long question (8)

Question 1

what mechanism do cells use for DNA repair and give an example of a mutation and a repair mechanism

Question 2

mechanisms cell use for DNA repair

Answer 2

• deoxydipyrimidine photolyses
• DNA methyl transferase
• glycosylase

Question 3

what is deoxydipyrimidine photolyses

Answer 3

• light-dependent, only works in UV-exposed skin
• photolyase (enzyme) cleaves thymine dimers
• DNA is not cut therefore error free
• damage not repaired leads to skin cancer

Question 4

what is DNA methyl transferase

Answer 4

• enzyme
• reverse alkylating agents
• transfers methyl/ethyl groups from guanine and thymine to an internal cysteine residue
• error free

Question 5

what is glycosylase

Answer 5

• removes alkyated or foreign bases
• cleaves nucleoside bonds between deoxyribose sugar and base
• leaves apurinic/apyrimidinic i.e. no base
• base excision repair
• helix distortion
• error (mutation) = adds incorrect base

Question 6

example of a mutation

Answer 6

multisite mutations

Question 7

what are multisite mutations

Answer 7

• contain more than one base change
• deletions + insertions

Question 8

what are deletions

Answer 8

• most common
• don’t back mutate
• extensive changes will have severe effect
• small changes can have small e.g. 3 bases can delete one amino acid
• small changes can have small and big effects e.g. 4 bases can delete one amino acid & change reading frame

Question 9

what are insertions

Answer 9

• usually in the form of a recombination event
• can move around genome “jumping genes”
• possibly from viruses
• genetic elements
• disrupt transcription/translation
• important in evolution

Question 10

phenylketonuria

Answer 10

• autosomal recessive inheritance i.e. 2 mutant copies are needed
• mutations in the phenylalanine hydroxylase gene on chromosome 12q
• variation in PKU is due to heterogeneity in the mutant alleles