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Genetics III > Lecture 3 - CO and mating type switch > Flashcards

Lecture 3 - CO and mating type switch Flashcards

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1
Q

What proteins are newly relicated sister chromatids held together by?

A

protein loops = cohesins

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2
Q

When do cohesins form?

A

form in G1 before replcation

replication fork synthesises in S phase within cohesin loops

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3
Q

When sister chromatids are separated, what are the cohesin loops cleaved by?

A

Cohesin loops cleaved by separase (only active when needed, anaphase) which cuts at Scc1 part of cohesin loop

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4
Q

What are the four parts of the cohesin loop?

A

Smc1
Smc2
Smc3
Scc1 (separase cuts)

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5
Q

When chromosome attach to the mitotic spindle, where are the cohesins found?

A

At ends of chromosomes not at the centromeres

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6
Q

What is needed for migration of sister chromatids to opposite poles?

A

cohesin (not at centromeres) to create tension through the correct attachment of sister chromatids, stable arrangement

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7
Q

Where are cohesins involved in meiosis?

A

hold together homologous chromosomes after crossing over

1) crossing over occurs at metaphase 1
2) separase cleaves cohensins at chiasma’s where crossing over has occured, freeing the held together homolgous chromosomes
3) Anaphase 1 chromosomes pulled to opp ends
4) after metaphase 2, separase cleaves cohesins at centromere freeing homologs
5) anaphase 2

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8
Q

Why does crossing over contribute to correct segregation of homolgous chromosomes>

A

not held together by cohesins before crossing over, cohesins neccessary by generation tension formed when correct attachment of microtubles

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9
Q

What does the synaptomenal complex hold together?

A

two pairs of sister chromatids aka homolgous chromosomes

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10
Q

When do synaptomenal complexes form?

A

when homolgous chromosomes are aligned after dsb formation, before crossing over

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11
Q

What is a bivalent?

A

pair of homologous chromosomes

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12
Q

What is the structure of the synaptomenal compex?

A
  • traverse filaments down the middle
  • connected to the axial cores of the homologs
  • connected to cohesin compex
  • cohesin complex connected to chromatid loops of sister chromatids of one homolog
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13
Q

What happens in the absence of crossing over?

A

chromosomes may segregate incorrectly lead to aneuploidy

  • MI, non disjunction
  • MI leads to Disomic gametes and nullsomic gametes
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14
Q

What is aneuploidy?

A

abnormal number of chromosomes within a cell

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15
Q

What are disomic gametes?

A

having one or two chromosomes present in two copies

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16
Q

What are euploid gametes?

A

correct number of chromosomes per cells

17
Q

When should crossing over not occur and what is this called?

A

In mitosis

between sister chromatids (sister chromatid exchange)

18
Q

How can sister chromatid exchange be visualised?

A

1) grow cells in presence of BrdU, used instead of dT
2) grow cells for two generations without BrdU
3)when SCE takes place a mosiaic pattern of label on both chromatids are seen
Normally, the label remains with one sister chromatid

19
Q

How are harlequin chromosomes formed?

A

high frequency of SCE e.g. in Blooms syndrome

20
Q

What are teh roles of BLM helicase?

A

acts as an antirecombinase

  • dissolution of HJ leading to nonCO
  • HJ resolution leading to crossing over
21
Q

What is the process fo HJ dissolution?

A

1)BLM helicase facilitaes branch migation, shorting the dHJ into a hemicatenane
2) Topo IIIa leads to decatenation
results in resolution of HJ with no crossing over

22
Q

What does non allelic (unequal crossing over) xrossing over lead to?

A

genome rearrangements

23
Q

What is shown in non allelic crossing over of direct repeats interhomologs?

A

duplication on one strand

deletion on one strand

24
Q

What is shown in non allelic crossing over of direct repeats between sister chromatids?

A

duplication in one strand

deletion in one strand

25
Q

What is shown in non allelic crossing over of direct repeats within a chromatid?

A

deletion one strand

formation of an acentric fragment

26
Q

What is shown in non allelic crossing over of indirect repeats between homoglous chromosomes?

A

an isodicentric chromosome

and an acentric fragment

27
Q

What is shown in non allelic crossing over of indirect repeats between sister chromatids?

A

an isodicentric chromosome

acentric fragment

28
Q

What is shown in non allelic crossing over of indirect repeats within a chromatid?

A

inversion

29
Q

What might SSA at DSBs between repeated sequences lead to?

A

deletions
1)DSB between repeated sequences
2)resection by 5’ to 3’ exonuclease, gerate 3’ overhangs, deletes one repeating sequence on each strand
3) annealing joins repeated sequences together
4) 3’ ends processsed following ny ligation
heteroduplex formed, section between repeats deleted

30
Q

What is the mating type switch in yeast?

A

where a sequence from either of the silent loci (cassette) inserted into the active locus in SDSA

31
Q

What is the mechanism of MAT switching?

A

1)homolgous regions flanking the active locus and silent locus line up (x and z line up on HMLAalpha and MATa)
2)right side of boundary of MATa cut by HO endonuclease
3) 3’ to 5’ resection of active cassette by exonucleases and helicases (unwind dna)
4) Rad52 binds leading to strand invasion of region containing silent locus
5) donor template (Yalpha) copied
6)newly syntheised DNA displaced by bubble migration and helicases
7) non homolgous section (Ya, active cassette) removed
8) second strand sythesis
switch to MAT alpha

32
Q

How can MAT switching be visualised and monitered?

A 
by southern blot analysis
-cells engineered to carry inducible HO endonuclease, distance travelled measured over time, MATa middle, cut MATa end, MATalpha beginning
-expected fragments were:
long MATalpha
short MATa cut by HO endonuclease
middle MATa
-visulalised by labelled probe

Genetics III (12 decks)

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