A3 MUTATIONS AND CANCER

What is a gene mutation?

a change in the sequences of bases in a gene

What can a mutation in DNA cause?

a change in the sequence of amino acids in the polypeptide chain, so may alter the function of the protein

What is the genetic code?

consists of base triplets which code for amino acids

How is the genetic code described?

• universal

• degenerate

• non-overlapping

What occurs during a deletion/addition mutation?

• loss/gain of a base in a region of DNA coding for a gene

• results in an alteration of the base triplets from the mutation

• reading frame has been shifted to the left, so is known as a frame shift

What is a substitution mutation?

where one base can be swapped for a different base

What are the 3 outcomes of a substitution mutation?

• missense mutation

• silent mutation

• nonsense mutation

What occurs in a missense mutation?

• a single amino acid is changed in the polypeptide chain

• will affect the primary structure of the protein so will affect the tertiary structure

What occurs in a silent mutation?

• no effect on the protein produced

• due to the degenerative nature of the genetic code, more than one base triplet can code for the same amino acid

• the mutation does not alter the amino acid coded for

What occurs in a nonsense mutation?

• a production of a truncated protein

• mutation causes a triplet to code for a stop codon

• the polypeptide chain will terminate prematurely so will not be able to perform its proper function

What is an inversion mutation?

• a group of bases becomes separated from the DNA sequence and rejoin at the same position but back to front

• changes the amino acids in the portion of the protein, so may affect the shape and function of the protein

What is a duplication mutation?

• one or more bases are repeated

• a doubling of a part of a chromosome, of an entire chromosome or the whole genome

What is a translocation mutation?

• a group of bases become separated from the DNA sequence on one chromosome and becomes inserted into DNA sequence of a different chromosome

• potential to create new genes or completely disrupt genes

When do mutations occur?

• spontaneously during DNA replication

• natural and random

• cause permanent changes to the DNA

• occur at a set rate

What’re mutagenic agents?

• increase the rate of mutation

• high energy radiation

• carcinogens

What is the rate of cell division controlled by?

• proto-oncogenes (which stimulate cell division)

• tumour suppressor genes (which slow cell division)

What occurs if a proto-oncogene is mutated?

becomes a oncogene, resulting in an over-stimulation of cell division so cell division is permanently switched on, resulting in a tumour

What occurs if a mutation occurs in a tumour-suppressor gene?

gene becomes activated, so it stops inhibiting cell division and the rate of cell division increases

What can tumours be?

• malignant

• benign

What is a benign tumour?

encapsulated by connective tissue, slower growing and do not metastasise and are usually treatable with surgery

What is a malignant tumour?

fast growing, non-capsulated and can spread

What’re the different ways that the rate of cell division will increase?

• mutation occurs in a proto-oncogene, altering it to become an oncogene which results in an over-stimulation of cell division so cell division is permanently switched on

• mutation occurs in a tumour-suppressor gene so the gene becomes activated and stops inhibiting cell division

• tumour suppressor gene is over methylated, so the gene becomes inactivated and stops inhibiting cell division

• oncogene is undermethylated, so the gene becomes more activated and cell division is switched on

How is breast cancer stimulated by oestrogen?

• cancer cells have ER receptors in the membrane

• oestrogen stimulates them to divide