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Lecture 15

HMB265H1 Lecture Notes - Lecture 15: Mutation Rate, Indel, Mutagen


Department
Human Biology
Course Code
HMB265H1
Professor
Maria Papaconstantinou
Lecture
15

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HMB265 Lecture 15 - Mutations
Lecture Outline
DNA mutation
Consequences of mutation
Use of mutations to determine enzyme pathways
Mutations are the source of new alleles
- DNA sequence determine phenotypes
- Changes to code affect how genes are expressed
- Changes caused by mutations might or might not be apparent
- Generate alleles of different genes that can give us diversity
- Have mutations that occur spontaneously and randomly all the time
- Can induce them using mutagens
- If have mutation in skin cell, will not be transmitted to next generation
- Only mutations in germline cells are transmitted to progeny
EX: Cancer
- Mutations that occur in later stages of left in somatic cells won’t be transmitted
- But are certain genes that predispose you to cancer
- = people are more likely to develop cancer
- = only mutations in germline cells can be transmitted
- Somatic mutations can’t be transmitted
-
General observations of mutation rates
- Mutation genes change from one allelic form to another = create new alleles
- Mutations that affect phenotype occur very rarely
- Out of all the DNA, only have 20 000 genes
- = lots of DNA sequences that aren’t coding region of protein
- Different genes mutate at different rates
- Has to do with length of gene (longer gene = more likely it is to be hit by
mutation)
- Genes more mutable than others might be because of location on DNA
- But in generally mutate randomly = at any time and in any cell of organism
- Mutation can occur during normal DNA replication
- Many proteins involved
- Mutation rate can increase after exposure to a mutagen
- Can increase rate of mutation by exposure to mutagen
- Affect DNA structure
Classification of mutations
- Classify mutations according to the effect they cause (nature of change of DNA)
- Start with WT starting sequence
- Then have mutations:
[1] Substitution
- Base is substituted by one of the other 3 bases = base substitution
[2] Deletion (Indel) can be huge or single base-pair
[3] Insertion (Indel) can be huge or single base-pair
[4] Inversion
- 180 degree rotation of segment of DNA
[5] Reciprocal translocation
- Part of 2 nonhomologous chromosomes change places
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DNA Structure
- Has sugar backbone, bases
- Mutation can affect sugar backbone or bases
Classification of mutations
Base Substitution:
- Transitions: Change purine to purine or pyrimidine to pyrimidine
- A ↔ G
- C ↔ T
- Transversion: Change purine to pyrimidine or vice versa
- A ↔ T
- A ↔ C etc.
Why study mutations?
- Mutations = markers for genes
- If DNA absolutely identical, can’t put finger in one spot vs another
- Variability = can identify that spot
- = use them as markers
- Study how WT gene works
- When discover mutations that disrupt enzyme function, can learn about enzyme
function
Definitions
WT:
- Lots of variability in phenotypes and in available alleles
- Many SNPs make us very different
- Hard to say which is WT and which isn’t
- = everything is WT as long as it’s more than 1% of population
Mutant:
- Rare, and occur in less than 1% of population
- Of course all SNPs are from mutations, but we call mutants the rare ones
Forward mutation:
- Change WT allele to different allele
Reverse mutation:
- Reverse novel mutation to revert back to WT allele
- Fix new mutation
Easier to knock out gene and few ways to fix back = reverse mutations are more rare
- Except for transposable elements
- Can jump into genome
- Reverse mutations of TEs are more frequent
Causes of Mutations
Spontaneous:
- Arise in absence of known mutagen
- Provide “background rate” of mutation (i.e. how many mutations per gene per gamete)
Induced:
- Action of mutagen alters nucleotide sequence
What causes mutations?- Natural processes
Depurination
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