AQA Biology Unit 5

Organisms need to respond to stimuli to increase their chances of survival

Taxis- directional movement in response to a stimulus (e.g. woodlice move away from light- less chance of predation and reduces water loss)

Kinesis- non-directional (random) movement in response to a stimulus (e.g. woodlice move slowly and turn less often in high humidity, so they stay where they are. As air becomes drier, they move faster and turn more often allowing them to move to a more humid area thus reducing water loss)

Receptors- detect stimuli. Can be cells or proteins on cell surface membranes. E.g. baroreceptors detect blood pressure changes

Effectors- cells that bring about a response to a stimulus, to produce an effect. Effectors include muscle cells and cells found in glands (e.g. pancreas)

Receptors communicate with effectors via the nervous system or hormonal system

Neurones (nerve cells):

• Sensory- transmit electrical impulsese from receptors to CNS
• Relay- transmit electrical impulses between sensory and motor neurones
• Motor- transmit electrical impulses from CNS to effectors

Stimulus ----> Receptors ----> CNS ----> Effectors ----> Response

                ^sensory neurone             ^motor neurone

When electrical impulse reaches end on neurone, chemicals called neurotransmitters take information across gap (synapse) to the next neurone.

Rapid, involuntary response to a stimulus.

Does not go through conscious parts of the brain so automatic

Simple reflexes are protective- help organisms to avoid damage to body as response is so quick

Hormonal system in mammals made up of glands and hormones

Gland- group of cells that are specialised to secrete a useful substance, such as a hormone

Hormone- 'chemical messengers'

Hormones are secreted when a gland is stimulated. Glands can be stimulated by change in concentration of specific substance or by electrical impulses

Hormones diffuse directly into the blood, then taken around the boy via the circulatory system. They diffuse out of blood all over the body but each hormone will only bind to specific receptor for that hormone, found on the membranes of some cells (target cells). Hormones trigger response in the target cells (effectors)

Stimulus ----> Receptors ----> Hormone ----> Effectors ----> Response

Nervous system:

• Electrical impulse carried by neurones
• Fast response
• Localised effect
• Short-lived effect

Hormonal system

• Hormones carried in the blood
• Slow response
• Widespread effect
• Long-lasting effect

Chemical mediator- chemical messenger that acts locally (on nearby cells). Cells release chemicals that bind to specific receptors on target cells to cause a response.

Chemical mediators differ from hormonal response in the following ways

• Secreted from cells that are all over the body (not just glands)
• Target cells right next to where chemical mediator is produced (local response)
• Travel short distance to target cells (quicker response)

Histamine:

Chemical mediator stored in mast cells and basophils (cell in immune system). Released in response to body being injured or infected. Increases permeability of capillaries nearby to allow more immune system cells to move out of blood to injured/infected area

Prostaglandins:

Group of chemical mediators produced by most cells in the body. Involved in inflammation, fever, blood pressure regulation, blood clotting etc.

Receptors are specific- only detect one particular stimulus

Receptors in the nervous system convert the energy of the stimulus into the electrical energy used by neurones

Resting potential: Inside of cell is negatively charged relative to the outside

Generator potential: When stimulus is detected, cell membrane becomes excited and more permeable, allowing more ions to move in/out of the cell- altering the potential difference.

Action potential: If generator potential is big enough, an action potential is triggered (electrical impulse along neurone). Action potential only triggered if generator potential reaches threshold level. If stimulus is too weak, generator potential will not reach threshold, so no action potential.

Mechanoreceptors- detect mechanical stimuli (e.g. pressure and vibrations). Found on skin. Contain end of sensory neurone (sensory nerve ending) which is wrapped in lamellae (connective tissue)

When pacinian corpuscle is stimulated (e.g. by tap on arm), lamellae are deformed and press on sensory nerve ending. This causes deformation of stretch-mediated sodium channels in sensory neurone's cell membrane. Sodium ion channels open, allowing sodium ions to diffuse into cell, creating a generator potential. If generator potential reaches threshold then action potential is triggered.

