo Cloning

  • Step 1
    The DNA fragment is placed within vector DNA (Something that can transfer DNA into a cell). They can be small cirular plasmid DNA or bacteriophages (Viruses that infect bacteria). The Vector DNA Is opened by the Same restriction endonuclease that was used to isolate the DNA fragment. This is so that the sticky ends of both DNA sections will be complementary. The vector DNA and the DNA fragment are mixed with DNA ligase. The ligase joins the ends together. This Process is called Ligation. The New combined DNA is called Recombinant DNA.
  • Step 2
    The vector with the recombinant DNA is used to transfer the gene into host cels. Plasmid vectors are inserted into a cell by heat shockin the cells, as this encourages cells to take in the plasmids. With bacteriophage vectors the cell is infected by the injection of DNA, which then combines with the bacterial DNA. Host cells which take up vectors are said to be transformed.
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In Vivo Cloning (Continued)

  • Step 3
    Not all cells take up the vector. Marker genes can be used to identify the transformed cells, they are inserted into vectors a the same time as the target gene. Host cells are grown on an agar plate. The marker gene could be for antibiotic immunity, so any cells which were not transformed are killed by antibiotics. It could also be fluorescence. Identified cells are allowed to reproduce to gain copies of the cloned gene.
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Pro's and Con's of both types of cloning

In Vivo Cloning:

  • This can produce mRNA and protein, and DNA, cause its done in a living cell, which has the ribosomes and enzymes needed to produce them.
  • It can produce modified DNA, mRNA and proteins.
  • Large fragments of DNA can be produced this way.
  • It can be relitively cheap, depending on how much DNA you want to produce.
  • However DNA must be isolated, and it can be a slow process, due to bacteria.

In Vitro Cloning:

  • Can produce lots of DNA
  • Only replicates desired DNA, so you don't have to isolate the desired gene.
  • It is a fast process.
  • However it can only replicate small fragments, it can't produce mRNA or proteins, and it can be expensive.
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Genetic Engineering

  • Genetic engineering is the manipulation of an organisms DNA.
  • Genetic engineering is also known as recombinant DNA technology.
  • Organisms that have been genetically transformed are called transformed.
  • These organisms have recombinant DNA, this is DNA formed by joining DNA from different sources.
  • Microorganisms, plants and animals can be transformed to benefit humans.
  • Transformed organisms can be made using the same technology as in vivo cloning.
  • The DNA fragment with the desired gene is isolated, then it is inserted into a plasmid vector, the vector is transfered into a bacterium. Transformed bacteria are identified and grown.
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Benefits of Genetic Engineering


  • Crops can be made to have higher crop yields, and better nutritional value., reducing famine and malnutrition, and can reduce costs.


  • Industrial processes often use enzymes, which can be produced by transformed organisms, this is low cost, and many can be produced.


  • Many drugs and vaccines are produced by transformed organisms, using recombinant DNA technology. This is cheap, and produces large quantities.
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Concerns about Genetic Engineering


  • Farmers may plant only one type of crop (monoculture) which is more vulnerable to disease. Also weeds may become resistant to herbicides.


  • Without proper labelling people won't have a choice in consuming transformed crops.
  • Purification may lead to toxins in food.


  • Companies may limit use of genetic engineering, even though it may be life saving.
  • This could be used unethically, eg. designer babies.
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  • A genome is all the genetic material in an organism.
  • Not all of the genome codes for proteins, some of it is repetitive non-coding bases, sometimes thousands over.
  • The number of repetitions varies from person to person.
  • The repeated sequences appear multiple times in the genome, this can be compared to identify an individual, this is called genetic fingerprinting.
  • The probability of two people having the same genetic fingerprint is very low.
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  • Eloctrophoresis seperate DNA fragments to make a genetic fingerprint.
  • A sampple of DNA is obtained.
  • PCR is used to make lots of copies of the DNA that contains repeated sequences, primers are used to bind to either side of these repeats, so the whole repeat is amplified.
  • You end up with DNA fragments, where the length corresponds to the number of repeats the person has at a specific position.
  • A fluorescent tag is added to all the DNA fragments, so they can be viewed under UV light.
  • The DNA then goes through electrophoresis:
    The DNA is placed  into a well in gel, and covered in a buffer solution that conducts electricity. Then an electric current is passed through the gel, DNA fragments are negative, so the move towards the positive electrode. Small DNA fragements move futher, so DNA seperates according to size. The DNA fragments are viewed as bands under UV light. Then genetic fingerprints can be compared.
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Uses of Genetic Fingerprinting

