AQA Biology Unit 2


B2.1- Plant and Animal Cells

Typical features of animal cells

  • Nucleus- contains chromosomes made of DNA, controls the activities of the cells
  • Cytoplasm- jelly-like substance in which the chemical reactions of the cell occur
  • Cell membrane- controls what can leave/enter the cell
  • Mitochindria- rod shaped structures which release energy during aerobic respiration
  • Ribosomes- ball shaped structures in the cytoplasm where proteins are made

Typical features of plant cells 

Plan cells have all the features listed above, as well as some additional ones:

  • Cell wall- layer outside the membrane that keeps the cell in the correct shape
  • Chloroplasts- structures in the cytoplasm which trap light energy for photosynthesis
  • Permanent vacuole- cavity filled with cell sap, which helps support the cell
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B2.2- Bacterial and Fungal Cells

Common features of a bacterial cell

  • Cytoplasm
  • Cell membrane
  • Cell wall
  • Ribosomes
  • Loop of DNA
  • Capsule
  • Tail

Common features of a fungal cell

  • Cell membrane
  • Cell wall
  • Cytoplasm
  • Mitochondria
  • Vacuole
  • Nucleus
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B2.3- Specialised Cells

All cells have the same basic structure, but they differentiate to carry out certain functions. 

Specialised animal cells

  • Red blood cell- no nucleus, large surface area, very small, carries haemoglobin 
  • Nerve cell- short extensions at the end of the nerve, one long nerve fibre extension insulated with a fatty sheath
  • Muscle cell- long and thin, full of proteins to make it shorten and contract
  • Sperm cell- head containing DNA to combine with egg cell, tail to swim to egg cell with
  • Ciliated epithelial- tall, column shaped, cilia on top to move particles out of windpipe

Specialised plan cells

  • Palisade mesophyll cell- column shaped to allow a lot of chloroplasts to be packed in
  • Root hair cell- has a long extension to protrude into soil and extract nutrients
  • Xylem- hard wall to allow transport of water
  • Phloem- living neighbouring cells provide energy for transport of sugars, hollow for transport
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B2.4- Diffusion

Particles move from an area of high concentration to an area of low concentration. This process, called diffusion, is the one by which most particles move into and out of cells. 

There are several factors that can affect the rate of diffusion:


The shorter the distance that the particles have to travel, the higher the rate of diffusion will be 

Concentration gradient

The greater the difference in concentration between two areas, the higher the rate of diffusion

Surface area

The greater the surface area the particles can diffuse across, the higher the rate of diffusion

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B2.5- Animal Tissues and Organs

Muscle tissue

Comprised of a group of muscle cells. When the cells contract, the muscle tissue contracts. This brings about movement

Glandular tissue

Comprised of a group of cells able to produce and then secrete a substance

Epithelial tissue

Comprised of a group of cells that work together to form a protective layer over the body


The stomache is made up of muscle tissue (to bring about movement to break up the food inside the stomach), glandular tissue (which produces acids and enzymes to digest the food) and epithelial tissue (to protect the stomach from the high acidity of its contents).

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B2.6- Animal Organ Systems

Organs in the digestive system: mouth, salivary gland, oesophagus, stomach, liver, pancreas, small and large intenstines, rectum and anus

There are three categories that all the organs in the digestive system fall into: 

  • releasing digestive juices (pancreas, salivary glands, liver) to lubricate the food, produce enzymes to aid digestion and neutralise acidity
  • digestion (mouth, stomach and small intestine) which is the break down of foods so that the particles containing nutrients can be absorbed into the blood 
  • absorption (small intestine, large intestine, anus) which is when the nutrients in digested food particles and the remaining water are absorbed, then the waste material is excreted
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B2.7- Plant Tissues and Organs


  • Stem- supports plant, transports substances
  • Leaf- produces energy through photosynthesis
  • Root- anchors the plant, takes up water and minerals from soil
  • Flower- reproduction


  • Epidermal tissue- protects organs
  • Xylem- vascular bundle of dead cells that transports water
  • Phloem- vascular bundle of live cells that transports sugars and minerals
  • Palisade mesophyll- contain chloroplasts for photosynthesis
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B2.8- Photosynthesis

carbon dioxide + water > glucose + oxygen 

6CO2 + 6H2O > C6H12O6 + 6O2

Photosynthesis requires carbon dioxide from the air and water from the soil. 

Products of photosynthesis:

Oxygen- waste gas released into the air

Glucose- carbohydrate used for energy in the plant. Can be stored as insoluble starch until needed, such as at night when photosynthesis cannot take place but respiration is still taking place. 

