Mutation Flashcards
1
Q
Mutation
Definition
A
a change in the DNA sequence of a genome
e.g. a change in chromosome number, a change in chromosome structure, a change in DNA sequence
2
Q
Point Mutations
Definition
A
a single base changes
3
Q
Neutral Mutation
Definition
A
a point mutation that results in the substitution of one amino acid for another with similar chemical properties
4
Q
Mis-sense Mutation
Definition
A
- a point mutation that results in a different amino acid in the sequence
- might cause protein structure or activity to be altered
- this could result in a novel phenotype e.g. change from a basic to an acidic amino acid resulting in a change in charge
5
Q
Non-sense Mutation
Definition
A
- point mutation resulting in a premature termination codon
- causing formation of a truncated protein
- the protein may have no or partial function depending on where in the sequence the termination occurs
6
Q
Frameshifts
A
-insertion or deletion of one or two bases resulting in a shift in the reading frame of the DNA strand
7
Q
Larger Scale Mutations
A
-changes in large sequences of DNA
e.g. loss of part or all of a gene
genes disrupted by insertion of large sequences e.g. transposons
8
Q
Causes of Mutation
Spontaneous Mutations
A
- errors in replication
- chemical changes to bases
- insertions resulting from transposon activity
9
Q
Causes of Mutation
Induced Mutations
A
- chemical change caused by mutagens
- changes caused by radiation (x-ray, UV or gamma)
- insertions resulting from transposon activity
10
Q
Errors in Replication
A
- DNA polymerase accidentally inserts wrong base, this is possible because non standard bases can occur
- -this is usually corrected by proof reading of DNA polymerase but if not it will lead to mutation
- looping out of template strand, a base is missed by DNA polymerase resulting in a deletion
- looping out of new strand, two bases are accidentally added instead of one
- hairpins, insertion or deletion of multiple bases i.e. looping out of a bigger loop
- looping out and hairpins are associated with runs of the same base
11
Q
Frameshift Mutations and the Discovery of the Triplet Code
A
- analysis of frameshift mutations and second site reversions were instrumental in the discovery of the triplet code
- insertion or deletion of one or two bases into bacteriophage genes resulted in frame shift mutants
- but insertion or deletion of three bases usually did not
- this tells us that each amino acid is coded for by three bases
12
Q
Reversion of Frameshift Mutations
A
- the exact opposite to the original mutation can occur at the site of the original mutation
- OR a second mutation can occur at a second site that results in a reversal of the effects of the original mutation
13
Q
Chemical Modification of DNA
Deamination of Cytosine Residues
A
- one of the most common chemical modifications
- the carbon 4 residue of the cytosine is deaminated, the -NH2 is replaced with an =O
- this converts the cytosine into a uracil
- the C-G base pair becomes a T-A base pair in the next round of replication
- but because uracil is not usually found in DNA, this chemical change is usually detected and repaired by the cell
14
Q
Chemical Modification of DNA
Methylation of Cytosine
A
- when cytosine is methylated, a methyl group is added to carbon 5
- the –H on carbon 5 becomes -CH3
15
Q
Chemical Modification of DNA
Deamination of Methylated Cytosine
A
- converts a cytosine base into thymine
- this is not easily detected and repaired so conversion of C to T frequently results in C-G to T-A mutations
16
Q
Chemical Modification of DNA
Depurination of DNA
A
- the removal of a purine base (A or G) from the DNA
- during replication DNA polymerase will insert any base into the gap
- this means there is a 1/4 chance that the correct base will be reinserted and a 3/4 chance of mutation
17
Q
Chemically Induced Mutation
Nitrous Acid
A
- converts -NH2 groups to =O
- cytosine is deaminated resulting in a C-G to T-A mutation
- adenine is deaminated to produce hypoxanthine which base pairs with C resulting in an A-T to G-C mutation
18
Q
Chemically Induced Mutation
Hydroxylamine
A
- converts -NH2 to -NOH
- cytosine is converted to hydroxylaminocytosine which base pairs with A resulting in a C-G to T-A mutation
19
Q
Chemically Induced Mutation
Alkylating Agents
A
- add -CH3 on to =O residues
