W8 Molecular basis of mutation

Question 1

Point mutations: Base-pair substitutions

Answer 1

1) Transitions: one purine/pyrimidine base pair to the other
2) Transversions: mutation from purine/pyrimidine base pair to the other
3) Missense mutations: Change of mRNA codon = change of one amino acid = altered protein function (If the amino acid substitution causes no change in function = neutral mutation)
4) Nonsense mutation: Change of mRNA codon to code for a STOP codon

Question 2

Point mutations: Base-pair addition/deletion

Answer 2

1) Frameshift mutation (addition/deletion): Alteration of mRNA reading frame so incorrect amino acids incorporated into the polypeptide chain

Question 3

Consequences of mutation:

Answer 3

1) Germ-line mutation:
-Occurs in germ-line cells
-Present in gametes
-Potentially passed on to next generation

Novel mutation:
-A mutation within a newborn individual not present in parents
-Mutation will be present in germ-cells and potentially passed on

Somatic mutation:
-Occurs in a somatic cell after birth
-Mutation affects individual and can be localised (not passed to offspring)

Question 4

Causes of mutations:

Answer 4

-Mutagenesis: creation of mutations
1) Spontaneous - naturally occurring in cells
2) Induced - Cause by external mutagens

Question 5

Spontaneous mutations: Depurination

Answer 5

-Spontaneous loss of a purine in the DNA strand due to hydrolysis of the covalent bond between the purine and the deoxyribose sugar
-Leaves an apurinic site
-During replication any base can be inserted opposite an apurinic site
-Leads to transversion or transition

Question 6

Spontaneous mutations: Deamination

Answer 6

-Deamination of cytosine produces Uracil (U pairs with A instead of G)
-Deamination causes UA base pairing during replication. If not repaired will lead to CG > TA transition mutation

Question 7

Induced mutations:

Answer 7

1) Ionising radiation: Emission of electrons/positrons breaks covalent bonds
2) UV light:
-Causes photochemical reactions
-Formation of covalent bonds between pyrimidines
-Can cause replication to stall = cell death
-Incorrect repair of Thymine dimers = Mutations
3) Chemical mutagens:
-Act to modify DNA by modifying chemical structure of bases
-Cause point mutations
4) Intercalating agents:
-Inset themselves between adjacent bases in one or both DNA strands
-Leads to addition or deletion of bases (frameshift mutation)