Starch does not affect the concentration in cells and as it is insoluble, it cannot be carried out of the plant in water

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B2.9- The Leaf and Photosynthesis

Adaptations of the leaf for photosynthesis

  • Broad and flat- greater surface area for light to enter 
  • Thin- short diffusion pathway 
  • Chloroplasts- inside cells, containing chlorophyll to absorb light energy
  • Upper palisade- area which receives most light has most chloroplasts
  • Palisade cells- packed in tightly to fit a lot in
  • Veins- carry water to leaf and glucose away
  • Veins- support leaf 
  • Stomata- allow oxygen and carbon dioxide in/out
  • Spony mesophyll- air spaces for diffusion
  • Air space- large surface area : volume 
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B2.10- Rates of Photosynthesis

Factors affecting rate:

  • Light intensity
  • Temperature
  • Carbon dioxide levels

When a process requires several factors, such as photosynthesis, the one at the lowest level will limit the rate of the process. This lowest factor is known as the limiting factor. If the limiting factor in photosynthesis is increased, the rate of photosynthesis will increase until another factor becomes the limiting factor, and the rate will no longer be able to increase.

Carbon dioxide is usually the limiting factor of photosynthesis.

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B2.11- Controlling Photosynthesis

In greenhouses, people artificially control the factors which most affect photosynthesis. 

Light-The sun and electric lighting systems used to provide light all year round, and removable netting/whitewash is used when natural light is too strong

  • Advantage: growth all year round | Disadvantage: cost of lighting

Temperature- Glass traps heat inside greenhouse, shields plant from wind, heaters used inside, allows growth out of season/in wrong climate, ventilators open when too hot

  • Advantage: growth all year round, tropical plants in UK | Disadvantage: cost of fuel for heaters

Carbon dioxide- fossil fuels such as paraffin burned in heaters

  • Advantage: speeds up photosynthesis | Disadvantage: cost of fuel 

Water- automatic water systems ensure correct amount

  • Advantage: constant water supply | Disadvantage: cost of electricity, water and running
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B2.12- Sampling Techniques

  • Distribution- the location and spread of a particular species (affected by temperature, nutrients, carbon dioxide levels, light, water, oxygen)
  • Population- number of organisms in a given area
  • Community- different populations living together in one area


Square frames placed on ground. The number of organisms inside is counted, and an average is taken

Transect lines

Tape laid across an environment. The amount of organisms touching the line is counted, or the it is used to evenly distribute quadrats

  • Accurate- large enough apparatus to make results reproducible
  • Reliable- repeat readings offer reliablility of results 
  • Fair- same equipment each time, equal distribution of equipment without bias
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B2.13- Handling Environmental Data

The mean is the total of all readings taken divided by the amount of readings taken to get an average value. A single anomalous result can change the mean a lot

The median is the middle value of data when all readings are arranged in order. This is largely affected by data points that are much higher or lower than the others

The mode is the most common reading taken. Its advantage is that it is not affected much by anomalous result, as it does not take into account the spread of data

Steps scientists take to carry out research

1: Make a hypothesis

2: Test hypothesis

3: Analyse data, take averages and plot on graphs

4: Interpret evidence, look for correlations 

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B2 Part 1 Catch up

  • Living organisms are comprised of cells. Each part of a cell has a special function
  • There are similarities and differences between plant and animal cells
  • Fungal cells have a cell wall made of chitin 
  • Bacterial cells have a wall, ribosomes, a membrane and cytoplasm, but no nucleus
  • All cells have the same basic structure but become specialised for different functions
  • Substances move into and out of cells by diffusion, which is affected by concentration gradient, distance, surface area and temperature
  • Groups of cells work together as tissues, which work together as organs, which work together as organ systems, which work together as an organism
  • In plants the roots, stems, flowers and leaves are the organs 
  • Xylem and ploem are plant tissue. Xylem transports water and minerals, phloem dissolved substances
  • Plants make energy from photosynthesis. The leaves are well adapted for this 
  • Humans can control the rate of photosynthesis by controlling its limiting factors
  • Biologists use sampling techniques to evaluate the distribution of organisms
  • The data they collect from techniques such as quadrats or transect lines is analysed, averages are taken and correlations are looked for
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B2.14- Proteins

Proteins are made of smaller molecules called amino acids. These are made of carbon, hydrogen, oxygen and nitrogen, and join together in long chains to form proteins. The chains fold into a particular shape, and each type of protein has this specific shape that allows it to carry out certain processes. 

