Mutation and DNA Repair Flashcards
What is a mutation?
- 2 ways they can result
- How does it cause a problem?
Mutation: An inheritable change in DNA
-Results from damage or chemical changes to DNA bases - can occur spontaneously or be induced.
- Mutations change bases’ binding specificity (i.e. so A binds to G)
- next round of replication introduces a mutation (CG pairing that wasn’t present before)
- mutations are fixed in the genome of the cell
How often do mutations occur?
- In E. coli
- For a double mutant - spontaneous mutation frequency
- In a broth culture of E.coli, there will be mutants.
- to isolate a lacZ- mutant, need to screen approx. 10^7 cells
- spontaneous mutation frequency = 10^-7
- To find a lacZ- LacA- double mutant, would need to screen approximately 10^14 cells
Spontaneous Mutation Frequency among different species
- In humans
- In immunoglobulin genes
- Frequency & exposure to mutagens
- Mutagen definition
- Varies among species
- In humans; is approximately 10^-5
- Some genes have different spontaneous mutation rates
e. g. Immunoglobulin genes (antibody genes) = 10^-3- Hypermutable because you need diversity in antibodies for protection against virus, bacteria etc.
- The frequency can be increased by increasing exposure to DNA damaging agents (called MUTAGENS)
- Mutagens = chemical or physical agents that cause chemical changes in DNA bases)
How do we know mutations are spontaneous?
-experiment involving E. coli
- E.coli onto streptomycin agar and incubated -> if replicates are made all plates will have the same pattern/distribution of growth on the plates
- demonstrates that mutations pre-existed in the culture and were not caused by the exposure to streptomycin
*defines the nature of mutations -> random
Effect of mutagen/mutation
-depends on 2 factors
Depends on;
- Type of mutation
- Location of mutation
*Don’t necessarily see an effect of the mutation
2 Types of mutations
- Missense: Change of one amino acid for another
2. Nonsense: Change of one amino acid codon with a stop codon
Nature of codons & effect of mutation
-Which of the two types is worse?
- Are approximately 4 codons for every a.a.
- Changing first or second base in most cases will change the a.a.
- if a stop codon is introduced early on, it will stop translation and the protein won’t be made = MAJOR EFFECT
- Nonsense will have a much greater effect as the protein won’t get made!
Missense mutation Effect
-conserved region -> what is it and why is it important when thinking about mutations?
- The effect of missense mutation depends on where the mutation is
- protein sequence tends to be different between species, even though the protein may be the same
- Conserved region = small portion where all proteins have same a.a. sequence
- this region is therefore critical to function of that protein
- rest of protein may be involved in other less critical functions
- mutation in conserved region likely to be more disastrous than in other regions
Effects of mutations within cells
- only 2.8% of human genome encodes proteins
- 28% is transcribed (e.g. tRNA, rRNA & regulatory RNAs)
- Approx. 70% of our genome don’t encode for anything - therefore a very small amount of mutation would affect our proteins
- Being diploid also protects us - effect of a mutation can be masked by copy on sister chromosome
Frameshift mutations
- what are they
- why are they significant
-what can induce a frameshift mutation?
- are the insertion or deletion of 1 or more bases in mRNA that changes the reading frame and therefore completely changes the protein
- have a profound effect on the gene
- Frameshift mutagens can induce this change
What else can also cause mutations?
- what this does
- likely effect in humans
- but what it can do to a gene that has significant effects on expression
- DNA arrangements can also cause mutations
- are quite common b/w DNA molecules that are quite similar - they can undergo cutting and joining (i.e. recombination)
-if small copy of a DNA sequence is similar but in reverse orientation, it may cause pairing
-net result is the inversion of the sequence between the reacting sites
*as human genomes have small amount of coding genes, it’s unlikely to effect a protein
BUT, inversions can move a gene from one place to another & the position of a gene on the chromosome can have significant effects on expression
How location on a chromosome effects expression of a gene
- length of telomere
- e.g. of eye colour in Drosophilia
Telomeres induce the formation of heterochromatin (as are the DNA around the centromeres (tightly packed DNA)
- genes in heterochromatin are not expressed (even when the right activators are present) * most of chromosome exists as euchromatin (which is not as tightly packaged)
- Length of telomere, and hence length of the heterochromatin can very from cell to cell -> gene may be expressed in some cells but not in others
- e.g. eye colour in Drosophilia (inversion = patchy red colour compared to red colour in wildtype = position effect variegation)
What inversions do (in general)
-Inversions exert an effect by moving genes into or out of regions of heterochromatin
Result of directly repeated sequence in DNA
- Result = DNA that lies between the sequence can be deleted
- deletion can range from 100s of kB
Recombination & chromosome number
-effects on humans and plants
- Recombination can fuse chromosomes together so that 1 of the daughter cells at cell division receive an extra copy of part or all of a chromosome
- trisomy
- occurs in plants a lot, but plants can deal with it well
- in humans, trisomies can have a very significant and negative outcome (i.e. Down syndrome)
