16.2. Mutations Flashcards
Gene Mutation
A change in the structure of a DNA molecule, producing a different allele of a gene
Environmental factors that can increase chances of a mutation
- All types of ionising radiation (alpha, beta and gamma radiation) can damage DNA molecules, altering the structure of the bases within them.
- Ultraviolet radiation has a similar effect, as do many chemicals
- These are all mutagens (increases the chances of mutation occurring)
When do mutations occur?
Not known. They are random
Ways in which the sequence of bases in a gene may be altered:
1) base substitution
2) base addition
3) base deletion
base substitution
where one base simply takes the place of another
- Often have no effect at all (silent mutation).
- Base substitutions are often silent mutations because many amino acids have more than one triplet code
- However, base substitutions can have very large effects.
base addition
where one or more extra bases are added to the sequence
- very significant effect on the structure, and therefore the function
base deletion
where one or more bases are lost from the sequence
- very significant effect on the structure, and therefore the function
Frameshifts
Base additions or deletions are said to cause frameshifts in the code. Ofen, the effects are so large that the protein that is made is totally useless. Or the addition or deletion may introduce a ‘stop’ triplet part way through a gene, so that a complete protein is never made at all.
Sickle Cell Anemia
- The small difference in the amino acid sequence makes little difference to the haemoglobin molecule when it is combined with oxygen.
- But when it is not combined with oxygen, the ‘unusual’ β-globin polypeptides make the haemoglobin molecule much less soluble.
- The molecules tend to stick to each other, forming long fibres inside the red blood cells.
- The red cells are pulled out of shape, into a half-moon or sickle shape.
- When this happens, the distorted cells become useless at transporting oxygen.
- A person with this unusual β-globin can suffer severe anaemia (lack of oxygen transported to the cells) and may die.
Sickle Cell Anemia Mutation Process
Example of base substitution
- HBA is the normal blood allele
- HBS is the sickle cell allele
- CTT (Glu) base sequence is replaced by CAT (Val)
Albinism
- Recessive Allele
- In albinism, the dark pigment melanin is totally or partially missing from the eyes, skin and hair.
- In humans this results in pale blue or pink irises in the eyes and very pale skin and hair.
- The pupils of the eyes appear red.
- The condition is often accompanied with poor vision, jerky eye movements and a tendency to avoid bright light.
Albinism Mutation Process
- A mutation in the gene for the enzyme tyrosinase results in either the absence of tyrosinase or the presence of inactive tyrosinase in the cells responsible for melanin production.
- Tyrosine is usually converted into DOPA then into dopaquinone by the enzyme tyrosinase and then dopaquinone is converted into melanin.
Huntington’s Disease
HD is a neurological disorder resulting in involuntary movements (chorea) and progressive mental deterioration. Brain cells are lost and the ventricles of the brain become larger
Huntington’s Disease Inheritance
- Huntington’s disease (HD) provides an example of a mutation that is inherited as a dominant allele.
- This means that most people with the condition are heterozygous and have a 1 in 2 chance of passing on the condition to a child.
Huntington’s Disease Mutation
- The mutation is an unstable segment in a gene on chromosome 4 coding for a protein, huntingtin.
- In people who do not have HD, the segment is made up of a small number of repeats of the triplet of bases CAG.
- People with HD have a larger number of repeats of the CAG triplet. This is called a ‘stutter’.
