Lecture 12 Flashcards
1
Q
Mutations:
A
- Alterations in DNA structure that can produce permanent changes in genetic information encoded if they are not repaired
- Rarely, mutations serve as a biological advantage – adaption and evolution
2
Q
List 4 mechanisms of DNA damage:
A
- Spontaneous mutations
- Error during replication
- Chemical mutagens
- Ionizing radiation
3
Q
How often does DNA damage occur in the cell?
A
- Continually
- 10^12 depurinations occur in your body within the time it takes you to read a sentence
- Deamination and depurination can occur spontaneously
4
Q
What are the consequences of spontaneous mutation
A
- Depurination can lead to the deletion of one or more nucleotides when DNA replicates
- Deamination can lead to alterations in DNA sequence when DNA replicates
- These mutations are spontaneous
5
Q
Errors during replication:
A
- Errors during replication are rare, but do occur despite the proof-reading ability of DNA polymerase
- The rate is approx. 1 mistake for every 10^7 correctly added nucleotides
6
Q
Consequences of errors during DNA replication
A
- If uncorrected, mismatches will lead to permanent mutations in one of the two DNA molecules produced during DNA replication
7
Q
How does nitrous acid function as a chemical mutagen?
A
- Example: deamination’s can occur at higher frequencies when cells are exposed to nitrous acid
- Nitrous acid is used as a food preservative
- Hypoxanthine pairs with cytosine instead of thymine
8
Q
Ionizing radiation
A
- UV lights can cause adjacent thymine residues to form covalent bonds with one another
- Which introduces a kink in the DNA that blocks polymerization past this point
9
Q
Xeroderma pigmentosum:
A
- A rare skin disease where a mutation in a gene necessary for dealing with thymine dimers is not functional (nucleotide excision repair is needed)
10
Q
What are the 5 types of DNA repair?
A
- Mismatch repair: used to fix errors that were missed during DNA replication
- Base excision repair: removes offending base and replaces it
- Nucleotide excision repair: removes offending nucleotide and replaces it
- Direct repair: does not remove damaged base, but fixes the damage on spot
- Homologous recombination: takes advantage of a homologous chromosome to fix double stranded breaks
11
Q
What 3 type of repair are indirect and what mechanism do they use?
A
- Mismatch repair
- Base excision repair
- Nucleotide excision repair
Mechanisms:
- Use an endonuclease to create a nick in the DNA near the damage
- Remove a section of the damaged DNA with an exonuclease
- Fill in the gaps with a polymerase and seal it with DNA ligase
12
Q
What are the 3 steps that mismatch repair uses to correct errors during replication?
A
- A complex of proteins binds to mismatched base pairs and an endonuclease cuts of damaged strand
- An exonuclease degrades the DNA from cut site to the mismatch
- DNA polymerase fills in the missing nucleotides and DNA ligase seals the gap
13
Q
Describe DNA methylation
A
- DNA is normally methylated
- It takes a few minutes for the newly synthesized DNA strand to become methylated
- Shortly after replication, DNA is hemimethylated… meaning that the template strand is methylated while the newly synthesized strand is not
14
Q
Base excision repair:
A
- Removes offending base and replaces it
- Multiple nucleotides can be replaced
- Generally, follows the same 3 steps as MMR
1. Excision of damaged region
2. Endonuclease cuts DNA
3. DNA polymerase makes new top strand by using bottom strand as a template
4. DNA ligase seals the nick (gap) - Example: deamination of cytosine
15
Q
What are double stranded DNA breaks?
A
- Both strands are damaged at the same time
16
Q
What is nonhomologous end joining?
A
- Nucleases remove nucleotides from broken ends and then is ligated together
- Leads to loss of nucleotides
- Can lead to dysfunctional genes
17
Q
Homologous recombination
A
- Takes advantage of a homologous chromosome to fix double stranded breaks without a loss of nucleotides
- Occurs shortly after DNA replication when there are two copies of the DNA double helix
- Also seen during genetic recombination that occurs during meiosis
18
Q
Outline the process of homologous recombination:
A
- DNA double strand break occurs
- Exonuclease degrades 5’ end of DNA
- Strand invasion of unbroken homologous chromosome
- Invading strand is extended by DNA polymerase
- Invading strand is released
- DNA polymerase fills in gaps
- DNA ligase seals the gaps
19
Q
Compare consequences of damage in germ cells vs. somatic cells
A
- Mutations in germ cells can result in the passing of mutations to future offspring’s
- Mutations in somatic cells over time can lead to abnormalities like cancer
20
Q
Fidelity of DNA replication
A
- Despite millions of years of divergent evolution, DNA sequences between many species remain very similar
- This is a consequence of the faithfulness of DNA replication and repair