In the human body, proteins are the basic structural material for skin, muscle, bone, ligaments, cartilage and cell membranes. However, they also have some other important functions:

  • forming antibodies
  • creating and being hormones and receptors
  • making up the channels in cell membranes
  • acting as biological catalysts (enzymes)
  • creating structural components 
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B2.15- Enzymes

Enzymes are a type of protein designed to speed up chemical reactions inside an organism. Some examples of processes they speed up include:

  • photosynthesis
  • respiration
  • proteinsynthesis

Because the enzyme itself does not get used up when speeding up these reactions, it can be used multiple times, provided the shape of its active site does not change. 

Enzymes, like all proteins, are folded into a particular shape, and the shape of one part of the enzyme is called its active site. This is very important as it is the area where the substrate molecule (the molecule that needs joining/breaking) fits in to be catalysed. Each enzyme is designed for a specific type of substrate molecule, and if the shape changes due to high/low temperatures/pH levels, the enzyme will no longer work because the substrate molecule will not fit in it. 

When the active site does change shape, this is known as denaturing. The enzyme is denatured

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B2.16- Enzymes and Digestion

There are several enzymes that are necessary for the digestion of food. They are made in specialised cells in the lining of the gut, where they pass through the gut wall and come into contact with the food molecules. 

Amylase | made in salivary glands, pancreas, small intestine | breaks starch into sugar | works in mouth and small intestine

Protease | made in stomach, pancreas, small intestine | breaks proteins into amina acids | works in stomach and small intestine 

Lipase | made in pancreas, small intestine | breaks lipids into fatty acid and glycerol | works in the small intestine

The stomach produces hydrochloric acid to kill harmful bacteria and allow protease to work

Bile is made in the liver and is stored in the gall bladder before being released into the small intestine. It provides the alkaline conditions required for the enzymes in the small intestine to work. 

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B2.17- Enzymes in the Home- Detergents

Biological washing powders contain enzymes in them to break down stains on clothes, which may contain

  • protein (eg blood, egg) or
  • fats (eg grease, sweat).

The enzymes added to washing powders are proteases and lipases. 

Because washing powers tend to be very highly alkaline, the enzymes added to biological washing powders must be able to work under conditions with very high pH levels.

They also need to be able to work within the temperature ranges of a normal washing machine, which is between 10 and 90 degrees celsius. 

The enzymes added to biological wahsing powders are usually obtained from bacteria or fungi, which companies grow in large vats. The bacteria or fungi pass out enzymes from their cells, and these are collected and used. 

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B2.18- Enzymes in Industry

Many reactions in industrial processes can take place without enzymes, but normally only at a higher temperature and under a lot more pressure. For this reason, it is cheaper to use enzymes in industry. 

Properties of industrial enzymes

  • Long shelf life 
  • Withstand high temperatures
  • Wide pH tolerance
  • Work in presence of other chemicals

Proteases are used in industry to pre-digest baby food to allow babies to absorb the amino acids

Carbohydrases break down starch into sugar, which is used to flavour foods

Isomerase catalyses glucose into fructose, which is sweeter than glucose, so less of it is needed to make foods sweet. It is used in a lot of slimming food

Enzymes can still denature at high temperatures, and do contaminate the foods sometimes

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B2.19- Energy and Life Processes

All living things need energy for severeal reasons:

  • to build large molecules 
  • for muscle contraction (in animals)
  • to control body temperature (in animals)

Building molecules plants use nitrates, sugars and other nutrients to make amino acids. These are joined to make proteins during proteinsynthesis. Plants join sugar to make starch, and animals join sugar to make glycogen

Muscle contraction animals need movement to escape from predators, find food and mate

Controlling body temperature is necessary for some animals because it allows them to be active at night and during winter. However, it means they need to eat more food

Energy is released by cells when they undergo respiration, either aerobically or anaerobically. 

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B2.20- Aerobic Respiration

Respiration takes place mostly in the mitochondria. Aerobic respiration can be summarised in the equation:

glucose + oxygen > carbon dioxide + water (+ energy)

Energy and exercise

During exercise the muscles need to contract more. For this they need to be supplied with more energy, so they need more glucose and oxygen. To meet those requirements, the breathing rate and depth increases to obtain more oxygen, the heart rate increases to supply the muscles with oxygenated blood faster and the muscle cells break down stores of glycogen into glucose.The muscles release more heat and more carbon dioxide, which has to be removed from the body.

Storing glycogen

When carbohydrates are digested, they are digested into suar by enzymes. This passes into the bloodstream, then to the muscles, where it is converted to a large molecules calle glycogen

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B2.21- Anaerobic Respiration

Anaerobic respiration occurs when there is not enough oxygen

glucose > lactic acid (+ energy)

A much smaller amount of energy is produced from anaerobic respiration than aerobic, and the lactic acid produced is also toxic, so can harm the muscles. Lactic acid is carried to the liver where it is processed. 