Receptors in your eye that detect light. Light enters eye through pupil. Amount of light that enters controlled by iris muscles. Light rays focused by lens onto the retina, which lines inside of eye. Retina contains photoreceptor cells.

Fovea is an area of retina where there are many photoreceptors. Nerve impulses from photoreceptor cells carried from retina to brain by optic nerve (bundle of neurones). Where optic nerve leaves neurone is called "blind spot" (no photoreceptors, so not sensitive to light)

How photoreceptors work:

Light enters eye, hits photoreceptor and is absorbed by light-sensitive pigments. Light bleaches pigments, causing chemical change and altering membrane permeability to sodium. Generator potentiial created and if it reaches threshold, a nerve impulse is sent along a bipolar neurone (connect photoreceptors to optic nerve) which takes impulse to brain.

Rods mainly found in peripheral parts of retina. Cones found packed together in fovea.

Rods only give information in black and white (monochromatic vision).Cones in colour (trichromatic vision). Three types of cones: Red sensitive, green sensitive, blue sensitive. Stimulated in different proportions so you see different colours.

Sensitivity:

Rods very sensitive to light as many rods join one neurone, so many weak generator potentials combine to reach threshold and trigger action potential. Cones less sensitive as one cone joins one neurone, so more light is needed to reach threshold and trigger action potential

Visual acuity (ability to tell apart points that are close together):

Rods give low visual acuity as many rods join one neurone, so light from two objeccts close together cannot be told apart. Cones give high visual acuity as cones are close together and one cone joins one neurone. When light from two points hits two cones, two action potentials go to the brain- so you can distinguish two points that are close together as two separate points.

The resting membrane potential: In resting state, outside of membrane is positively charged compared to inside- so membrane is polarised  Resting potential -70mV.

Resting potential created and maintained by sodium-potassium pumps and K+ channels in neurone's membrane. Sodium-potassium pumps use active transport to move 3Na+ out of neurone for every 2K+ moved in. ATP is needed for this. K+ channels allow facilitated diffusion of K+ out of neurone, down concentration gradient

1. Sodium-potassium pumps move Na+ out of neurone, but memrane not permeable to Na+ so cannot diffuse back in. Creates sodium ion electrochemical gradient as there are more positive sodium ions outside the cell than inside.

2. Sodium-potassium pumps also move K+ into the neurone

3. When at rest, most K+ channels open. This means membrane is permeable to K+, so some diffuse back out through K+ channels.

More +ve ions move out of the cell than enter, so outside of cell positively charged compared to inside

When a neurone is stimulated, Na+ channels open. If the stimulus is big enough, it will triger a rapid change in potential difference. This causes cell to be depolarised.

1. Stimulus excites neurone cell membrane, causing Na+ channels to open. Na+ diffuse into neurone down sodium ion electrochemical gradient. Inside of neurone now less negative

2. Depolarisation- If potential difference reaches thresshold (-55mV), more Na+ channels open, so more Na+ diffuse into the neurone.

3. Repolarisation- Na+ channels close and K+ channels open. K+ diffuse out of the neurone.

4. Hyperpolarisation- K+ channels are slow to close so slight 'overshoot' where too many K+ diffuse out of neurone. Potential difference becomes more negative than resting potential (less than -70mV).

5. Resting potential- Ion channels reset. Sodium-potassium pump returns membrane to resting potential by pumping Na+ out and K+ in. Maintains resting potential until membrane is excited by another stimulus. Refractory period ensures action potentials are unidirectional.

Three factors affect speed of conduction of action potentials:

1. Myelination: some neurones have a myelin sheath which is an electrical insulator. It is made of a type of cell called a Schwann cell. Between the Schwann cells are tiny patches of bare membrane called the nodes of Ranvier. Na+ channels are concentrated at the nodes of Ranvier

Saltatory conduction: in myelinated neurones, depolarisation only happens at nodes of Ranvier- impulse 'jumps' from node to node. This is called saltatory conduction and is very fast. In a non-myelinated neurone, the impulse travels as a wave along whole length of axon membrane. This is slower than saltatory conduction.

