TOPIC 5 - DNA MUTATIONS AND REPAIR

Question 1

Effects of mutations within genes and proteins

Answer 1

reduced protein forming, quantity, efficiency

change in protein function

Question 2

Mutation overall impacts on genes

Answer 2

silent (no observable change)

loss of function

gain of function

conditional (changed regulation due to environmental conditions)

Question 3

3 types of base substitution + result

Answer 3

ONE BASE SUBSTITUTED

missense: alternate aa
nonsense: premature stop codon
silent: no aa effect (wobble)

Question 4

Single base deletion/insertion

Answer 4

ONE BASE ADDED/DELETED CAUSES FRAMESHIFT

could lead to immediate nonsense

extensive missense

inflame mutation: multiple of 3 nucleotides no frameshift, only bases added/deleted

Question 5

Types of larger scale mutations

Answer 5

Duplication: region of chromosome duplicated.

Inversion: part of chromosome reversed

Deletion: region of chromosome deleted.

Insertion: arm of one chromosome inserted into another

Translocation: swap between two chromosomes

Question 6

causes of mutations

Answer 6

retroviruses and transposons

induced

spontaneous

Question 7

explain induced cause of mutations + UV

Answer 7

require a mutagen,

UV irradiation causes covalent linkage between 2 adjacent pyrimidine bases. results in a melanoma

Question 8

Spontaneous hydrolytic damage

Answer 8

deprivation: losing purine base, still have rest of nucleotide, read as deletion, causing frameshift
deamination: amine group removed from cytosine producing uracil. CG is then replaced with UA

Question 9

spontaneous alkylation damage

Answer 9

methylated guanine results in altered base of guanine pairing with thymine. Base substitution of T instead of the complimentary C.

Question 10

types of repair during replication

Answer 10

proofreading polymerase

mismatch repair

Question 11

proofreading polymerase

Answer 11

fixes majority of errors

3’-5’ exonuclease activity removes several bases, replication resumes

during S phase

Question 12

mismatch repair

Answer 12

MutS binds to wrong base

MutL connected to MutS feeds DNA through finding a nick

degradation to nick by exonuclease, gap refilled

during S phase

Question 13

Base excision repair

Answer 13

FIXES DEPURINATION, DEAMINATION

single base repair

DNA glycosylases recognise altered base

excision by hydrolysis

AP endonuclease cuts backbone

normal replication

Question 14

Nucleotide excision repair

Answer 14

FIXES DIMERS

multiple base repair

multienzyme finds distortion

cleaves backbone of both strands

DNA helices unwinds strand containing lesion

normal replication

Question 15

Direct reversal repair

Answer 15

FIXES METHYLATION

Most efficient

Methyltransferase accepts methyl group on cysteine from alkylated guanine

Question 16

Transcription coupled DNA repair

Answer 16

excision repair systems coupled with RNA polymerase

polymerase detects errors and stalls

repair machinery comes to error site

Question 17

Emergency repair of heavily damaged DNA

Answer 17

In emergency, less accurate polymerase used, lack exonucleaolytic proofreading activity, only used for a few seconds, mutations likely to occur

Question 18

enzymes involved in DNA repair in cell cycle

Answer 18

ATM protein signals delay

p53 arrests at G1 checkpoint, directs repair, can apoptosis

Chk1 kinase arrests at S and G2 checkpoints, same as above

Question 19

NHEJ

Answer 19

Ku proteins recognise ds breaks, grasp broke ends, complex forms holding two ends together, nucleases create sticky ends, normal replication,

LOSS OF NUCLEOTIDES
SOMATIC CELLS

Question 20

Homologous recombination fixing

Answer 20

occurs between sister chromatids

exonuclease degrade 5’ ends producing 3’ overhang.

RecA (pro)
Rad 51 (euk)
intertwine 3' overhang strand producing a duplex.

homologous sequence found

extension of invading strand by DNA poly

Question 21

error in homologous repair

Answer 21

could use homologous chromosome instead of sister chromatid, allele could be lost if different on other chromosome

LOSS OF HETEROZYGOSITY

Question 22

Homologous repair for crossing over

Answer 22

Spo11 and Mre11 create ds breaks in one chromatid.

Normal process

invasion into chromatid on homologous chromosome, replication, and back

double holliday junction, branch migration to include more cross over

cleave to resolve junction and chromatids

Question 23

error in crossing over

Answer 23

mismatches cannot be properly fixed as parent strand cannot be distiquished, randomly selected

Question 24

Sickle cell anaemia alleles

Answer 24

2 alleles HbA and HbS

1 HbS = carrier
2 = disease

Question 25

explain recessive inheritance

Answer 25

people carry disease if they only have 1 copy of mutated gene. lots of carriers due to heterozygous advantage.

Question 26

3 Changes in DNA resulting in new genes

Answer 26

Intragenic mutation gene duplication DNA segment shuffling gene mixing (horizontal transfer)

Question 27

what is a gene family

Answer 27

similar genes with different mutations are part of gene families

Question 28

Restriction Fragment Length Polymorphism (RFLP)

Answer 28

PCR section of DNA digest with restriction enzymes gel electrophoresis blotting technique: region upstream of additional restriction site will be in different position on gel, identify specific regions of a gene and therefore identify presence of alleles, genes, fragments etc.