Mutations

Question 1

What is a mutagen?

Answer 1

A mutagen is an agent, such as a chemical or radiation that causes a genetic mutation.

Question 2

What is a point mutation? And what are the two types?

Answer 2

A point mutations is a single base change.
Transitions change purines to purines(A and G)/ pyrimidines to pyrimidines (C and T)
Transversions convert purine to pyrimidines and vice versa

Question 3

What are the different types of point mutation?

Answer 3

Nonsense mutation - nonsense mutation changes a codon for an amino acid into a STOP codon. Causes a shorter protein.

Missense mutation - missense mutation changes a codon for an amino acid into a codon for a different amino acid

Silent mutation - silent mutations change a codon for an amino acid into a different codon for the same amino acid

Question 4

What is a transposon?

Answer 4

A transposon is an autonomous mobile DNA sequence (DNA sequence that can change its position within a genome, sometimes creating or reversing mutations).

Question 5

What are the different types of mutation?

Answer 5

Point mutations

Insertions and deletions

Rearrangements - inversions (section of DNA being flipped so the beginning of the section is now the end)/translocations

Question 6

What are the consequences of small insertions or deletions?

Answer 6

INDELs can cause a frame shift

• no frame shift when INDELs are multiples of 3
• if splicing acceptor site is deleted from intron and the spliceosome will not be able to remove intron and so intron is translated which will often kill protein

Question 7

Describe the different allele classifications for diploids.

Answer 7

Dominant allele - a dominant allele expresses its phenotypic effects when heterozygous. Most wild type alleles are dominant.

Recessive allele - the phenotype of a recessive allele is masked by the presence of a wild type allele. Most mutant alleles are recessive.

Question 8

What are the three types of dominant mutation?

Answer 8

Hypermorph - a mutant allele which produces an exaggerated version of the wild type alleles. The phenotype of a Hypermorph is worsened by increasing wild type gene dose. E.g. Gene for making blue eyes, normal gene would make blue eyes but Hypermorph would make really bright blue eye. Increased protein activity.

Neomorph - a mutation that causes a dominant gain of function that is different from the original function. E.g. New protein/gene activity. E.g. Ectopic expression - over expression of gene eyeless in other tissues induces eye formation.

Antimorph - dominant mutations that act in opposition to normal gene function. Anti morphs are also called dominant negative mutations. A type of mutation in which the altered gene product possesses an altered function that antagonises the wild type allele. E.g. Makes the protein not work as well, fly eyes get smaller if antimorph but increasing the wild type function reduces the severity of an antimorph

Question 9

What are the two types of recessive mutation?

Answer 9

Hypomorph - causes a partial loss of function e.g. Reduced protein activity/reduced gene expression. The phenotype of a hypomorphic allele is stronger when the other allele is null than when homozygous

Null (amorph) - total loss of function e.g. Eliminates protein activity

Question 10

What is a reversion and what are the three causes of this?

Answer 10

A reversion is the restoration or partial restoration of an allele back to its wild type state
The three causes of a reversion include;
- back mutation - the original mutation has been reversed
- the gene acquires a second mutation that counteracts the first mutation
- mutation in the second gene that counteracts the mutation in the first gene - inter genie suppression

Question 11

What is synthetic lethality?

Answer 11

Synthetic lethality occurs when a combination of deficiencies of two or more genes leads to cell deaths where as a deficiency in only one of these genes does not.

Question 12

What are the two types of genetic interaction pathway with respect to synthetic lethality?

Answer 12

Within-pathway genetic interaction

• if both genes are normal this yields the wild type phenotype
• if only one of either gene is mutant, the cell is viable
• if both genes are mutant then this is lethal

Between-pathway genetic interactions

• both pathways bring about essential biological functions
• if you mutate one of either pathways the essential biological function can still be brought about the other
• need at least one non-mutated pathway to bring about biological function