Ch.19: Gene mutation, DNA repair, and recombination

Question 1

What are gene mutations?

Answer 1

molecular changes in the DNA sequence of a gene

Question 2

What is a transition?

Answer 2

a change of a pyrimidine (C, T) to another pyrimidine (T, C) or a purine (A, G) to another purine (G, A)

Question 3

What is a transversion?

Answer 3

a change of a pyrimidine (C, T) to a purine (A, G) or a purine (A, G) to a pyrimidine (C, T)

Question 4

Silent Mutations are?

Answer 4

base substitutions that do not alter the amino acid sequence of the polypeptide due to the degeneracy of the genetic code

Question 5

Missense mutations are?

Answer 5

base substitutions in which an amino acid change occurs

Question 6

Nonsense mutations are?

Answer 6

base substitutions that change a normal codon to a stop codon

Question 7

Frameshift mutations involve?

Answer 7

the addition or deletion of a number of nucleotides that is not divisible by three

Question 8

What mutations are characterized by their differential ability to survive?

Answer 8

Deleterious mutations: decrease the chances of survival. The most extreme are lethal mutations.

Beneficial mutations: enhance the survival or reproductive success of an organism

Question 9

What do suppressor mutations do?

Answer 9

reverse the phenotypic effects of another mutation

Question 10

What does an intragenic suppressor mutant do?

Answer 10

the second mutant site is within the same gene as the first mutation

Question 11

What does an intergenic suppressor mutant do?

Answer 11

the second mutant site is in a different gene from the first mutation

Question 12

What are the two types that animal cells are classified as?

Answer 12

Germ-line cells: Cells that give rise to gametes such as eggs and sperm

Somatic cells: All other cells

(slide 17 has a good picture for this)

Question 13

What does Tonr stand for?

Answer 13

T one resistance

Question 14

What did the Lederberg bacteriophage T1 tonr experiment prove?

Answer 14

By placing random mutants on plates that contained T1 and one that didn’t, it showed that most cells were killed by T1; this showed that spontaneous mutants could occur that provided selective advantages

Question 15

What are the 3 types of spontaneous mutations?

Answer 15

• Depurination
• Deamination
• Tautomeric shift

Question 16

What is Depurination? What mutations does it cause?

Answer 16

the removal of a purine (guanine or adenine) from the DNA forming an apurinic site; depurination of A or G can cause both transition and transversion mutants

Question 17

What is deamination? What mutations does it cause?

Answer 17

deamination of cytosine is the removal of an amino group from the cytosine base
- Base pairing causes the cytosine to be converted to a thymine which results in a C:G to T:A transition mutation
- deamination of 5-methylcytosine causes a C:G to T:A transition mutation

Question 18

What is a Tautomeric shift and the common stable forms? What mutations does it cause?

Answer 18

A tautomeric shift is a temporary change in base structure
-Tautomeric shifts cause both T:A to C:G and C:G to T:A transition mutations

• The common, stable form of thymine and guanine is the keto form. Rarely, T and G convert to an enol form

-The common, stable form of adenine and cytosine is the amino form/ Rarely, A and C can convert to an imino form

Question 19

What does oxidative stress do?

Answer 19

Causes a G:C to T:A transversion mutation to counter transition mutation

Question 20

What are deamination agents?

Answer 20

replace amino groups
with keto groups (–NH2 to =O) ex. nitrous acid

-can change cytosine to uracil and adenine to hypoxanthine
- modified bases do not pair with the appropriate nucleotides in the daughter strand during DNA replication and mutations occur
-Uracil base pairs with adenine and hypoxanthine base pairs with cytosine

Question 21

What are alkylating agents? What mutations does it cause?

Answer 21

disrupt the appropriate pairing between nucleotides by alkylating bases within the DNA, ex. nitrogen mustards and ethyl methanesulfonate (EMS); cause both cause both G:C to A:T and T:A to C:G transition mutations

Question 22

What do deamination agents cause?

Answer 22

both C:G to T:A and A:T to G:C transition mutations

Question 23

What do intercalating agents cause?

Answer 23

frameshift mutations

Question 24

What do base analogues cause?

Answer 24

transition mutations

Question 25

What does UV light cause and what mutations?

Answer 25

the formation of cross-linked thymine dimers and C:G to T:A transition mutations

Question 26

What can allow covalent modifications of nucleotides to be reversed?

Answer 26

three specific enzymes

Question 27

What does Photolyase do?

Answer 27

repairs thymine dimers (It splits the dimers restoring the DNA to its original condition using the energy of visible light for photoreactivation)

Question 28

What does Alkyltransferase do?

Answer 28

repairs alkylated bases (It transfers the methyl or ethyl group from the base to a cysteine side chain within the alkyltransferase protein. This action permanently inactivates
alkyltransferase)

Question 29

What do base excision repair (BER) involve?

Answer 29

a category of enzymes known as DNA N-glycosylases that recognize an abnormal base and cleave the bond between it and the sugar in the DNA
- can eliminate abnormal bases such as uracil, 3-methyladenine and 7-methylguanine depending on species

Question 30

What does Nucleotide excision repair (NER) do?

Answer 30

repair many types of DNA damage:
- Thymine dimers and chemically modified bases
- Missing bases
- Some types of crosslinks
-NER is found in all prokaryotes and eukaryotes, however, its molecular mechanism is better understood in prokaryotes

Question 31

What do Mismatch repair systems do?

Answer 31

recognize and correct base pair mismatches

Question 32

What are the 2 systems that can repair DNA double-strand breaks?

Answer 32

homologous recombination repair (HRR): sister chromatids are genetically identical; can be an error-free repair mechanism

nonhomologous end joining (NHE): repair may result in the deletion of a small region of the DNA.

Question 33

What happens when a replicative DNA polymerase encounters a damaged translesion DNA region?

Answer 33

DNA polymerase is swapped for a TLS polymerase and the region is duplicated with error-prone replication

Question 34

What happens in the Holliday model for homologous recombination?

Answer 34

a break occurs at the identical location of two chromatids that are aligned together

Question 35

What happens in a double-strand break model?

Answer 35

a double-strand break creates a gap in the DNA of a single chromatid that must be repaired

Question 36

What are the 2 ways gene conversion can occur?

Answer 36

-DNA mismatch repair (can convert one allele to another (branch migration))
-DNA gap repair synthesis (can convert one
allele to another (uses double-strand break model))