2. Axon diameter: Action potentials are conducted quicker along axons with bigger diameters as there is less resistance to flow of ions than in cytoplasm of smaller axon.

3. Temperature: Speed of conduction increases as temperature increases as ions difuse faster.

Synapse- junction between a neurone and another neurone and an effector cell

Synaptic cleft- gap between cells and synapse

Presynaptic neurone- before synapse, has a swelling called a "synaptic knob" that contains synaptic vesicles filled with neurotransmitters.

Effect of an action potential:

• Action potential reaches end of neurone
• Neurotransmitters released into synaptic cleft
• Diffuse across to postsynaptic membrane and bind to specific receptors
• May trigger an action potential, cause muscle contraction or cause hormone to be secreted

Receptors only on postsynaptic membrane, so impulses are unidirectional. Neurotransmitters are removed from the cleft so response does not keep happening

1. Action potential arrives at synaptic knob of presynaptic neurone. The action potential stimulates Ca2+ channels in the presynaptic neurone to open. Ca2+ diffuse into the synaptic knob

2. Influx of Ca2+ causes synaptic vesicles to fuse with presynaptic membrane. The vesicles release acetylcholine (neurotransmitter) into the synaptic cleft by exocytosis

3. Acetylcholine diffuses across synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane, causing Na+ channels in postsynaptic membrane to open. Influx of Na+ causes an action potential on the postsynaptic membrane. Acetylcholine is removed from synaptic cleft so response does not keep happening. It is broken down bby acetylcholinesterase and the products are reabsorbed by the presynaptic neurone and used to make more acetylcholine.

A specialised cholinergic synapse between a motor neurone and a muscle cell.

Neuromuscular junctions use acetylcholine which binds to cholinergic receptors called nicotinic cholinergic receptors.

They work in the same way as cholinergic synapses but have a few differences:

• Postsynaptic membrane has lots of folds that form clefts which store acetylcholinerase
• Postsynaptic membrane has mre receptors than other synapses
• When a motor neurone fires an action potential, it always triggers a response in muscle cells. This is not always the case for a synapse between two neurones

Excitatory neurotransmitters- depolarise the postsynaptic membrane, making it fire an action potential if a threshold is reached

Inhibitory neurotransmitters- hyperpolarise the postsynaptic membrane preventing it from firing an action potential

Summation- the effect of neurontransmitters released from many neurones is added together

Spatial summation:

Two or more presynaptic neurones release their neurotransmitters at the same time onto the same postsynaptic neurone.

Temporal summation:

Two or more nerve impulses arrive in quick succession from the same presynaptic neurone.

Drugs can affect synaptic transmission in various ways:

• Some are the same shape as neurotransmitters so they mimic their action at receptors (agonists). More receptors are activated (e.g. nicotine mimics acetylcholine)
• Some block receptors so they cannot be activated by neurotransmitters (antagonists). Fewer receptors, if any, can be activated
• Some inhibit the enzyme that breaks down neurotransmitters. More neurotransmitters in the synaptic cleft bind to receptors and they are there for longer
• Some stimulate the release of neurotransmitter from presynaptic neurone so more receptors are activated (amphetamines)
• Some inhibit the release of neurotransmitters from presynaptic neurone so fewer receptors are activated

Made up of large bundles of long cells called muscle fibres. Cell membrane of a muscle fibre cell is called the sarcolemma. Bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm. The folds are called transverse (T) tubules and help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.

Sarcoplasmic reticulum (network of internal membranes) runs through sarcoplasm. Sarcoplasmic reticulum stores and releases Ca2+ needed for muscle contraction.

Contain bundles of thick and thin myofilaments that move past each other to make muscles contract. Thick myofilaments are made of the protein myosin and the thin myofilaments are made from the protein actin.

