lecture 12

Question 1

Mutations

Answer 1

alterations in the DNA structure that can produce changes in the genetic info if they are not repaired

Question 2

do mutations give a biological advantage?

Answer 2

rarely! usually that comes from adaptation and evolution

Question 3

sources of DNA damage (4)

Answer 3

• spontaneous mutations
• errors during replication
• chemical mutagens
• ionizing radiation

Question 4

Spontaneous mutations

Answer 4

DNA damage occurs continually in cells
Depurinations and deaminations can occur spontaneously!!!

• ex. 10^12 depurinations occur in your body within the time it takes to read this

Question 5

Depurination could lead to

Answer 5

a deletion of one or more nucleotides when DNA replicates
(spontaneous)

Question 6

Deamination could lead to

Answer 6

alterations in the DNA sequence when DNA replicates
(spontaneous)

Question 7

Errors during replication

Answer 7

rare, but do occur despite proofreading ability of DNA polymerase

Question 8

consequences of errors during replication

Answer 8

if uncorrected, mismatches will lead to permanent mutations in ONE of the two DNA molecules produced during DNA replication

Question 9

Chemical mutagens

Answer 9

example: deaminations occur at higher frequencies when cells are exposed to nitrous acid

• nitrous acid is used as a food preservative
  -hypoxanthine pairs w cytosine instead of a thymine

Question 10

Ionizing radiation

Answer 10

UV light can cause adjacent thymine residues to form covalent bonds with eachother

Introduces a kink in the DNA that blocks polymerization past this point

example: Xeroderma pigmentosum

Question 11

xeroderma pigmentosum

Answer 11

rare skin disease where a mutation in a gene necessary for dealing with thymine dimers is not functional (no nucleotide excision repair)

Question 12

DNA repair pathways (5)

Answer 12

1. Mismatch repair (MMR)
   = fixes errors that were missed during DNA replication
2. Base excision repair (BER)
   = removes offending base pair and replaces it
3. Nucleotide excision repair (NER)
   = removes an offending nucleotide and replaces it
4. Direct repair= does not remove the damaged base, but fixes the damage on the spot
5. Homologous recombination= homologous chromosome fixes double-stranded breaks

Question 13

MMR, BER and NER are all forms of

Answer 13

indirect repair!!
They fix the mistake by replacing a stretch of nucleotides

Question 14

MMR, BER and NER all have similar mechanisms

Answer 14

1. Use an endonuclease to create nick(s) in the DNA near the damage
2. Remove a section of the damaged DNA with an exonuclease
3. Fill in the gap with a polymerase and seal it with DNA ligase

Question 15

Mismatch repair (MMR) 3 syeps

Answer 15

=corrects errors made during replication in 3 steps

1. A complex of proteins binds to the mismatched bp, and an endonuclease cuts the damaged strand
2. An exonuclease degrades the DNA from the cut site to the mismatch
3. DNA polymerase fills in the missing nucleotides and DNA ligase seals the gap

Question 16

Base excision repair (BER)

Answer 16

=repairs DNA when the bases are altered

Ex. Deamination of cytosine
- Generally follows the same three steps as MMR
- Multiple nucleotides are replaced
- Remove offending base, DNA polymerase replaces nucleotides, endonuclease cuts DNA, DNA ligase seals nick

Question 17

Double-Strand (ds) DNA Breaks

Answer 17

= both strands are damaged at the same time
- No more redundancy from the complementary base pairing!

Question 18

Nonhomologous end joining

Answer 18

• Nucleases remove nucleotides from the broken end and fragments are ligated together
• Leads to a loss of nucleotides
• Can lead to dysfunctional genes

Question 19

Homologous recombination

Answer 19

• Repairs dsDNA breaks without the loss of nucleotides
• Most often occurs shortly after DNA replication when there are two copies of the double helix
• Also seen during genetic recombination that occurs during meiosis

Question 20

Homologous recombination: steps

Answer 20

a) Double stranded DNA break occurs
b) Exonuclease degrades 5’ ends of DNA
c) Strand invasion of unbroken homologous chromosome
d) Invading strand is extended by DNA polymerase
e) Invading strand is released
f) DNA polymerase fills in gap
g) DNA ligase seals gaps
h) Success!!!

Question 21

mutations in germ (sperm and egg) cells results in

Answer 21

the passing of the mutation to future offspring

Question 22

mutation in somatic (not sperm and egg: diploid) cells over time can lead to

Answer 22

abnormalities like cancer
- many cancers have an inherited genetic component