Mutation

Question 1

Mutation

Definition

Answer 1

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

Question 2

Point Mutations

Definition

Answer 2

a single base changes

Question 3

Neutral Mutation

Definition

Answer 3

a point mutation that results in the substitution of one amino acid for another with similar chemical properties

Question 4

Mis-sense Mutation

Definition

Answer 4

• 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

Question 5

Non-sense Mutation

Definition

Answer 5

• 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

Question 6

Frameshifts

Answer 6

-insertion or deletion of one or two bases resulting in a shift in the reading frame of the DNA strand

Question 7

Larger Scale Mutations

Answer 7

-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

Question 8

Causes of Mutation

Spontaneous Mutations

Answer 8

• errors in replication
• chemical changes to bases
• insertions resulting from transposon activity

Question 9

Causes of Mutation

Induced Mutations

Answer 9

• chemical change caused by mutagens
• changes caused by radiation (x-ray, UV or gamma)
• insertions resulting from transposon activity

Question 10

Errors in Replication

Answer 10

• 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

Question 11

Frameshift Mutations and the Discovery of the Triplet Code

Answer 11

• 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

Question 12

Reversion of Frameshift Mutations

Answer 12

• 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

Question 13

Chemical Modification of DNA

Deamination of Cytosine Residues

Answer 13

• 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

Question 14

Chemical Modification of DNA

Methylation of Cytosine

Answer 14

• when cytosine is methylated, a methyl group is added to carbon 5
• the –H on carbon 5 becomes -CH3

Question 15

Chemical Modification of DNA

Deamination of Methylated Cytosine

Answer 15

• 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

Question 16

Chemical Modification of DNA

Depurination of DNA

Answer 16

• 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

Question 17

Chemically Induced Mutation

Nitrous Acid

Answer 17

• 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

Question 18

Chemically Induced Mutation

Hydroxylamine

Answer 18

• converts -NH2 to -NOH

- cytosine is converted to hydroxylaminocytosine which base pairs with A resulting in a C-G to T-A mutation

Question 19

Chemically Induced Mutation

Alkylating Agents

Answer 19

• 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

Question 20

Physical Mutagens

X-rays, Gamma and Fast Neutrons

Answer 20

• 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

Question 21

Physical Mutagens

UV radiation

Answer 21

• 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

Question 22

Nucleotide Excision Repair

Answer 22

• controlled by enzymes

- remove conjoined bases and replace them with new ones

Question 23

Repairing Double Stranded Breaks

Answer 23

• 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

Question 24

Auxotrophic Mutant

Definition

Answer 24

• 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

Question 25

Minimal Medium

Definition

Answer 25

• a nutrient medium lacking in a certain metabolite

- usually the metabolite that the auxotrophic mutant cannot synthesise

Question 26

Mutagenesis in a Haploid Organism (e.g. E.coli)

Answer 26

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

Question 27

Mutagenesis in a Diploid Organism (e.g. Arabidopsis)

Answer 27

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

Question 28

Transposon

Definition

Answer 28

• length of DNA with inverted repeats at either end (2-3kb in length)
• can change position within genomes

Question 29

Trasposase Enzymes

Answer 29

• 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

Question 30

What are the conditions for a mutation to be significant in evolutionary terms?

Answer 30

-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

Question 31

Recombination and Mutation

Answer 31

• mutation gives rise to changes

- recombination shuffles these changes

Question 32

Recombination

Definition

Answer 32

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

Question 33

Meiosis

Answer 33

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

Question 34

Recombination in Meiosis

Answer 34

-crossing over between homologous chromosomes

Question 35

Exchange of DNA by Recombination…

Answer 35

…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

Question 36

Recombination in Bacteria

Answer 36

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