6.1.1 Gene Mutations

Question 1

What is a gene mutation?

Answer 1

• A change in the sequence of base pairs in DNA
• Resulting in altered polypeptide

Question 2

What increases the probability of mutations?

Answer 2

• Ionising radiation (X rays can break DNA strands, which are altered during repair process)
• Deaminating chemicals (can convert 1 base to another)
• Methyl or ethyl groups added to bases (Causes incorrect base pairing)
• Viruses can insert sections of viral DNA into DNA of cells, changing the nucleotide sequences, altering the polypeptide

Question 3

Why is the DNA sequence described as ‘degenerate’?

Answer 3

Several different triplets code for the same amino acid

(so mutations often have no effect, sequence is altered slightly so the structure and function doesn’t change)

Question 4

3 methods of mutation

Answer 4

• Insertion
• Deletion
• Substitution

(of one or more nucleotides)

Question 5

How does ‘insertion’ mutation work?

Answer 5

• When a new base is inserted in the DNA sequence
• Changes the amino acid that would have been coded for originally
• Knock on effect by changing triplets further down DNA sequence
• ‘frameshift’ mutation
• Can drastically change the amino acid sequence and so affect the polypeptide’s ability to function

Question 6

How does ‘deletion’ of nucleotides work?

Answer 6

• Mutation occurring when nucleotides are deleted
• frameshift effect
• Knock on effect on bases further down DNA sequence, hence altering the polypeptide function a lot

Question 7

How does ‘substitution’ of nucleotides work?

Answer 7

• When the base is swapped for another
• No frameshift effect, so no knock on effect

Can cause 1 of 3 things
- Silent mutation (mutation does not alter amino acid sequence, genetic code is degenerate)
- Missense mutation (1 amino acid is changed in the whole polypeptide chain, (sickle cell anaemia)
- Nonsense mutation (creates premature stop codon, causing polypeptide chain to be incomplete so protein’s structure and function is affected a lot (cystic fibrosis)

Question 8

What are beneficial mutations?

Answer 8

• Mutations which alter the characteristic of an organism causing beneficial effects

Eg. Production of melanin provide protection from harmful UV radiation, whilst still letting vitamin D be synthesised. Also at lower sunlight intensities, paler skin lets vitamin D be synthesised more easily than darker skin.

Question 9

What are harmful mutations? What is cystic fibrosis?

Answer 9

• Harmful effects

Eg. Haemophilia, sickle cell anaemia, cystic fibrosis

Cystic fibrosis is caused by a deletion mutation, and is a nonsense mutation. Causes lung and pancreatic problems due to extreme thickness of mucus.

Question 10

What are neutral mutations?

Answer 10

• No adv or disadv
• Mutation doesn’t alter polypeptide chain
• Or only slightly so that it has no effect on structure or function

Eg. Bitter tasting testing ability in Brussel sprouts