Lactic acid damages muscles by lowering the pH. This causes enzymes in the muscles to become denatured, hence why they can no longer work efficiently. 

Even when a person has stopped exercising and has a normal oxygen supply again, the breathing rate takes a while to return to normal. This is because the body continues taking in additional oxygen to oxidise the lactic acid that is inside the body. This continuation of heaving, deep breathing even after exercising and the need for more oxygen after exercising is called oxygen debt

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B2.22- Cell division (Mitosis)

Body cells have to divide to replace worn-out cells, repair damaged tissues and to allow the organism to grow. They divide by a process called mitosis

Step One The genetic material in the cell is copied so that there are two identical sets of it

Step Two The copies of the chromosomes move to opposite sides of the cell, and new nuclei form around them 

Step Three The cell splits in two, with each half containing a new nucleus with the correct amount of chromosomes inside 

Some organisms reproduce asexually. This means the only process they use to produce offspring is mitosis. 

Mitosis is generally only used in more mature organisms for repair of cells and tissues, because the organisms no longer need to grow. However, mature plants still have some areas in which mitosis may occur, such as root and shoot tips. 

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B2.23- Cell Division (Meiosis)

Gametes are sex cells. They contain only 23 chromosomes, which is half the amount of a normal body cell. Gametes are produced through meiosis. 

Step One The genetic material inside the cell is copied, just as in mitosis

Step Two The original cell splits in two with the identical chromosomes at each end, like mitosis. Again like mitosis, a new nucleus forms around each set of genetic material, forming two identical new cells

Step Three Each of these cells the divides a second time, without copying its genetic material. This produces four haploid cells, which contain half the amount of normal chromosomes

When two gametes join and fertilise, they produce a zygote. This zygote has 46 chromosomes- the normal amount for a body cell- and so divides itself repeatedly to begin to form a human. Because the zygote contains genetic information from a male and female gamete, it will be genetically different to both of the parents. 

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B2.24- Inheritance

What Mendel Did

  • He took several varieties of pure-breeding pea plants with varying characteristics and cross-pollinated two two varieties, one with the characteristic of short stems, and one with tall
  • Mendel found that the offspring from cross-pollinating these two varieties all had tall stems
  • He then allowed this generation to interbreed. In this second generation, some of the pea plants were short-stemmed, and some were tall-stemmed. He found that the ratio of tall to short was 3:1

From these findings, Mendel deduced several things. Firstly, inheritance could not be a blending mechanism, as was previously thought- if this were the case, the first generation would be medium-stemmed, but they were not. Secondly, he realised that the characteristic of being tall-stemmed must have had some dominance over being short-stemmed, because in the first generation there were only tall-stemmed pea plants. However, the reappearance in the second generation led Mendel to believe that the characteristic had not disappeared, but was merely recessive next to the tall-stemmed characteristic. This, combined with the ratio of 3:1, all led Mendel to believe the cells in the pea plant contained two inheritance factors for each characteristic.

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B2.25- Importance of Mendel's Work

Mendel's findings can be presented in genetic diagrams to show the inheritance of characteristics:


These are two ways of displaying inheritance. A genetic diagram (left) and punnett square (right)

Mendel's work allowed scientists after him to discover the existence of DNA, chromosomes, genes and alleles. Mendel was not awarded any prizes for his discoveries, but his work greatly influenced modern science

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B2.26- Inheritance of Sex

Females have two X chromosomes. Males have one X and one Y chromosome. In every human, one of the chromosomes to code for sex comes from the mother, and one comes from the father. This results in every human having an equal 50/50 chance of being born male or female. 

Half of the male gametes (sperm) carry the X chromosome from the father. The other half carry the Y chromosome. It is the fact that fertilisation is completely random that gives the inheritance of sex a perfectly even 50% chance. 

    (                  (

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B2.27- How Genes Control Characteristics

DNA carries coded genetic material. Every cell in the human body contains 46 molecules of DNA. Each of these molecules is called a chromosome. 

Within each DNA molecule (chromosome) shorter sections of it code for certain characteristics by containing the right code for a specific combination of amino acids, which make up a protein. These sections of the chromosomes are called genes. 

Book Analogy

One whole book is all of your DNA.

The chapters are organised sections of the book. Chapters, in this analogy, represent chromosomes.

Inside these chapters are paragraphs. Each paragraph is one gene and gives across a particular 'meaning' (characteristic). 

