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Lecture 8; 2007
Biology 207; Section B2; Good
Lecture#8 - Mutations originate as damage to DNA
Readings:
Griffiths et al (2004) 8th Edition: pp 451-469; 190-192
Griffiths et al (2000) 7th Edition: Ch. 15 pp 462-467; Ch. 16 pp 495-519
Problems: Ch. 14: Problems 1-5, 16, 19, 23, 25-26
Concepts:
What are mutations?
1. Mutations are changes in the DNA sequence.
2. Mutations can arise without external influence (spontaneous) or can be caused by
mutagenic agents (induced).
3. Properties of mutagens and repair systems influence the mutations induced.
Mutation
Process: the change in structure of a gene from one form (commonly the normal or wild
type) to a variant form (mutation).
Mutant - a cell or organism bearing a mutant gene that expresses itself in the phenotype
Mutations affect the organisms phenotype, depending on the location of the change.
Overview
There are many ways (mechanisms) by which genetic change can occur. Mutation is a
catch-all category for any form of change in DNA sequence.
The textbook has subdivided mechanisms of DNA change into 4 broad categories:
1) Gene mutation - Chapter 14 - this lecture & next
2) Chromosomal Rearrangements
Chapter 15 - changes in structure
Chapter 15 - changes in number
3) Recombination - Chapter 4 & 14 - to come
4) Transposable Genetic Elements - Mobile elements Chapter 13 – Covered by Dr.
Locke’s section, not in detail in Section B2.
Mutations can be described at different levels
- DNA, chemical level - sequence change
- gene product level -function
- phenotypic level, how the mutant appears
Mutations - chemical changes to DNA
- change in quality, quantity, or order of DNA
1) Substitutions - (G) (A) (T) (C) Fig 6-7(8th) 15-1(7th)
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Lecture 8; 2007
Biology 207; Section B2; Good
(A) Protein coding sequence
1) no amino acid change - silent substitutions
2) change an amino acid - may abolish, reduce, increase or change its activity missense mutation
3) stop codon - abolishes the function of the truncated product – nonsense mutation
(B) Transcribed but not translated (Non-protein coding genes)
1) Alter RNA sequence - affect function of RNA molecules (e.g. rRNA, tRNA)
(C) Non-transcribed sequences
1) change sequences that regulate gene expression - such as the promoter sequence
2) change DNA sequence in region that has no phenotypic effect - DNA between genes
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Lecture 8; 2007
Biology 207; Section B2; Good
2) Addition or Deletion of bases
- changes (+2, +1, -1, -2) the amino acid reading frame (triplet codon)
(A) altered protein
(B) shortened protein if codon is stop - truncated polypeptide
3) Rearrangements - which are: Translocations, Inversions, mobile elements
Gene Mutation
1) Spontaneous Mutations - arise without external influence - naturally occurring
2) Induced Mutations - derived by exposure to mutagenic agents or procedures
- occur at a much higher frequency than spontaneous
Spontaneous Mutations
1) Errors during DNA Replication
The mismatches that do occur are normally either
1) Edited by DNA pol III
2) Repaired by other DNA repair systems
- error in base pairing - forms during DNA synthesis
- if not repaired (by exonuclease of pol III or repair systems, lead to a base
substitution
Transitions - purine=>purine; pyrimidine=>pyrimidine
Tranversions - purine=>pyrimidine; pyrimidine=>purine
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Lecture 8; 2007
Biology 207; Section B2; Good
2) Frame shifts during replication
Frame shifts can be either additions or deletions
Both are thought to:
- occur during DNA replication
- occur at repeated sequences
See Fig 14-21(8th) 16-4(7th)
Typically detected in protein coding stretches of DNA because they alter the reading
frame of triplet codons. Model to account for frame shifts based on work of
Streisinger in 1960's on the lysozyme gene of bacteriophage T4 (before DNA
sequencing technology) Frame shift mutations account for mutation hot spots in some
genes. Hot spots are sites in a gene that are much more mutable than other sites.
Example
lac I gene studied by Miller –Fig 14-22(8th) 16-5(7th).
Shows the distribution of 140 spontaneous mutations
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Lecture 8; 2007
Biology 207; Section B2; Good
Note:
a) cluster - at site of 3 tandem CTGG repeats in coding region
- some mutants are due to addition of one repeats
- other mutants are due to deletion of one repeat
b) mutant that gains additional repeat
- has a high rate of reversion to wild type
- due to loss of extra repeat
In humans: CGG triplet repeat -> fragile -X syndrome
Fig14-24(8th) 16-12(7th)
Spontaneous Lesions
- due to naturally occurring damage to the DNA
- not during DNA replication
1) Depurination - Figure 16-7(7th)
- break in the glycosidic bond between the base and the deoxyribose sugar
- results in the loss of an A or G base from the DNA -> called apurinic site - no base
-> at replication, a template that is apurinic can not specify a base -> replication error ->
mutation
2) Deamination - Fig 14-18(8th) 16-8(7th)
- removal of an amine -NH2 group
- cytosine -> deamination -> uracil
- uracil will pair with A at replication
- result: C -> U acts like T-> transition GC -> AT
3) Oxidatively damaged bases - Fig 14-20(8th)
- active oxygen (O2 - or H202 - or -OH radicals)
- These are normal by products of cellular metabolism
- damage Thymine -> Thymine glycol
Guanine -> 8-oxodG (GO) pairs with A (not C)
Note - in these cases the miss-match will result in substitutions
Induced Mutations
-various chemicals or treatments induce mutations
Mutational specificity
- mutagen produces a characteristic type of mutation
Note - Properties of the mutagen influence the type of mutation induced
Example Figure 16-13(7th) compare EMS, UV light, Aflatoxin B, mutants in lac I gene
shows
the categories of base substitutions
1) EMS - favors GC -> AT - it is an alkylating agent - modifies the base -> causes misspairing
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Lecture 8; 2007
Biology 207; Section B2; Good
2) UV light - favor GC -> AT - produces a photo dimer in adjacent thymine residues
- abnormal base pairing
3) Aflotoxin - favors GC -> TA - generates apurinic sites by addition to N7 position of
guanine (purine).
4) Base analogs - chemicals that are similar to normal bases
- can be incorporated into DNA like normal bases but
- don't have pairing properties of normal bases
- incorrect pairing at replication -> substitution
5) Intercalating agents - planar molecules that intercalate (slip in) themselves between
stacked bases in the double helix
- cause single base pair insertions or deletions
- intercalated molecules may stabilize Streisinger model and result in frame shifts ->
base pair addition/loss
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