Mutation, Repair, And Recombination

Front 1

What mutations is more likely to produce a severe rather than mild phenotypic consequence

Back 1

Nonsense mutations

Front 2

Why do mutations in non-coding regions of a gene still have major effects on phenotypic expression?

Back 2

These non-coding regions have binding sites such as promoters and enhancers for and RNA polymerase and transcription factors respectively, the splice sites, and sites for ribosome binding during translation. These can all impact gene expression and lead to phenotypic consequences.

Front 3

Loss-of-function mutations are significantly more frequent than gain-of-function mutations. Do you agree with this statement? Justify your answer

Back 3

Mutations are more likely to reduce or eliminate gene function than to enhance it. As stated in the text, “By randomly changing or removing one of the components of a machine, it is much easier to break it (that is, loss of function) than alter the way it works (that is, gain of function)

Front 4

What are the two major processes responsible for genetic variation?

Back 4

mutation and recombination

Front 5

Define “transition mutations” and “transversion mutations.” Give one example of each.

Back 5

Transitions: Change from a purine to a purine or a pyrimidine to a pyrimidine. Examples: A to G

Front 6

Missense mutations can be classified as either “conservative substitutions” or “nonconservative substitutions.” These two subclasses differ in that in a conservative mutation:

Back 6

a functionally similar amino acid is encoded by the mutated codon.

Front 7

What is a nonsense mutation?

Back 7

(i) A point mutation that converts an amino acid codon into a stop codon.

Front 8

(ii) What effect do nonsense mutations have on the length of mRNA?

Back 8

(ii) Nonsense mutations have no effect on mRNA length

Front 9

(iii) What effect do nonsense mutations have on the length and function of a protein?

Back 9

(iii) Nonsense mutations cause premature termination of translation. Thus, the protein will be shorter and probably non-functional.

Front 10

(iv) The following DNA sequence is from the non-template strand of a gene. List all possible single point mutation events that would create nonsense mutations.

5’ ATG CTG AGA TGC GAA CAG GAC 3’

Back 10

(iv) Any point mutation that converts an amino acid codon into a stop codon (TAA, TAG, or TGA) is a nonsense mutation. For this particular sequence, the point mutations that would cause nonsense mutations are underlined, with the “new” base written in parentheses: 5’ ATG CTG A(to T)GA TGC(to A) G(to T)AA C(to T)AG GAC 3’

Front 11

Is transversion or translation more likely to occur?

Back 11

Transitions occur at a higher frequency than transversion, thus the C to T mutation is the most likely to occur.

Front 12

Suppose you have a strain of E. coli that is resistant to streptomycin due to the presence of the strR gene. You are interested in obtaining several mutants that are susceptible to streptomycin due to point mutations in the strR gene. Design an experiment to select for such streptomycin susceptible strains.

Back 12

Streptomycin-resistant E. coli should be exposed to a mutagenic agent, such as EMS, to induce point mutations. These will then be plated on an agar medium without the antibiotic. A replica plate that contains streptomycin will also be made. By comparing the two plates (colonies that die in the plate containing the antibiotic are susceptible), new mutants can be isolated.

Front 13

Salvador Luria and Max Delbruck designed the flunctuation experiment to show that

Back 13

resistant cells were selected by phage infection

Front 14

The FMR-1 gene undergoes ________________ to produce susceptibility to fragile X syndrome

Back 14

tri-nucleotide repeat expansion

Front 15

Deamination,Trinucleotide expansion, Depurination, Oxidation Are all examples of what type of mutation

Back 15

spontaneous mutations

Front 16

How are spontaneous mutations created?

Back 16

1) Errors in DNA replication (base substitutions due to tautomeric shifts or incorporation of incorrect base and base insertions and deletions due to replication slippage).

Front 17

Design an experiment to differentiate between spontaneous mutations and induced mutations. What is the basis of your experiment?

Back 17

Smaller and larger cultures will be started separately but plated on culture media with a selective agent such as an antibiotic (for example, to test for spontaneous mutations that will give resistance to the antibiotic) with the same concentration of cells per plate at the same time. Over time, varying number of resistant colonies per plate will be found depending on when the mutation occurred. (Early mutations give more colonies.)

Front 18

What enzyme directly involved in base excision repair mechanism of mutations?

Back 18

Glycosylases

Front 19

Photoreactivation repair is effective only in the presence of visible light. What other repair mechanism is available to repair pyrimidine dimers if visible light was not available? Describe the steps of this mechanism

Back 19

Nucleotide excision repair can be used to repair pyrimidine dimers in the dark. This system detects distortions in the double helix created by abnormal bases. Uvr ABC exinucleases cleave flanking bases at the damaged area to remove the abnormal bases. DNA polymerase I then repairs the missing segment and DNA ligase seals the newly synthesized bases to the original DNA strand.

Front 20

Examples of homology-dependent repair systems

Back 20

Base excision repair

Front 21

What repair system exists in cells to repair damage of a single base via homology dependent repair?

Back 21

Base excision repair

Front 22

What is the mismatch repair system?

Back 22

It is a post-replication repair mechanism.

Front 23

What mutations is more likely to produce a severe rather than mild phenotypic consequence

Back 23

Nonsense mutations Why do mutations in non-coding regions of a gene still have major effects on phenotypic expression?