Chapter 13 lecture (Homologous recombination) Flashcards

1
Q

why is Homologous recombination Essential in bacteria and in eukaryotic mitosis

A

to repair DNA damage and stalled replication forks

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2
Q

why is homologous recombination essential in eukaryotic meiosis

A

for generating diversity and for chromosome segregation

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3
Q

Recombination enzymes can use … as substrates

A

any pair of identical or nearly identical sequences

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4
Q

how is homologous recombination initiated

A

a double stranded break

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5
Q

in homologous recombination, how is the DbSB enlarged?

A

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

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6
Q

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

A

stand invasion
heteroduplex DNA

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7
Q

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

A

undamaged copy of DNA

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8
Q

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

A

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

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9
Q

in homologous recombination, High fidelity DNA polymerases will

A

fill in remaining gaps and all strands will be ligated

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10
Q

If it occurs after S phase

A

there will be identical sister chromatids to use as a template

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11
Q

If it occurs in G1,

A

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

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12
Q

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

A

a sister chromatid is present

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13
Q

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

A

RecA protein

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14
Q

RecA protein is similar to what in eukaryotes

A

Rad51

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15
Q

RecA binds to

A

the single stranded 3’ overhang produced after DbSB

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16
Q

RecA forces ssDNA into a

A

“double stranded” helical conformation

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17
Q

The RecA/ssDNA complex can displace

A

single strands from duplexes

18
Q

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

A

Complimentary strand will pair with ssDNA to form heteroduplex

19
Q

Homologous recombination is necessary in eukaryotes
for

A

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

20
Q

failure of homologous pairing leads to

A

nondisjunction in anaphase 1

21
Q

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

A

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

22
Q

what two proteins coat ssDNA overhangs and promote strand invasion

A

dmc1 and rad51

23
Q

dcm1 is only active in

A

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

24
Q

rad51 is active

A

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

25
Spo11 protein produces the
DbSB needed for meiotic recombination
26
how does Spo11 protein produce the DbSB needed for meiotic recombination
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
27
Sequence specificity of Spo11 activity is ... but timing is ...
low highly regulated
28
Nuclease activity of Spo11 is related to existence of
recombination hotspots and coldspots in genome
29
hotspots
open active chromatin promotors
30
coldspots
centromeres, telomeres constitutive heterochromatin
31
holliday junction
where two homologous helices are held together by the exchange of two of the four strands
32
90% of the time, the DbSB are resolved as
non-crossover (patch) products
33
10% of the time, the DbSB are resolved as
The second strand is captured Another Holliday junction is formed The strands are cut and resolved as crossover (splice) products
34
what end of the strands are used as primers to extend the heteroduplexes
3'
35
When the extended single strands reaches the end of the gap, the single strands are ligated and
the second holiday junction is formed
36
The Holliday junction can move along the DNA using
branch migration
37
Upon resolution of both Holliday junctions using cleavage at different sites,
crossover products will be produced
38
Crossover control
The small number of crossovers on a chromosome will inhibit the resolution of nearby DbSB as crossovers
39
Even if a DbSB is resolved as a non-crossover a small heteroduplex patch remains that is a potential site of
gene conversion
40
gene conversion
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
41
How does DbSB-coupled repair lead to gene conversion?
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