7.2 Molecular mechanisms of mutation Flashcards
What corrects mutations/base changes
DNA repair
true or false: there is a race between DNA repair and DNA replication
true. DNA repair occurs before DNA replication, before the mutation is incorporated into the genome
What are the 8 natural processes that cause spontaneous mutatons through DNA damage
- Depurination
- Deamination
- X-rays
- Ultraviolet light
- Oxidative damage
- Proofreading
- Base tautomerization
- Trinucleotide repeats
What is depurination
is a process in which a purine base (adenine or guanine) is lost from a DNA molecule due to the breaking of the glycosidic bond between the base and the sugar. This leaves an empty site in the DNA strand called an AP (apurinic) site. If not repaired, this can lead to mutations during DNA replication, as DNA polymerase may insert an incorrect base opposite the missing site. Depurination is one of the most common types of spontaneous DNA damage. it happens 1000/hr in every cell
What is deamination of C
The removal of an amino group (-NH2). It changes cytosine to uracil. Hence, deamination followed by replication may alter a C:G base pair to a T:A pain in future generations of DNA molecules.
Explain X rays
naturally occuring radiation which breaks the sugar phosphate backbone of a DNA molecule
Explain ultraviolet light
It causes adjaent thymine residues to become chemically linked into thymine dimers (DNA lesions formed when two adjacent thymine bases bond covalently)
Explain oxidative damage
(of any of the four bases)
8-oxodG(Guanine) mispairs with Adenine
so normal G-C becomes mutant A-T after replication
Explain Proofreading (function of DNA polymerase)
It decreases mistakes during replication. Proofreading function of DNA polymerase—3’-to-5’ exonuclease recognizes and excises mismatches. Mutations may occur if the polymerase misses an error, allowing it to remain uncorrected, or if it mistakenly removes a correct nucleotide and replaces it with an incorrect one. Additionally, in regions of repetitive sequences, polymerase slippage can lead to insertions or deletions, causing frameshift mutations if not corrected. Though proofreading is generally protective, these rare events highlight its potential to introduce mutations.
Explain what Base tautomerization
Another reason DNA polymerase may make mistakes is due to base tautomerization. Each of the 4 bases has two tautomers (similar chemical forms that interconvert continually). This causes mutations when DNA bases temporarily shift to alternative forms, or tautomers, with slightly different structures. These rare forms change the base-pairing properties: for example, adenine may pair with cytosine instead of thymine. If a tautomeric form occurs during DNA replication, the incorrect base pairing can result in the incorporation of the wrong nucleotide. When DNA replicates again, this mismatch becomes a permanent mutation, introducing an error into the DNA sequence (point mutations)
Explain how trinucleotide repeats cause mutation
Trinucleotide repeats are sequences in DNA where a three-nucleotide motif (like CAG) repeats multiple times. During DNA replication, slippage by DNA polymerase in these regions can lead to mutations by changing the repeat count. Repeats above/under a certain number result in disease causing alleles
Differentiate between expansion and contraction of trinucleotide repeats
Expansion: When slippage causes the repeat number to increase (e.g., CAG becomes longer), it results in more repeated motifs. This can lead to disorders if the repeat count exceeds a certain threshold (e.g., Huntington’s disease).
Contraction: When the slippage reduces the repeat count, it’s called contraction, leading to fewer repeats than the original sequence. While less common, it can still impact gene function if repeats are critical for normal expression.
What is a mutagen
It is a physical or chemical agent that raises the frequency of mutations above the spontaneous rate.
What are the three ways in which mutagens alter DNA (chemical action)?
- Replace a base
- Alter base structure and properties:
a) Hydroxylating agents add an -OH group
b) Alkylating agents add ehtyl or methyl groups
c) Deaminating agents remove amine groups - Insert between bases: Intercalating agents- cause double strand breaks
What are the four accurate repair systems
- correction of DNA replication errors
- Double-strand break repair
- Reversal of DNA base alterations (excision repair)
- Homology-dependent repair of damaged bases or nucleotides
What are the 2 error-prone repair systems
SOS systems
Microhomology-mediated end-joining (MMEJ)
base excision repair fixes damage caused by what two processes
Depurination and Deamination
Explain the process of Base excision repair
- Deamination of C
- DNA glycosylase removes U and creates an AP site
- AP endonuclease cuts the backbone (inside)
- DNA exonuclease removes nucleotides
- DNA polymerase synthesises new DNA to fill gaps
- DNA ligase seals everything together
Nucleotide excision repair fixes damage caused by what processe
Ultraviolet light
Explain the process of Nucleotide excision repair
- Exposure to UV light causing a thymine dimer to form.
- UvrA and UvrB scan the DNA to look for mistakes
- UvrB and UvrC endonucleases nick (cut) the strand containing the dimer
- Damaged DNA fragment is released
- DNA polymerase fills gap with new DNA
- DNA ligase seals everything together
What are the two mechanism that repair double strand breaks
1, nonhomologous end-joining (NHEJ)
2. Homologous recombination (similar to process during meiosis)
Unrepaired double-strand breaks can lead to ____________
Deletions and chromosome rearangement
Explain the process of homologous recombination
- Double strand break
- Resection
- Strand invasion
- Formation of double Holliday junction
- Branch migration
- Resolution of Holliday junction
Explain NHEJ
- Double strand breaks
- PKcs+ KU80+ KU70 binds to the double strand breaks and protects the ends from nucleases
- This bridges the two ends., allowing DNA ligase to bind loose ends
In bacteria what mechanism corrects mistakes in replication
methyl-directed mismatch repair
Explain the process of methyl-directed mismatch repair
- G/C in parental strand is mutated to G/T in the new strand.
- To distinguish the G/C and the G/T, an enzyme called adenine methylase puts a methyl group on the A near a GATC site
- MutL and MutS detect and bind to the mismatched nucleotides (G/T)
- MutL and MutS direct another enzyme- MutH to nick the G/T mutation
- DNA endonuclease removes all the nucleotides between the nick and a position just beyond the mismatch, leaving a gap
- DNA polymerase fills the gap with new DNA
- DNA ligase seals everything together
Explain the SOS system-bacteria
It is used at replication forks that stalled because of unrepaired DNA damage. “Sloppy” DNA polymerase used instead of normal polymerase. It adds random nucleotides opposite damaged bases
Explain microhomologous mediated end-joining (MMEJ)
Similar to NHEJ, except nucleotides are removed at double-stranded breaks leading to small deletions
What are three examples of human diseases that result due to defects in DNA repair genes
Xeroderma pigmentosum
colorectal cancer
breast cancer
Xeroderma pigmentosum results from?
Mutations in any one of the 7 genes involved in nucleotide excision repair
Hereditary forms of colorectal cancer results from
Mutations in mismatch repair genes
Hereditary forms of breast cancer results from
Breast cancer genes BCRA1 and BCRA2 involved in double-strand break repair by homologous recombination
Differentiate between germ line mutations and somatic mutations
Germ line mutations: occurn in gametes/gamete precursor cells
They are transmitted to the next generation and do not affect the transmitter. They provide raw material for natural selection
Somatic mutations: occur in somatic cells
They are not transmitted to the next generation of individuals, hence they affect the person that has them. Theycan affect survival of an individual and can lead to cancer.
Genetic variation is due to a balance between 3 things:
- Continous introduction of new mutations
- Loss of deleterious mutations due to selective disadvantage
- Increase in frequency of a few mutations with selective advantage
