Edexcel Categories Archives: Topic 3: Voice of the Genome



Continuous variation is when individuals in a population vary within a range with no distinct categories; these can be things like height, mass or skin colour.

Discontinuous variation is when individuals are in a specific category, and will fall into only one category, these can be things like sex, eye colour and blood group.

Some characteristics are controlled by only one gene, and are called monogenic, these tend to show discontinuous variations.

Most traits are polygenic which is when they are controlled by a number of genes are different loci. When one gene is affected by another it is known as epistasis.

Some characteristics are only influenced by genotype such as blood group, but some are influenced by the environment and genotype. Some of these include Height, monoamine oxidase A, Cancer and animal hair colour.


Since 1850 the human population has on average got 8cm taller. This can be due to many reasons such as:

  • Taller men have more children, resulting in a gradual change
  • Greater movements of people have lead to less inbreeding
  • Improved health, especially in protein
  • The end of child labour has allowed more energy to be put into growth.

Hair colour

In animals they make melanin using the enzyme Tyrosinase. This is the first step along a chemical pathway. Some animals have light hair in the summer and dark in the winter this is due to a temperature trigger.


Monoamine oxidase A is an enzyme that catalyses the breakdown of neurotransmitters in the brain involved in the regulation of behaviour. It has been found that in some individuals they have a rare mutation of the MAOA gene, where no enzymes are produced; these people portray a very aggressive behaviour.



It is the uncontrolled division of cells that leads to a lump of cells (tumours) forming. It is made up of both an environmental and genetic threshold.



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Cell differentiation

Cell differentiation

Stem cells are unspecialised and they can develop into any type of cell.

Totipotency which is when it has the ability to produce all cell types including the specialised cells in an organism and extra-embryonic cells.

The second type is Pluripotency which has the ability to produce all the specialised cells, but not extra-embryonic cells.

Totipotency cells are only available in very early stages of the embryo in humans. When there are only 8 cells, this occurs after three divisions.

Five days after conception, a hollow ball of cells called the blastocyst forms. The outer cells of the blastocyst form the placenta. The 50 or so cells inside this form the tissue of the developing embryo, these are known as pluripotent embryonic stem cells.

Multipotent stem cells are those which still have the capacity to give rise to different cell types, such as neural stem cells.

How stem cells work:

  1. Stem cells all contain the same gene, but not all of them are expressed because some are ‘switched’ off.
  2. Under the right conditions some genes are activated and others inactive.
  3. mRNA is only transcribed from the active gene.
  4. The mRNA from the active genes is then translated into proteins.
  5. These proteins modify the cell- they determine the cell structure and control cell processes.
  6. Changes to the cell produced by these proteins causes the cell to become specialised. These changes are difficult to reverse.

Some stem cell treatment already exists, such as treatment for leukaemia, where bone marrow transplants are given. Scientists are researching treatments for injuries such as: spinal cord injuries, and heart disease. The treatments can save many lives, and also improve the quality of lives for many people.


There are two methods from where stem cells can come from, these are Adult stem cells and embryonic stem cells.

How society makes decisions about the use of stem cells.

There are many for and against arguments.

To help these arguments many authorities have been set up.  These include looking at the proposals of research to see if it should be allowed. This ensures that any research involving embryos is carried out for a good reason. The licensing and monitoring centres involved in embryonic stem cell research. There are many guidelines which are involved. They also provide evidence and advice to governmental officials. There are no ethical objections to a person using multipotent stem cells derived from adults. In the UK the HFEA regulates research on human embryos.

Different genes are switched on and off. Such as the b-galactosidase which is an enzyme, it breaks down the carbohydrate lactose when it is presented in the surroundings.  When lactose is not present in the environment a lactose repressor molecule binds to the DNA, this prevents the transcription of the b galactosidase gene.

Eukaryotes: genes in uncoiled, accessible regions of the eukaryote DNA can be transcribed into messenger RNA. The enzyme RNA polymerase binds to the section of DNA adjacent to the gene to be transcribed. This section is known as the promoter region. Only once the enzyme has become attached to the DNA will the transcription proceed. The gene remains switched off until the enzyme attaches to the promoter region successfully. “


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Fertilisation is where male and female gametes fuse.

In mammals fertilisation occurs in the oviduct, and in flowering plants it occurs in the embryo sac.

  1. Sperm is deposited high up in the vagina, near the cervix. Once there the sperm makes its way up the cervix and uterus and into one of the two oviducts fertilisation occurs.
  2. The sperm swims towards the egg cell in the oviduct.
  3. Once the sperm contacts the zona pellucida of the egg cell, the acrosome swells and bursts, releasing digestive enzymes, these digestive enzymes digest the zona pellucida, so it can reach the cell membrane of the egg cell.
  4. Once the sperm head fuses with the cell membrane, this causes a cortical reaction occurs, which is when the egg cell releases cortical granules in a space between the cell membrane and the zona pellucid.
  5. The chemicals from the cortical granules make the zona pellucid thicken, making it impenetrable to other sperm cells.
  6. Once the nuclei fuse the tall is discarded.