Under a microscope a myofibril will have dark (myosin) and light (actin) bands

A-band (dark): contain actin and myosin filaments

I-band (light): contain actin filaments only

Myofibril made up of many short units called sarcomeres. Ends of each sarcomere are marked with a Z-line. In the middle of each sarcomere is an M-line (middle of myosin filaments). Around M-line is H-zone (only contains myosin filaments).

Myosin and actin filaments slide over one another to make the sarcomeres contract. The myofilaments themselves do not contract. The simultaneous contractoin of lots of sarcomeres means the myofibrils and muscle fibres contract. Sarcomeres return to their original length as the muscle releases

A bands are the only one that stay the same length.

Ca2+ binds to tropomyosin, causing it to change shape and reveal the binding site on the actin. The myosin binds to the binding site on the actin, forming an actin-myosin cross bridge. Ca2+ also activates ATPase which breaks down ATP to provide the energy needed for muscle contraction. ATP is used to detach the myosin from the actin and reattach it onto the next actin in a "rowing" motion. ATP also provides energy to break the actin-myosin cross bridge. A new actin-myosin cross bridge is formed and the cycle is repeated

Slow twitch muscle fibres:

Contract slowly and can work for a long time without getting tired. Energy is released slowly through aerobic respiration. They have lots of mitochondria and blood vessels to supply the muscles with oxygen. Mitochondria mainly found near edge of muscle fibres so that there is a short diffusion pathway for oxygen from the blood vessels to the mitochondria. Rich in myoglobin

Fast twitch muscle fibres:

Contract very quickly but get tired quickly. Energy released quickly through anaerobic respiration using glycagen. They also have stores of PCr so energy can be generated very quickly when needed. Have few mitochondria or blood vessels. Do not have much myoglobin

Nervous system:

• Central nervous system (CNS)- brain and spinal cord
• Peripheral nervous system- neurones that connect the CNS to the rest of the body

Peripheral nervous system:

• Somatic nervous system- controls conscious activities
• Autonomic nervous system- controls unconscious activities

Autonomic nervous system:

• Sympathetic nervous system- 'fight or flight'
• Parasympahetic nervous system- 'rest and digest'

The autonomic nervous system is involved in the control of heart rate.

The SAN generates electrical impulses that cause the cardiac muscles to contract. The rate at which the SAN fires (heart rate) is unconsciously controlled by the medulla (in brain)

Baroreceptors- pressure receptors in the aorta, vena cava and carotid arteries. They are stimulated by high and low blood pressure

Chemoreceptors- chemical receptors in the aorta, carotid arteries and medulla. They monitor the oxygen level in the blood and also carbon dioxide and pH (indicators of oxygen level)

Electrical impulses from receptors are sent to the medulla along sensory neurones. The medulla processes the information and sends the impulses to the SAN along the sympathetic or parasympathetic neurones.

The response of a plant to a directional stimulus. Plants respond to stimuli by regulating their growth. A positive tropism is growth towards the stimulus, whereas a negative tropism is a growth away from the stimulus.

Phototropism- the growth of a plant in response to light. Shoots are positively phototropic, roots are negatively phototropic

Geotropism- the growth of a plant in response to gravity. Shoots are negatively geotropic, roots are positively geotropic

Plants respond to stimuli using growth factors- chemicals that speed up/slow down plant growth. Plant growth factors are prouced in the growing regions of the plant (e.g. shoot tips, leaves) and move to where they are needed in other parts of the plant.

Auxins (growth factors) are produced in the tips of shoots and diffuse backwards to stimulate the cell just behind the tips to elongate. If the tip of a shoot is removed, no auxin is available so shoot stops growing

IAA:

An important auxin that is produced in the tips of shoots of flowering plants. Moved around the plant to control tropisms by diffusion/active transport for short distances, and via the phloem for long distances. This results in different parts of the plant having different amounts of IAA. Uneven distribution of IAA means uneven growth of plant.

The maintenance of a constant internal environment. The internal system is kept in a state of dynamic equilibrium.

