Mutations Genetic Diseases and DNA Repair

Question 1

DNA Polymorphisms v Mutations

Answer 1

Polymorphism- Variation at a locus in 2 alleles that occur at a frequency > 1% (Common in population)
Ex) SNPs

Mutation- stable change in DNA sequence <1% (RARE- deviation from normal population)

Question 2

What are 2 possible ways mutations can lead to splicing errors?

Answer 2

1- alteration of conserved splice signals (AKA mutation of already existing splice donor or splice acceptor sequence)

2- direct activation of cryptic splice sites (mutation in normal portion of intron or exon that changes its sequence so it now mimics the consensus splice donor or splice acceptor sequence) New sites now recognize by spliceosome

Question 3

Sources of Mutation (4)

Answer 3

1- spontaneous alteration of nucleotides
2- chemical modifications of bases
3- radiation damage
4- polymerase errors during replication

Question 4

Thymine Dimer

Answer 4

• caused by UV exposure from sun

- carbons of neighboring thymines will attach directly at base- not by normal phosphodiester bond

Question 5

Deamination of DNA Nucleotides

Answer 5

• Deamination= remove amino group from base
• products of deaminated A & G are recognized as unnatural and repaired, T has no amino group so not deaminated, and product of C is uracil which is also recognized and repaired b/c U should not be in DNA
• BUT…when methylated C is deaminated –> thymine (NOT RECOGNIZED) so now have T-G which is mismatch

Question 6

Mechanisms of DNA Repair (3)

Answer 6

1- Base excision repair
2- NER (nucleotide excision repair)
3- Mismatch repair

Question 7

Base Excision Repair

Answer 7

-SINGLE base or part of single base

1-DNA glycosylase recognizes and removes altered base
2-sugar phosphate of missing base is removed by endonuclease and phosphodiesterase
3- gap filled by DNA polymerase
4- nick sealed by DNA ligase

Question 8

NER

Answer 8

• Nucleotide excision repair
• generally for correction of mult nucleotides

1- multi-enzyme complex recognizes bulky lesion
2-Break phosphodiester bonds on ea side of lesion (BY ENDONUCLEASE)
3-DNA helicase removes entire portion of damaged strand
4- polymerases and ligases fill it in

Question 9

2 sub-pathways of NER

Answer 9

1- The GGR-NER

2- Transcription-Coupled Repair (TCR-NER) - more efficient b/c used when DNA is being transcribed (needs to be more efficient b/c actively being turned into mRNA to be made into proteins)

**both use same set of proteins and general steps

Question 10

Translesion DNA Synthesis

Answer 10

• Alternative to NER
• DNA replication can occur despite DNA damage by using DNA polymerase eta to copy the bases at the damage site so normal DNA polymerase can cont
• DNA poly eta is error free BUT same thing can happen using DNA poly zeta but more errors

Question 11

Mismatch Repair

Answer 11

• Often directed by methyl groups; mismatch repair system recognizes that the methylated strand must be the template and thus the correct strand
• As a result it corrects the new/un-methylated strand

Question 12

Why do repeat expansions occur?

Answer 12

• DNA polymerases have proofreading capabilities but are not as good in regions of repeated base sequences
• New strand will have extra repeat and mismatch repair systems not functional in those w/ disorder –> these additional bases remain in daughter cell so daughter cell is expanded

Question 13

Double Strand DNA Break Repair (2 ways)

Answer 13

Non-homologous end joining- quick and dirty; attach broken ends but usually suffer deletion of bases (MORE common)

Homologous recombination- longer and more precise; use sister chromatid as template to fill in missing bases at the break (LESS common)

Question 14

Xeroderma pigmentosum (XP)

Answer 14

• lack genes needed for NER (due to mutations in these genes)
• results in premature aging and inc risk of skin cancer
• have to stay out of the sun to avoid UV dimer formation b/c these cannot be repaired