Lecture 3 - CO and mating type switch Flashcards
What proteins are newly relicated sister chromatids held together by?
protein loops = cohesins
When do cohesins form?
form in G1 before replcation
replication fork synthesises in S phase within cohesin loops
When sister chromatids are separated, what are the cohesin loops cleaved by?
Cohesin loops cleaved by separase (only active when needed, anaphase) which cuts at Scc1 part of cohesin loop
What are the four parts of the cohesin loop?
Smc1
Smc2
Smc3
Scc1 (separase cuts)
When chromosome attach to the mitotic spindle, where are the cohesins found?
At ends of chromosomes not at the centromeres
What is needed for migration of sister chromatids to opposite poles?
cohesin (not at centromeres) to create tension through the correct attachment of sister chromatids, stable arrangement
Where are cohesins involved in meiosis?
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
Why does crossing over contribute to correct segregation of homolgous chromosomes>
not held together by cohesins before crossing over, cohesins neccessary by generation tension formed when correct attachment of microtubles
What does the synaptomenal complex hold together?
two pairs of sister chromatids aka homolgous chromosomes
When do synaptomenal complexes form?
when homolgous chromosomes are aligned after dsb formation, before crossing over
What is a bivalent?
pair of homologous chromosomes
What is the structure of the synaptomenal compex?
- 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
What happens in the absence of crossing over?
chromosomes may segregate incorrectly lead to aneuploidy
- MI, non disjunction
- MI leads to Disomic gametes and nullsomic gametes
What is aneuploidy?
abnormal number of chromosomes within a cell
What are disomic gametes?
having one or two chromosomes present in two copies
What are euploid gametes?
correct number of chromosomes per cells
When should crossing over not occur and what is this called?
In mitosis
between sister chromatids (sister chromatid exchange)
How can sister chromatid exchange be visualised?
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
How are harlequin chromosomes formed?
high frequency of SCE e.g. in Blooms syndrome
What are teh roles of BLM helicase?
acts as an antirecombinase
- dissolution of HJ leading to nonCO
- HJ resolution leading to crossing over
What is the process fo HJ dissolution?
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
What does non allelic (unequal crossing over) xrossing over lead to?
genome rearrangements
What is shown in non allelic crossing over of direct repeats interhomologs?
duplication on one strand
deletion on one strand
What is shown in non allelic crossing over of direct repeats between sister chromatids?
duplication in one strand
deletion in one strand
What is shown in non allelic crossing over of direct repeats within a chromatid?
deletion one strand
formation of an acentric fragment
What is shown in non allelic crossing over of indirect repeats between homoglous chromosomes?
an isodicentric chromosome
and an acentric fragment
What is shown in non allelic crossing over of indirect repeats between sister chromatids?
an isodicentric chromosome
acentric fragment
What is shown in non allelic crossing over of indirect repeats within a chromatid?
inversion
What might SSA at DSBs between repeated sequences lead to?
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
What is the mating type switch in yeast?
where a sequence from either of the silent loci (cassette) inserted into the active locus in SDSA
What is the mechanism of MAT switching?
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
How can MAT switching be visualised and monitered?
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