Temperature- rate of metabolic reactions increases when temperature increases. More heat, so more kinetic energy so molecules move faster and are substrate molecules are more likely to collide with enzyme's active site. If temperature becomes too high, the hydrogen bonds in the tertiary structure break, changing the shape of the active site so that the substrate can no longer bind to it. The enzyme is denatured. If temperature is too low enzyme actvity is reduced, slowing the rate of metabolic reactions.

pH- If blood pH is too high/low enzymes become denatured.

Blood glucose concentration- if it is too high, the water potential of blood is reduced to a point where water molecules diffuse out of cells into the blood by osmosis. This causes cells to shrivel up and die. If blood glucose concentration is too low, cells are unable to carry out normal activities as there is not enough glucose for respiration to provide energy. 

Homeostatic systems involve receptors, communication system and effectors. Receptors detect when level is too high/low, and information is communicated via nervous/hormonal system to effectors. The effectors respond to counteract the change- bringing the level back to normal. The mechanism that restores the level to normal is called a "negative feedback mechanism".

If the change is too big then effectors may be unable to counteract it.

Homeostasis involves multiple negative feedback mechanisms for each thing being controlled as this gives more control over changes in the internal environment. Having multiple negative feeback mechanisms means you can actively increase/decrease a level so it returns to normal. If you only had one negative feedback mechanism, you can only actively change a level in one direction so it returns to normal. One negative feedback means slower response and less control.

Positive feedback mechanisms amplify the change. The effectors respond to further increase the level away from the normal level. Positive feedback is not involved in homeostasis because it does not keep internal environment constant. It is useful to rapidly activate process in the body.

Ectotherms- cannot control their body temperature internally. They control their temperature by changing their behaviour. Internal temperature depends on external temperature. Have a variable metabolic rate as they are unable to keep internal temperature constant. They generate little heat themselves. Activity level depends on external temperture- more active at higher temperatures.

Endotherms- control their body temperature internally by homeostasis, as well as altering their behaviour. Internal temerature less affected by external temperature. High metabolic rate as they can keep internal temperture constant. Generate a lot of heat from metabolic reactions. Activity level of ectotherms largely independent of external temperature- can be active at any time.

Heat loss:

• Sweating- sweat evaporates from surface of skin and takes heat from the body. Skin is cooled
• Hairs lie flat- erector pili muscles relax so hairs lie flat. Less air trapped, so skin is less insulated and heat can be lost more easily
• Vasodilation- arterioles near surface of skin dilate. More blood flows through capillaries in surface layers of the dermis. More heat lost from skin by radiation and temperature is lowered.

Heat production:

• Shivering- muscles contract in spasms. Body shivers, more heat produced from increased respiration
• Hormones- body releases adrenaline and thyroxine. Increase metabolism, so heat produced

Heat conservation:

• Less sweat- less sweat secreted, reducing amount of heat loss
• Hairs stand up- erector pili muscles contract so stand up. More air trapped, prevents heat loss
• Vasoconstriction- arterioles constrict. Less blood flows through capillaries, reducing heat loss

Body temperature maintained by part of brain called "hypothalams". It receives information about internal and external temperature from thermoreceptors.

Information about internal temperature comes from thermoreceptors in hypothalamus that detect blood temperature. Information about external temperature comes from thermoreceptors in the skin that detect skin temperature.

Thermoreceptors send impulses along sensory neurones to the hypothalamus, which sends impulses along motor neurones to effectors (muscles and glands). Neurones part of autonomic nervous system, so all done unconsciously. Effectors respond to restore body temperatures back to normal.

Blood glucose concentration monitored by cells in pancreas. Concentration rises after eating food containing carbohydrate, falls after exercise as more glucose used in respiration to release energy.

Hormonal system controls blood sugar using insulin and glucagon which are secreted by cells in pancreas called 'islets of Langerhans'. Islets of Langerhans contain beta and alpha cells. Beta cells secrete insulin into blood, alpha cells secrete glucagon into blood. Insulin and glucagon act on effectors, which respond to restore blood glucose concentration to the normal level.

