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Chapter 15&16

HMB265H1 Chapter Notes - Chapter 15&16: Dna Mismatch Repair, Rhodopsin, Tautomer

Human Biology
Course Code
Naomi Levy-Strumpf, Stephen Wright

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Pg. 256-258; 262-263; 267-271; 278-280; 285-289
Pg 317-325
Mutations at the Molecular Level
Eg. Rhodopsin, a light-sensitive protein found in rod cells and found on Chromosome 3
- Determines perception in low light
- A mutation in the rhodopsin gene is enough to lead to night blindness
- Other alterations can completely destroy rod cells, leading to blindness
3 Key Ideas About Mutations
1. Mutations are heritable changes in base sequence that can affect phenotype
2. Physically, a gene is usually a specific protein-encoding segment of DNA in a discrete
region of DNA
3. Genes are divisible, and each individual nucleotide pair can mutate independently and
can recombine with each other
- Mutations modify the structure or amount of a gene’s protein product; these
modifications influence phenotype
- All other mutations either alter genes in a way that doesn’t affect function or
change the DNA between genes
- Mutation rates are very slow; very few mutations actually affect gene function
- The rate of forward mutation is higher than the rate of reverse mutation (there are
more ways to disrupt gene function than restore it)
- Mutation rates in multicellular organisms are higher than in bacteria
- Diploid genome → tolerates mutations better than bacteria
- Many cell divisions between the zygote and meiosis, allowing for mutations to
- There are more mutations in sperm than in eggs
- Sperm undergo many more mitotic divisions, leading to higher chances of
- Base tautomerization → SNP if mismatch-repair can’t fix it
- He was lucky; the alternative alleles for the 7 particular pea genes had arisen through
forward mutations
- These mutations were also germ-line mutations, making them heritable
Key terms
- Common variants → “wild type alleles” for polymorphic genes

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- Forward mutation → mutation that converts a wild-type allele to a mutant (mutant can be
dominant or recessive to the wild-type allele
- Reverse mutations (reversion) → mutation that converts a novel mutant allele to revert
back to wild-type
- Substitution
- Transition (purine to purine or vice versa) vs. transversion
- Insertion/Deletion (InDel)
- Chromosomal rearrangements
- Mutagen → any physical or chemical agent that raises the frequency of mutations
- Base analogues → have a chemical structure similar to normal nitrogenous bases
- Tautomeric forms
- Intercalators → flat, planar molecules that can sandwich themselves between successive
base pairs and disrupt machinery for replication, recombination or repair
- Stabilizes slipped mispairing & causes single base InDel mutations
- Eg. proflavin, tobacco smoke, ethidium bromide
Molecular Mechanisms of Mutations
- Alteration of DNA
- Damaged by chemical reaction or irradiation
- Mistakes during replication
- Repair enzymes
- If the repair enzymes fix the errors before the next found of DNA replication,
everything is fine
- If not fixed, then the mutation is established permanently on both strands of the
double helix
- Types of Mutations
- Depurination (quite common) → DNA replication inserts a random base
- Deamination → changes cytosine to uracil; since U pairs with A rather than G, we
may have a transition mutation
- Cosmic rays/x-rays → break the sugar-phosphate backbone (causes deletions) &
improper stitching together
- UV light → thymine-thymine dimers form in adjacent thymine nucleotides
- Oxidative damage to any of the 4 bases
Proofreading Function of DNA polymerase
- DNA polymerase has proofreading ability
- 3’-5’ exonuclease → recognizes mispaired bases and excises it
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