U4 - Mutations

What is a mutation?

A gene mutation is a change in DNA base sequence (of chromosomes)

A chromosome mutation is where the number of chromosomes changes

What is a somatic mutation?

Muatations that happen in normal body cells - not gametes or the stem cells that will go on to make gametes

→ cannot be inherited by offspring

What is a germline mutaion?

Mutations that happen in gametes or stem cells that go on to make gametes

→ can be inherited by offspring

What causes mutations?

• Mistakes during DNA replication

• High energy radiation

• Chemicals (mutagens)

How do DNA replication errors cause mutation?

• DNA polymerase = 1 mistake every 100million bases it copies

• Despite this, each person receives around 60 new mutations from each parent

• These mutations are germline

• As you grow, you develop somatic mutations as your cells divide

• This is one reason people get cancer

How does high energy radiation cause mutations?

• Nuclear radiation can break the sugar phosphate backbone causing sections of genes to be moved around as the cell attempts to repair these breakages → translocation

• UV light can cause 2 adjacent thymine or cytosine molecules to bind permanently to each other, forming a “pyrimidine dimer”. This causes problems for DNA replication and transcription and is the leading cause of melanomas

• Ionisin radiation

What are the types of consequences of mutations?

• Neutral

• Harmful

• Beneficial

How can mutations be neutral?

• No effect on the organisms chances of surviving or reproducing

• Could be a “silent” mutation → still code for the same amino acid sequence (as code is degenerate).

• Could be in and INTRON or other non-coding region

• Amino acid is changed but has little effect on the shape of the protein

How can a mutation be harmful?

• Can reduce the chances of the organism surviving and reproducing

• Often they will change the shape of the protein so that is becomes non-functional

How can a mutation be beneficial?

• Can increase the chances of the organism surviving and reproducing

• Often they are actually in the regulating regions

• They affect how much of a protein is made, not the shape of that protein

What is a point/substitution mutation?

A mutation where one base is swapped for another

What is the effect of a substitution mutation?

• Silent: could still code for the same amino acid - or mutation in a non-coding part

• Mis-sense: one amino acid is changed - this may or may not be important

• Nonsense: the polypeptide produced is shorter than normal (truncated) - if a STOP codon is formed

What are addition/deletion mutations?

Adding or taking away a base or bases

What is the effect of addition/deletion mutations?

Could cause a “frame shift” - so every codon after the mutation is affected

What are duplication mutations? (PAPER 2 ONLY)

copying a short, repeated section of the base sequence

What is the effect of duplication mutations? (PAPER 2 ONLY)

May cause a “frame shift” - often these can happen in non-coding regions

What are inversion mutations? (PAPER 2 ONLY)

Flipping a section of the DNA - often caused by DNA being cut and repaired

What are inversion mutations effect? (PAPER 2 ONLY)

Many different codons

What are translocation mutations? (PAPER 2 ONLY)

Moving DNA sections form one place to another - often caused by DNA being cut and repaired

What is the effect of translocation mutations? (PAPER 2 ONLY)

May cause problems in two places:

• where it was cut from

• where it is added to