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B2.28- Genetic Disorders


This is a genetic disorder characterised by having additional digits. It is caused by a dominant allele, meaning only one parent needs to have polydactyly for the child to have a 50% chance of inheriting it. It is not possible to be a carrier of polydactyly. 

Cystic Fibrosis

CF is another genetic disorder that affects cell membranes. This results in frequent lung infections, failure to thrive, shorter lives, infertility and mucus lining the airways and pancreas. Unlike polydactyly, cystic fibrosis is caused by a recessive allele, meaning both parents must have or be carriers of cystic fibrosis in order for their child to inherit it. It is possible to be a carrier of cystic fibrosis and never be aware of it. 

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B2.29- Embryo Screening, Stem Cells, DNA Fingerpri

If two parents know they have a genetic disorder, they can undergo IVF and have the embryo formed screened for the disorder before it is implanted in the mother.

Advantages | Children will not have disorder- long life Children will not pass it on to their own children | Saves money in long term

Disadvantages | Embryos formed do not get the chance to develop into people | May be discrimatory towards people with genetic disorders

Stem cells are undifferentiated cells. Because they aren't specialised, they can take the form of any cell. They are found in early embryos, umbilical cords, bone marrow and amniotic fluid. They are used to treat Parkinson's disease, repair spinal cord injuries, grow tissues/organs for transplants and treat type 2 diabetes

DNA fingerprinting is used to establish family relationships, particularly paternity and maternity, and criminal convictions. 

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B2.30- Fossil Evidence

Fossils are the preserved remains of living organisms after they have died. There are several types of fossil

  • Original remains of parts - Bones and shells do not decay very easily so are often preserved as fossils for a very long time, even under suitable decay conditions
  • Replaced remains - Hard parts of the organism are sometimes replaced by minerals and other substances as they decay, so the fossil is the same shape as the remains, but is effectively a replica
  • Complete remains - This occurs when one or more of the conditions required for decay are not present- for example, in amber no air can reach the organism, so it is preserved in its entireity 
  • Evidence - Footprints, burrows or traces of where the organism was may be preserved as a fossil

Fossils provide evidence for evolution. However, it is rare for fossils to form and for some soft-bodied organisms, it is impossible. Erosion/weathering may also destroy fossils, so the records are often incomplete. 

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B2.31- Extinction

Causes of extinction 

  • Changes to the environment If the environment changes, many organisms will not be effectively adapted and will die out. 
  • Major catastrophic events Some events, such as volcanic eruptions or ice ages, change the environemt so rapidly and so much that most species do not have time to adapt by natural selection, and die out
  • New predators The arrival of an efficient new predator- such as humans- may kill all of a species because they are not adapted to escaping, or the predator is particularly good at hunting (eg tools and groups)
  • New diseases Some diseases are so virulent that an enire species is wiped out before immunity can be developed
  • New competitors The introduction of a new competitor may completely wipe out a food source or force the species to retreat to a very small territory
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B2.32- Forming New Species

STEP 1: Isolation

A population of animals become geographically separated (eg river, hill) and can no longer breed

STEP 2: Genetic Variation

Random mutations will lead to different variation on each side of the geographical barrier

STEP 3: Natural Selection

Over time, due to different conditions on each side of the barrier, the populations will differ slightly and adapt to their environment 

STEP 4: Speciation

The populations will eventually have become so different to each other through natural selection that they can no longer interbreed and produce fertile offspring even if they do meet again. They are now classed as separate species

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B2 Part 2 Catch up

  • Proteins are made of amino acids. They make up the structure of living organisms and perform vital functions in cells 
  • Enzymes are proteins that catalyse reactions in cells
  • Biological washing powders are ones which contain enzymes to break down stains
  • Enzymes are used in baby foods and slimming foods 
  • Respiration provides energy necessary for life processes
  • Aerobic respiration requires oxygen; anaerobic does not
  • Mitosis is for growth/repair or asexual reproduction. The offspring are genetically identical 
  • Meiosis produces gametes (sex cells) and therefore genetic variation
  • Mendel discovered how characteristics were inherited and we can now use genetic diagrams
  • Females have two X chromosomes; men have one X and one Y chromosome
  • Genes are sections of chromosoomes. Chromosomes are molecules of DNA
  • Genes control characteristics because they code for a set of amino acids
  • There are different versions of each gene, called alleles. Some alleles cause genetic disorders
  • Everyone (except identical twins) has different DNA. This can be used to identify people
  • Fossils provide evidence for evolution and give information on extinct animals
  • Mutations, isolation and competition contribue to the formation of a new species
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