Insulin lowers blood glucose concentration when it is high. Binds to specific receptors on cell membranes of liver and muscle cells and increases permeability of cell membranes to glucose, so cells take up more glucose. Insulin activates enzymes that convert glucose to glycogen (glycogenesis). Liver and muscle cells store glycogen in cytoplasm as an energy source.

Glucagon raises blood glucose concentration when it is low. Binds to specific receptors on cell membranes of liver cells and activates enzymes that break down glycogen into glucose (glycogenolysis). Glucagon also promotes formation of glucose from glycerol and amino acids (gluconeogenesis)

Rise in blood glucose concentration:

Pancreas detects change. Insulin is secreted and glucagon secretion stops. The liver and muscle cells respond: cells take up more glucose, glycogenesis is activated, cells respire more glucose.

Fall in blood glucose concentration:

Pancreas detects change. Glucagon is secreted and insulin secretion stops. Liver cells respond: glycogenolysis is activated, gluconeogenesis is activated, cells respire less glucose.

Cycle lasts 28 days in humans. It is a cycle of physiological changes in which the female body prepares for reproduction.

Menstrual hormones:

• Follicle-stimulating hormone (FSH)- stimulates the follicle to develop
• Oestrogen- stimulates the uterus lining (endometrium) to thicken
• Luteinising hormone (LH)- stimulates ovulation and stimulates corpus luteum to develop
• Progesterone- maintains thick uterus lining, ready for implantation of an embryo

FSH anf LH are secreted by the anterior pituitary gland in the brain. Oestrogen and progesterone are secreted by the ovaries.

FSH stimulates follicles to develop and oestrogen causes endometrium to thicken. LH causes ovulation, progesterone maintains a thick endometrium and remains high during pregnancy. It decreases when there is no pregnancy. Progesterone and oestrogen inhibit FSH. Progesterone has a negative feedback mechanism to FSH and LH. LH has a positive feedback mechanism to oestrogen.

DNA is a double helix- the strands are polynucleotides.

Nucleotide  made from a phosphate group, a pentose sugar and a nitrogenous base. Sugar in DNA is deoxyribose sugar. Base varies between adenine, thymine, cytosine and guanine.

Two DNA polynucleotide strands join together by hydrogen bonds between bases. A always pairs with T. C always pairs with G. This is called specific base pairing.

Genes are sections of DNA found on chromosomes. Genes code for proteins (polypeptides) and contain instructions to make them. Order of nucleotide bases in gene determines order of amino acids in a particular protein. Each amino acid is coded for by sequence of three bases (triplet) in a gene. Different sequence of bases code for different amino acids (genetic code). Sequence of bases in a section of DNA is a template that is used to make a protein during protein synthesis.

DNA molecules found in nucleus of cell. Ribosomes synthesise proteins and are found in cytoplasm. DNA too large to move out of nucleus, so a section is copied into a molecule called mRNA in a process called "transcription". mRNA leaves nucleus and joins with ribosome in cytoplasm where it can be used to synthesise a protein in a process called "translation".

Like DNA, RNA is a type of nucleic acid. RNA differs from DNA in the following ways:

• Sugar in RNA nulceotides is ribose sugar
• Nucleotides form a single polynucleotide strand
• Uracil replaces thymine as a base

mRNA is a single polynucleotide strand made in the nucleus during transcription. mRNA carries the genetic code from the DNA in the nucleus to the cytoplasm, where it is used to make a protein during translation. In mRNA, groups of three adjacent bases are called codons.

tRNA is a single polynucleotide strand folded into a clover shape. Hydrogen bonds between the specific base pairs hold the molecule in this shape. Every tRNA molecule has a specific sequence of three bases at one end called an "anticodon". They have an amino acid binding site at the other end. tRNA is found in the cytoplasm where it is involved in translation. It carries the amino acids that are used to make proteins to the ribosomes.