- guanine is converted to O6-methylguanine which base pairs with thymine resulting in a G-C to A-T mutation
- action on adenine results in an A-T to G-C mutation
20
Q
Physical Mutagens
X-rays, Gamma and Fast Neutrons
A
- can cause double and single strand breaks
- single strand breaks can become double strand breaks during replication
- if unrepaired, important cell information will be lost
21
Q
Physical Mutagens
UV radiation
A
- less damaging than x-ray and gamma
- can cause dimers to form between adjacent pyrimidines (especially TT) causing distortion of the double helix and disruption of the replication
22
Q
Nucleotide Excision Repair
A
- controlled by enzymes
- remove conjoined bases and replace them with new ones
23
Q
Repairing Double Stranded Breaks
A
- has to be done quickly
- if there are two breaks, it is possible for the section of DNA to be put in back to front
24
Q
Auxotrophic Mutant
Definition
A
- a mutant that can no longer synthesise a metabolite
- this means that it cannot grow on a minimal medium
- auxtrophic mutants need to be supplemented with the metabolite they aren’t able to synthesise
25
Minimal Medium
| Definition
- a nutrient medium lacking in a certain metabolite
| - usually the metabolite that the auxotrophic mutant cannot synthesise
26
Mutagenesis in a Haploid Organism (e.g. E.coli)
1) treat cells with mutagen an spread onto master plate with growth medium
2) leave to grow overnight
3) velveteen surface pressed onto master plate to make a stamp of the master plate
4) velveteen pressed onto the minimal medium plate
5) colonies left to grow on the minimal medium plate
6) compare the master and minimal medium plates
7) colonies that did grow on the master plate but not on the minimal medium plate contain auxotrophic mutants
27
Mutagenesis in a Diploid Organism (e.g. Arabidopsis)
1) treat seeds with mutagen
2) germinate seeds to produce M1 population
3) allow plants to self fertilise
4) plant the seeds, the plants that grow are the M2 population
5) screen the M2 progeny for mutant phenotypes (i.e. the seeds in the pods on the M2 plants
6) you can only see mutations that were in the germline genes since these are the ones that are passed on to offspring
28
Transposon
| Definition
- length of DNA with inverted repeats at either end (2-3kb in length)
- can change position within genomes
29
Trasposase Enzymes
- recognises the terminal inverted repeat sequences to cut the transposon out of the genome and reintegrate it elsewhere
- if the transposon relocated into another gene it will disrupt its coding, a form of mutation
30
What are the conditions for a mutation to be significant in evolutionary terms?
-in a sexually reproducing organism, a mutation must be in the cells that give rise to gametes for it to be significant in evolutionary terms
31
Recombination and Mutation
- mutation gives rise to changes
| - recombination shuffles these changes
32
Recombination
| Definition
the physical exchange of genetic material between genomes:
i) bringing together in the zygote of new combinations of parental chromosomes as a result of independent assortment during meiosis
ii) the physical exchange of DNA between two molecules by their breaking and rejoining
33
Meiosis
1) chromosomes in the nucleus
2) DNA replicated duplicating each chromosome to produce chromatid pairs
3) homologous pairs associate on the spindle
4) first meiotic division
5) second miotic division resulting in the production of a tetrad of haploid gametes
34
Recombination in Meiosis
-crossing over between homologous chromosomes
35
Exchange of DNA by Recombination...
...requires sequence homology between DNA strands that recombine
...is not a random process, it is undertaken by enzymes that carry out various steps in the process
...is very frequent compared with mutation and the most important means by which genetic diversity is generated
36
Recombination in Bacteria
i) uptake of DNA fragment that is integrated into the bacterial chromosome
ii) uptake of a plasmid that is integrated into the bacterial chromosome
iii) infection of the cell by bacteriophage and uptake of bacteriophage DNA in to the bacterial chromosome
iv) infection of the cell by two bacteriophages, recombination of the two bacteriophage genomes producing recombinant phages that are released upon lysis of the host cell
ALL OF THESE CASES REQUIRE HOMOLOGY BETWEEN THE TWO PIECES OF DNA
