DNA repair disorders

Question 1

Helicase

Answer 1

enzyme that unzips the double stranded DNA for replication

binds at OriC (origin replication)

Question 2

Topoisomerase

Answer 2

enzymes that relieve supercoiling problem by making single strand cuts in DNA to relieve twisting

Question 3

Primase

Answer 3

places RNA based template for DNA polymerase to extend

Question 4

DNA ligase

Answer 4

joins Okazaki fragments

Question 5

topoisomerase inhibitors

Answer 5

stop replication in rapidly dividing cells.

fluoroquinolones (cipro)

campothecin compounds - topotecan, etoposide (chemo)

Question 6

Chain terminators

Answer 6

remove 3’ hydroxyl group so that reverse transcriptase cannot add new nucleotides - inhibits replication of HIV

AZT (for HIV)

acyclovir

Question 7

telomerase

Answer 7

adds 6 base-pair repeat to the ends of DNA so that an RNA primer can be placed and the DNA can be transcribed by DNA polymerase rather than being lost every time the DNA is replicated

active in stem cells and cancer cells

Question 8

where is telomerase absent/why?

Answer 8

absent in terminally differentiated non-dividing cells - no DNA replication to help with

Question 9

Dyskeratosis congenita features

Answer 9

dysplastic nails, white patches in mouth, hyper/hypopigmentation, alopecia, bone marrow failure, myelopysplasia, squamous cell cancers

Question 10

Dyskeratosis congenita molecular basis

Answer 10

mutations in TERT and TERC and other telomere components - causes shortening of telomeres that worsens with each generation

Question 11

nucleotide excision repair

Answer 11

removal and replacement of nucleotide(s)

used for bulky lesions caused by UV light, smoke, PAH exposure

mutations in NER genes cause xeroderma pigmentosum

Question 12

Base excision repair

Answer 12

fixes non-standard bases or abasic sites one base at a time

usually fixes uracil bases in DNA as a result of deamination of cytosine

mutations in MUTYH (adenine glycosylase cause colon CA and polyps

Question 13

xeroderma pigmentosum

Answer 13

extreme sun sensitivity

freckle-like pigmentation

skin cancers within the first decade of life

mutation in nucleotide excision repair

Question 14

mismatch repair - what is it, what are the steps, what is the associated disease?

Answer 14

excises and replaces base pairs incorrectly put into the double helix that cause DNA to have a loop/bump in it.

MSH2/MSH6 bind mismatch -> MLH1/PMS2 bind and nick DNA -> nick extended by exonuclease, gap filled by polymerase, sealed by ligase

inherited mutation = Lynch syndrome

Question 15

Lynch syndrome

Answer 15

autosomal dominant disorder which presdisposes affected people to carcinomas (esp of the colon)

requires 2nd hit to become active

microsattelite instability

Question 16

microsattelite instability

Answer 16

hallmark of Lynch Syndrome

caused by failure to repair replication slippage at repeated sequences, resulting in changed # of repeats in DNA of tumor cells

Question 17

methods of double-strand break repair - what are they and what do they have in common?

Answer 17

non-homologous end joining

homologous recombination

both involve extensive phosphorylation of histones at the site of damage

Question 18

non-homologous end joining

Answer 18

simply joins broken ends together

can lead to loss of sequence

eliminates free DNA ends rapidly

error prone

Question 19

homologous recombination

Answer 19

uses another DNA molecule with homology (usually a sister chromatid) as template for repair

more accurate than non-homologous end-joining

leads to disease if damage occurs to normal gene and is repaired with a mutant template

Question 20

how do cancer cells continue proliferating without shortening telomeres?

Answer 20

activate telomerase

Question 21

hMutSa

Answer 21

recognizes single mismatches for mismatch repair

Question 22

hMutSB

Answer 22

recognizes insertion/deletion loops resulting from replication slippage for mismatch repair

Question 23

when else is homologous recombination used, and what diseases are associated with errors?

Answer 23

gametogenesis for meiotic recombination

errors occur when chromosomes out of alignment in presence of repeat sequences - leads to Williams and CATCH-22 syndromes

Question 24

direct reversal DNA repair

Answer 24

damage reverses without cleavage of the DNA backbone

when bases are alkylated, repair methyltransferases transfer the alkyl group to themselves - irreversible and “suicidal” repair

Question 25

ataxia telangiectasia

Answer 25

double strand break repair syndrome

cancer and neurodegeneration

Question 26

molecular basis of anticipation

Answer 26

triplet repeats are hard to copy due to slippage - the longer the repeat, the more likely slippage will occur

Question 27

EX triplet repeat expansion disorders

Answer 27

Fragile X

Myotonic dystrophy

Huntington’s Disease

Question 28

Fragile X

Answer 28

clinical features: protruded chin, mental retardation, large testicles

affected when greater than 200 triplet repeats

the expansion is not in the protein coding part of the gene, but rather, is up stream and causes FMR1 protein not to be expressed.

FMR1 is essential for normal cognitive development.

Question 29

Myotonic dystrophy

Answer 29

clinical features: gradually worsening muscle loss and weakness

affected range 50 (mild) to 5000+ (severe, congenital)

long mRNAs form aggregates

Question 30

Huntington’s disease

Answer 30

clinical features: mood changes, abnormal gait, progresses into dementia.

36-121 triplet repeats to be affected

polyglutamine tract in Huntington protein - creates sticky aggregates, which increase the degradation rate of neurons