BIOL2161 Lecture Notes - Lecture 1: Dna Mismatch Repair, Dna Replication, Mutation Rate
BIOL6: Genes: Replication and Expression
Block 1: DNA Structure, Replication and Repair
DNA Mutation and Repair
Replication Errors and Mismatch Repair
• Not all replication errors are detected by proof-reading
• Mismatch repair system operates soon after replication and distinguishes the newly replicated
strand (carrying the incorrect base) from the parental, correct strand
• Incorrect strand is repaired
• Replication accuracy
o Proofreading by DNA polymerases picks up 99% of incorrectly incorporated nucleotides
o The mismatch repair system corrects 99% of the remaining 1%
o The combined accuracy effect gives DNA replication its very high accuracy (error rate of
1 in a billion mutations)
• Mismatch repair assumes that the parental strand is correct
o Bacterial DNA is modified by methylation of some bases
o Newly synthesised DMA is methylated but not immediately
o Mismatch repair acts on the non-methylated strands, soon after DNA replication
o Replication errors include small insertions and deletions
▪ Replication slippage can lead to the misalignment of the template and newly
synthesised strand and results in the unequal daughter strands
▪ This is also repaired by mismatch repair
▪
o Mismatch repair and colon cancer
▪ Hereditary non-polyposis colon cancer (HNPCC) is due to inherited defects in
mismatch repair genes
▪ Accounts for 2-3% of colon cancer cases
▪ Somatic mutation of mismatch repair genes accounts for some non-familial colon
cancer cases
▪ Adenomas progress to carcinomas much more quickly because of the increased
mutation rate
Replication of chromosome ends
• The last RNA primer on the lagging strand cannot be replaced
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o
o To combat this, chromosomes have repeated sequences at each end, called telomeres
▪ These are G-rich on the strand with its 3' end at the chromosome end
▪ Short repeats od about 6-10bp
▪ Extends for 100s or 1000s of bps, depending on species
• Telomerase replicates chromosome ends and is a reverse transcriptase
o Last RNA primer a the 5' end of the lagging strand cannot be replaced by DNA
o Telomerase enzyme extends the chromosome end by adding telomere repeat sequences
o Reverse transcriptase: an enzyme that uses RNA template to make a complementary
DNA strand
▪ This is what telomerase is and it includes an RNA template homologous to
telomeres
o Telomerase extends the DNA, filling in the gap due to the removal of the RNA primer
o
o Cancer cells may have active telomerase, allowing the cells to live longer and not die due
to telomere shortening
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▪
Mutation and DNA Repair
• DNA repair
o Cells possess many DNA repair systems that detect and correct mutations
o Different types of mutations are replicated by different repair systems
o Many hereditary cancers arise as a result of mutations affecting DNA repair
o The human genome encodes >130 genes for DNA repair proteins
o Cells only become cancerous after accumulating more than one mutation (usually about
5)
o Mechanisms of DNA repair systems
1. Recognition of incorrect/modified base
2. Removal of damaged base/nucleotide/DNA region
3. DNA synthesis using correct strand as a template
4. Ligation of remaining nick
▪ Steps 1 and 2 different in each repair system, 3 and 4 the same in many repair
systems
o We all carry mutations, most have little to no effect
▪ Non-coding DNA
▪ I gees, utatios a e silet ad ay issese utatios do’t effet the
function of resulting protein
▪ Mutations can be responsible for the diff3erences between individuals
▪ Some mutations cause genetic diseases
o Causes of mutations:
▪ Replication error: mispairing or small insertions and deletions
▪ Spontaneous chemical damage to DNA
▪ Chemical mutagens: occurs via many different mechanism
▪ Radiation
UV light causes pyrimidine dimers to form - usually thymine
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Document Summary
Replication errors and mismatch repair: not all replication errors are detected by proof-reading, mismatch repair system operates soon after replication and distinguishes the newly replicated strand (carrying the incorrect base) from the parental, correct strand. Somatic mutation of mismatch repair genes accounts for some non-familial colon cancer cases: adenomas progress to carcinomas much more quickly because of the increased mutation rate. Uv light causes pyrimidine dimers to form - usually thymine. A single unrepaired thymine dimer can be lethal because progress of high fidelity dna polymerases is blocked: photoreactivation: detect repair. Prokaryotes and some eukaryotes have a light-dependant repair system that detects pyrimidine dimers. Photoreactivating enzyme (photolyase) uses energy from visible light to break the bonds between the two pyrimidine residues: splits the dimer to regenerate normal dna. Direct repair involves reversing the chemical reaction causing the dna damage: nucleotide excision repair (ner): detects and repairs bulky lesions in dna, like thymine dimers and modified bases.