  • Genetic fingerprinting is used to determine relationships and variability.
  • We inherit repetitive non-coding sequences from our parents, half from each parent. So the more bands that are the same on a genetic fingerprint, the closer the relation. This is used in paternity tests.
  • It can also be used to determine genetic variability within a population- The greater the number of non-matching bands, the more genetically different the people are. This means that you can compare band differences to see how genetically varied the population is.
  • Forensic science- It can be used to identify  criminals by comparing crime scene DNA with suspects DNA.
  • Medical diagnosis- It can be used to diagnose genetic disorders and cancer. It's useful when the specific mutation isn't known, and could be due to several mutations.
  • Animal and plant breeding- This is used to prevent inbreeding, which can lead to health problems, it can also reduce genetic disorders.
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Locating Genes

  • DNA probes can be used to locate genes, pr see if a persons DNA contains a mutated genes.
  • DNA probes are short strands of DNA. They have a specific base sequence, that's complimentary to a part of the target gene.
  • This means the DNA probe will bind to the target gene if it is present in the sample.
  • A DNA probe also has a label attached, so it can be detected.
  • The most common types of label are radioactive or fluorescent.
  • A sample of DNA is cut into sections by restriction enzymes, and separated by electrophoresis. The separated DNA fragments are then transferred to a nylon membrane and incubated with the fluorescently labelled DNA probe. If the gene is present, the DNA probe will hybridise (bind) to it. The membrane is then exposed to UV light and if the gene is present there will be a fluorescent band.
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Restriction Mapping

  • To find the sequence of a gene can be found by DNA sequencing.
  • Most genes are too long to be sequenced in one go, so they're cut into shorter sections, using restriction enzymes. Then the shorter sections are then put back together in the correct order, so the entire gene sequence can be read in the right order- restriction mapping can be used to do this.
  • Different restriction enzymes are used to cut labelled DNA into fragments, the DNA is then separated by electrophoresis. The size of the fragments is used to determine the relative locations of cut sites. A restriction map of the original DNA is made. A diagram showing the different cut sites , and so where the recognition sites of the restriction enzymes used are found.
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Gene Sequencing

  • There are 4 tubes containing: a single-stranded DNA template, DNA polymerase, DNA primer, free nucleotides, fluorescently labelled modified nucleotides (no more bases can be attached after this is attached) either A,T,G,C modified bases.
  • Each tube undergoes PCR to produce more DNA, The strands are different lengths because each one terminates at a different point, due to the modified nucleotides.
  • The DNA fragments in each tube a separated by electrophoresis and visualised under UV light.
  • The complementary base sequence can be read from the gel. The smallest nucleotide is at the bottom of the gel, each band represents one more base added.
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Gene Tharapy as a treatment for disease

  • Many human disease are caused by mutations.
  • Eg Sickle cell anemia:
  • this is a recessive disorder, caused by a mutation in the haemoglobin gene.
  • This causes cresent moon shaped haemoglobin, these block capilaries, which can cause organ damage. Some people are carriers. The disease protects against malaria, so it is more common in places where malaria is common.
  • DNA probes can be used to screen for mutated genes.
  • A DNA microarray is a glass slide with microscopic spots of different DNA attatched to it in rows. A sample of labelled human DNA is washed over the array. If the DNA sequences match the probes, it sticks. The array is washed, to removed any unstuck DNA. Then it is looked at under UV light. Any DNA that fluoresces means the person has that specific gene.
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Pro's and Con's of Gene Therapy

  • Scientific methods are continuously updated and automated, reducing labour, human error, costs and increasing productivity and accuracy.
  • The results of screening can be used for genetic counselling: Genetic counseling is advising patients and families about risks of genetic disorders. It involves advising people about screening, and if screening results are positive, giving advice about types of treatments.
  • And deciding treatment. Different mutations cause different cancers which respond to different treatments in different ways, using DNA screening can identify the mutation, and help decide the best course of treatment.
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The Sliding Filament Theory

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Muscle contraction

  • Muscle contraction is caused by an influx of calcium ions.
  • The calcium ions are released from the sarcoplasmic reticullum.
  • This happens when the motor neurone stimulates a muscle cell, which causes the sarcolemma to depolarise,the depolarisation passes through t tubes to the sarcoplasmic reticulum. 
  • These calcium ions bind to troponin, causing it to change shape, pulling tropomysin, out of the way of the actin-myosin binding sites.
  • The calcium ions also activate ATPase, which break down ATP, releasing energy.
  • This energy is used to move the myosin heads, and the break the actin-myosin bonds.
  • This movement happens rapidly.
  • When the muscle cell stops being stimulated, the calcium ions are removed, by active transport, back to the sarcoplasmic reticullum.
  • This causes troponin to return to its original shape, pulling tropomyosin over the binding site.
  • The actin molecules then slide back into place, and the muscle is then relaxed
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