Transcription is the first stage of protein synthesis. An mRNA copy of a gene is made in the nucleus:

1. RNA polymerase attaches to DNA- enzyme attaches to DNA double helix. Hydrogen bonds between two DNA strands in gene break, separating the strands. One strand used as template to make mRNA copy

2. Complementary mRNA is formed- RNA poolymerase lines up free nucleotides alongside template strand. T base replaced by U. Once RNA nucleotides have paired up with their specific bases on the DNA strand they are joined together, forming an mRNA molecule

3. RNA polymerase moves down the DNA strand- RNA polymerase moves along DNA, separating strands and assembling mRNA strand. Hydrogen bonds between uncoiled strands of DNA re-form back into double helix

4. mRNA leaves nucleus- when RNA polymerase reaches a particular of sequence of DNA called a "stop signal", it stops making mRNA and detaches from DNA. mRNA moves out through the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm

mRNA in eukaryotic cells is edited so that they do not contain introns (non-coding DNA). They only contain exons (coding DNA).

During transcription, introns and exons are both copied into mRNA. mRNA containing introns and exons are called pre-mRNA. Introns are removed by a process called "splicing" and the exons are joined together to form an mRNA strand. This takes place in the nucleus. The mRNA then leaves the nucleus for the next stage of protein synthesis- translation.

Translation is the second stage of protein synthesis. It takes place at ribosomes in the cytoplasm. During translation, amino acids are joined together by a ribosome to make a polypeptide chain (protein):

1. mRNA attaches itself to ribosome and tRNA molecules carry amino acids to the ribosome

2. tRNA molecule, with an anticodon that is complementary to the first codon on the mRNA, attaches itself to the mRNA by specific base pairing. A second tRNA molecule attaches itself to the next codon on the mRNA in the same way

3. The two amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecule moves away, leaving its amino acid behind

4. A third tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first two and the second tRNA molecule moves away. This process continues, producing a chain of linked amino acids (polypeptide chain), until there is a stop signal on the mRNA molecule

5. The polypeptide chain (protein) then moves away from the ribosome and translation is complete

The sequence of base triplets in mRNA which code for specific amino acids. Base triplets do not share their bases- they are non-overlapping.

The genetic code is also degenerate- there are more possible combinations of triplets than there are amino acids, so some amino acids are coded for by more than one base triplet. Start and stop signals are found and the beginning and end of the mRNA and are used to tell the cell when to start and stop the production of the protein.

The genetic code is also universal- the same specific base triplets code for the same amino acids in all living things

All cells in an organism carry the same genes but structure and function of different cells varies as not all genes in a cell are expressed (transcribed and used to make a protein). Because different genes are expressed, different proteins are made and these proteins modify the cell. Transcription of genes is controlled by protein molecules called "transcription factors".

Transcription factors move from cytoplasm to nucleus where they bind to specific DNA sites called "promoters" which are found near start of target genes. Transcription factors control expression by controlling rate of transcription. Activators increase the rate of transcription. They help RNA polymerase bind to start of target gene and activate transcription. Repressors decrease rate of transcription- they bind to start of target gene, preventing RNA polymerase from binding, stopping transcription.

Oestrogen is a hormone that can affect transcription by binding to a transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex. The complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene. The complex can either act as an activator or a repressor. Whether the complex acts as a repressor or activator depends on the type of cell and the target gene. So, the level of oestrogen in a particular cell affect the rate of transcription of target genes.

Gene expression also affected by a type of RNA called "small interfering RNA" (siRNA). They are short, double-stranded RNA molecules that can interfere with the expression of specific genes. Their bases are complementary to specific sections of a target gene and the mRNA that is formed from it.

SiRNA can interfere with both the transcription and translation of genes. siRNA affects translation through a mechanism called "RNA interference":

• In the cytoplasm, siRNA and associated proteins bind to the target mRNA
• Proteins cut up the mRNA into sections so it can no longer be translated
• So, siRNA prevents the expression of the specific gene as its protein can no longer be made during translation

siRNA is 20-25 nucleotides long