Chapter 13 lecture (Homologous recombination)

Question 1

why is Homologous recombination Essential in bacteria and in eukaryotic mitosis

Answer 1

to repair DNA damage and stalled replication forks

Question 2

why is homologous recombination essential in eukaryotic meiosis

Answer 2

for generating diversity and for chromosome segregation

Question 3

Recombination enzymes can use … as substrates

Answer 3

any pair of identical or nearly identical sequences

Question 4

how is homologous recombination initiated

Answer 4

a double stranded break

Question 5

in homologous recombination, how is the DbSB enlarged?

Answer 5

exonuclease will Degrade the 5’ end of both strands flanking the gap to form 3’-single stranded overhangs

Question 6

A 3’ overhang will perform a … of the other duplex to make …

Answer 6

stand invasion
heteroduplex DNA

Question 7

what will be used as template to synthesize new DNA through DbSB region

Answer 7

undamaged copy of DNA

Question 8

after the DNA is synthesized in homologous recombination, what happens next?

Answer 8

The invading strand is released and base pairs with the single strand from the original duplex

Question 9

in homologous recombination, High fidelity DNA polymerases will

Answer 9

fill in remaining gaps and all strands will be ligated

Question 10

If it occurs after S phase

Answer 10

there will be identical sister chromatids to use as a template

Question 11

If it occurs in G1,

Answer 11

it will have to use a non-identical homologous chromosome as a template

Question 12

Meiotic recombination uses a homologous sequence as a template, even though

Answer 12

a sister chromatid is present

Question 13

what is a key component of the homologous recombination mechanism in E. coli

Answer 13

RecA protein

Question 14

RecA protein is similar to what in eukaryotes

Answer 14

Rad51

Question 15

RecA binds to

Answer 15

the single stranded 3’ overhang produced after DbSB

Question 16

RecA forces ssDNA into a

Answer 16

“double stranded” helical conformation

Question 17

The RecA/ssDNA complex can displace

Answer 17

single strands from duplexes

Question 18

If RecA/ssDNA finds a complimentary sequence of >15 nt

Answer 18

Complimentary strand will pair with ssDNA to form heteroduplex

Question 19

Homologous recombination is necessary in eukaryotes
for

Answer 19

DNA repair
Restarting of collapsed replication forks
Needed for successful pairing of homologous chromosomes in prophase I

Question 20

failure of homologous pairing leads to

Answer 20

nondisjunction in anaphase 1

Question 21

Meiotic recombination begins in the same way as with general homologous recombination, except

Answer 21

The DbSB is induced by the Spo11 protein complex
The undamaged copy that is used as a template is a nonidentical homologous chromosome

Question 22

what two proteins coat ssDNA overhangs and promote strand invasion

Answer 22

dmc1 and rad51

Question 23

dcm1 is only active in

Answer 23

meiosis and has specificity for homologous chromosome
Prevents invasion of sister chromatid

Question 24

rad51 is active

Answer 24

throughout the cell cycle for all DsDB recombination in eukaryotes
Loaded onto the ssDNA by the Brca2 protein
Loss of Brca2 function leads to reduced DsDB repair

Question 25

Spo11 protein produces the

Answer 25

DbSB needed for meiotic recombination

Question 26

how does Spo11 protein produce the DbSB needed for meiotic recombination

Answer 26

A tyrosine side chain of Spo11 attacks the phosphodiester backbone of DNA and creates a covalent enzyme-DNA complex
Energy of phosphodiester linkage is stored in enzyme-DNA linkage
DbSB is reversible and DNA strands can be easily resealed if recombination does not proceed

Question 27

Sequence specificity of Spo11 activity is … but timing is …

Answer 27

low
highly regulated

Question 28

Nuclease activity of Spo11 is related to existence of

Answer 28

recombination hotspots and coldspots in genome

Question 29

hotspots

Answer 29

open active chromatin
promotors

Question 30

coldspots

Answer 30

centromeres, telomeres
constitutive heterochromatin

Question 31

holliday junction

Answer 31

where two homologous helices are held together by the exchange of two of the four strands

Question 32

90% of the time, the DbSB are resolved as

Answer 32

non-crossover (patch) products

Question 33

10% of the time, the DbSB are resolved as

Answer 33

The second strand is captured
Another Holliday junction is formed
The strands are cut and resolved as crossover (splice) products

Question 34

what end of the strands are used as primers to extend the heteroduplexes

Answer 34

3’

Question 35

When the extended single strands reaches the end of the gap, the single strands are ligated and

Answer 35

the second holiday junction is formed

Question 36

The Holliday junction can move along the DNA using

Answer 36

branch migration

Question 37

Upon resolution of both Holliday junctions using cleavage at different sites,

Answer 37

crossover products will be produced

Question 38

Crossover control

Answer 38

The small number of crossovers on a chromosome will inhibit the resolution of nearby DbSB as crossovers

Question 39

Even if a DbSB is resolved as a non-crossover a small heteroduplex patch remains that is a potential site of

Answer 39

gene conversion

Question 40

gene conversion

Answer 40

One allele converted into another
An additional way to produce genetic variation in meiosis
Is a possible result of any DbSB-coupled homologous recombination
Including DNA repair mechanism

Question 41

How does DbSB-coupled repair lead to gene conversion?

Answer 41

A polymorphic region is located inside the region that is involved in the strand invasion synthesis
The original allele will be replaced by one from the homologous duplex
If the homologous template is not identical, then it will create a heteroduplex mismatch
The heteroduplex mismatch randomly repaired by mismatch repair enzymes
Alters genotypic proportions of gametes