2.2 - Mutation and Repair

Question 1

what is a lesion in DNA?

Answer 1

-change/damage to a sequence of DNA
-is not automatically a mutation

Question 2

what classifies a mutation?

Answer 2

-a lesion that is not repaired prior to a new replication round (heritable change in nucleotide sequence)
-may impact a function (gene, promotor, binding site)
-changes due to mutations can be beneficial, detrimental, or neutral (most are detrimental)

Question 3

why is mutation observable in bacteria?

Answer 3

-genome is haploid (one gene copy)

Question 4

what are spontaneous mutations?

Answer 4

-natural basis of evolution
-occur during normal growth due to biological errors (without external intervention)
-errors are typically few and far between

Question 5

what are induced mutations?

Answer 5

-caused by external factors that cause a lesion (UV, chemicals, molecular techniques)
-these things can increase mutation rate (almost double)

Question 6

what is a wildtype strain?

Answer 6

-strain of an organism that is isolated from nature

Question 7

what is a mutant?

Answer 7

-a cell derived from the wild type whose genome carries a change in nucleotide sequence from that of the wild type genotype

Question 8

what is a phenotype?

Answer 8

-observable properties of a strain
-if changed from the wildtype, referred to as a mutant phenotype

Question 9

how are mutant derivative obtained?

Answer 9

-directly from the wildtype
-from another strain (parental)
-depending on the mutation a mutant may or may not differ in phenotype from its parent

Question 10

what is the parental strain?

Answer 10

-slight genetic differences that naturally develop over time
-strain of wildtype

Question 11

how many transcriptional units encode the metabolic pathway?

Answer 11

-4 monocistronic and 4 polycistronic units
-total of 8

Question 12

what is MalK?

Answer 12

-transport ATPase

Question 13

what is LamB?

Answer 13

-maltoporin

Question 14

what is MalM?

Answer 14

-periplasmic protein

Question 15

what is MalQ?

Answer 15

-amylomaltase

Question 16

what is a selectable mutation?

Answer 16

-the mutant strain gives a clear advantage under certain environmental conditions
-the progeny of the mutant cell are able to grow and replace the parent
-ex: antibiotic resistant mutant that can grow in the presence of an antibiotic which inhibits or kills the parents (selected under these conditions)

Question 17

what are non-selected mutations?

Answer 17

-neither an advantage or disadvantage over the parent cells
-can be detected by screening morphologies of colonies for differences

Question 18

what is preferred for genetic experiments, selection or non-selected?

Answer 18

-selection is preferred because selective conditions typically place severe restraints on the populations so mutants are easily detectable

Question 19

what are nutritional auxotrophs?

Answer 19

-mutant strains with an additional nutritional requirement above that of the wild-type or parental strain (the protothroph = normal growing) from which it was derived

Question 20

what are mutagens?

Answer 20

-chemicals that chemically modify DNA
-can be carcinogens

Question 21

what are the 3 categories of mutations?

Answer 21

-macrolesions (changes in large segments of DNA)
-frameshift mutations
-point mutations

Question 22

what are point mutations?

Answer 22

-mutations that change only one base pair
-change in phenotype depends on where in the genome the mutation occurs and the nature of it (may not have any phenotype change)
-can be a transition or transversion mutation
-typically reversible

Question 23

what is a transition mutation?

Answer 23

-change within the same base category (A to/from G or C to/from T)

Question 24

what is a transversion mutation?

Answer 24

-change between base categories (A/G to/from C/T)

Question 25

what impacts do point mutations have on polypeptides?

Answer 25

-if the point mutation is within the region of a gene that encodes a polypeptide then a change in the phenotype is most likely the result of an amino acid change in the polypeptide sequence -considered a misense mutation -likely a change in the first or second base of a codon -impact on the final protein depends on location of the changed amino acid and how it can affect folding and/or activity

Question 26

what are silent mutations?

Answer 26

-likely a change in the third base of a codon (because of genetic code degeneracy) -change does not affect the amino acid -change is at the DNA/RNA level not the protein level

Question 27

what amino acids can also see silent mutations in the first position?

Answer 27

-arginine and leucine

Question 28

what can also form as a result of a point mutation?

Answer 28

-formation of a stop codon -results in premature termination of translation, leads to an incomplete polypeptide -referred to as a nonsense mutation -unless the nonsense mutation is very close to the end of a gene, the product is made incompletely causing it to lack normal activity or be inactive

Question 29

what is a frameshift mutation?

Answer 29

-a deletion or insertion of one or two base pair(s) results in a shift in the reading frame -this scrambles the entire polypeptide sequence downstream of the mutation

Question 30

what happens when there is an insertion or deletion of 3 base pairs?

Answer 30

-simply results in an insertion or deletion of a codon -the actual reading frame itself does not change

Question 31

how are point mutations reversible?

Answer 31

-through a process called reversion -the second mutation can be spontaneous or induced -the strain where the original phenotype that was changed in the mutant is restored by a second mutation is called the revertant -there are 2 types of revertants if a structural gene is being impacted

Question 32

what are the 2 types of revertants?

Answer 32

-same-site revertant -second-site revertant

Question 33

what is a same-site revertant?

Answer 33

-the mutation that restores activity is at the same site as the original mutation if the back mutation also restores the original sequence then it is a true revertant (wild-type phenotype and genotype restored)