A flowering plant:

  1. A pollen grain lands on the stigma of a flower. The grain absorbs water and splits open.
  2. A pollen tube grows out of the pollen grain down the style. There are three nuclei in the pollen tube. One tube nucleus at the tube tip and two male gamete nuclei behind it that digest surrounding cells, making a way through for the pollen tube to go.
  3. When the tube reaches the ovary, it grows through the micropyle, and into the embryo sac within the ovule.
  4. In the embryo sac, the tube nucleus disintegrates and the tip of the pollen tube bursts releasing two male nuclei
  5. One male nucleus fuses with the egg nucleus to make a zygote. This divides by mitosis.
  6. The other male nucleus fuses with two other nuclei called polar nuclei, which is at the centre of the embryo sac. This produces a ell with a large nucleus. This provides a large food store, called the endosperm.
  7. So a double fertilisation has taken place.
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Production of a gametes

Production of a gametes

In animals the male gamete is sperm, and in female it is an egg cell (ova). In plants the male gamete is pollen grains, and the female gametes are ovules.

Gametes contain only half the normal amount of chromosomes, so in one gamete in humans it will be 23.

  1. The DNA replicates so there are two identical copies of each chromosomes called chromatids
  2. The DNA condense to form double armed chromosomes, made from two sister chromatids
  3. The chromosomes arrange themselves into homologues pairs, so 1 will match up with 1.
  4. First division the homologues pairs are separated, halving the chromosomes number.
  5. Second division – the pairs of sister chromatids are separated
  6. Four new gametes that are generically different are produced.



Genetic variation:

  • Crossing over of chromatids is when before the first division of meiosis the homologous pairs of chromosomes come together and pair up, two of the chromatids in each pair twist around each other. The twisted bit breaks off their original chromatid and rejoins onto the other chromatid recombining their genetically material. The point where the chromatids break is called the chiasmata.


  • Independent assortment of chromosomes is when the four daughter cells formed from meiosis have completely different combinations of chromosomes.
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The Cell cycle and Mitosis

The Cell cycle and Mitosis

The cell cycle consists of a period of cell growth and DNA replication called interphase and a period of cell division called mitosis.

Interphase is sub divided into three separate growth stages These are called G,S and G2. Mitosis is used for growth, repair and asexual reproduction.






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Cell organisation

Cell organisation

Genes are expressed through master genes. Such as in fruit flies once the main body segment have been determined, the cells in each segment become specialised for the appropriate structure, (Wings  Antennae) The master genes control the development of each segment. These genes were discovered by looking at mutations. The master gene produces mRNA which is translated into signal proteins. These proteins switch on the genes responsible for producing the proteins needed for the specialisation of the cell in each segment.

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Protein Transport

Protein Transport

  1. Transcription occurs when changing DNA to mRNA, where the mRNA leaves the nucleus via pores in the nuclear envelope.
  2. Proteins are made on ribosome’s
  3. The ribosomes sometimes attach to the rER where the protein made moves through making the 3-D shape en route. (ribosomes on the rough endoplasmic reticulum make proteins that are excreted or attached to the cell membrane, whereas the ribosomes which are free in the cytoplasm make proteins which stay in the cytoplasm)
  4. Vesicles pinch off the rER, these contain the proteins.
  5. Then they are transported to the Golgi apparatus where the vesicle fuses with the flattened sacs of the Golgi apparatus; here the proteins may undergo further processing/moulding.
  6. Vesicles’ containing the modified protein pinches off the Golgi apparatus, to be transported around the cell. Such as extracellular enzymes will be moved to the cell membrane ready to be excreted.
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The Human Genome


A genome is all the DNA of an organism or species.

In 2001, the Human Genome Project published a working draft of the sequence of bases in human cells. Work continues to identify specific genes and establish their function.

Part of the budget for the HGP has been set aside to address the ethical, legal and social issues which may arise from the project:


  • Should health insurance companies have access to information about genetic predisposition of potential clients to particular conditions?
  • When, and on whom should predisposition tests be carried out?
  • Who keeps this information confidential?
  • Should scientists have the right to patent particular sequences?
  • How will treatment made possible by the project be paid for?
  • Is it acceptable to destroy embryos found to contain mutant genes?
  • Is it acceptable to select embryos on the basis of desirable characteristics?
  • Inserting genes into embryos (germ line gene therapy) presents many risks
  • Should genes be transferred between species for transplantation purposes?



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Gene Transcription


The specialised function of a cell depends upon the proteins it synthesises ie which genes are expressed.

Transcription of a gene is initiated by RNA polymerase and transcription factors binding to a promoter region (section of DNA adjacent to gene).


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