Week 10: Chromsome Condensation

Question 1

Decatenation, compaction, & resolution

Answer 1

1. Decatenation occurs due to the fact that there are two replication forks which end up tangling up the DNA…so topoisomerase 2 needs to detangle the DNA in S phase
2. Compaction is the squishing chromosome vertically (prophase->metaphase)
3. Resolution ensures that during mitosis, sister chromatids are packaged into 2 distinct entities (prometaphase->early anaphase)

Question 2

Why is condensation and resolution important

Answer 2

1. W/o it DNA gets tangled, DNA can break etc.
2. Needs to occur for faithful genome transmission

Question 3

What is condensation largely mediated by?

Answer 3

1. The condensin complex (looks similar to cohesin)
2. The condensin complex’s structure is a SMC heterodimer which does structural maintenance of chromosomes
3. There are 2 condensin complexes in higher eukaryotes: Condensin 1 and condensin 2

Question 4

What is the working model for the action of the condensin complex

Answer 4

1. Multiple subunits joining together form rings or filaments
2. ATP driven head-head engagements or stalk-stalk interactions
   2a. ATP/intra: 1 ring
   2b. ATP/inter: multiple rings
   2c. Stalk/intra: 1 filament
   2D. Stalk/inter: multiples filaments

Question 5

Coordination between cohesion and condensin during the cell cycle

Answer 5

1. S phase: loading of cohesin and condensin
   1a. Cohesin added to keep together horizontally
   1b. Condensin 2 added to keep together vertically and oppose cohesins forces
2. Metaphase:
   2a. Cohesin has a force that keeps it together horizontally
   2b. Condensin 1 keeps it together horizontally
   2c. Condensin 2 keeps it together vertically
3. Anaphase: cohesin in middle removed by separase so force pulls chromatids apart

Question 6

How do we monitor/study condensation

Answer 6

1. In budding yeast, we apply red dye to label the genome, however we label the R DNA locus green (its a highly repetitive DNA that adopts specific morphology in the cell cycle so it’s easy to study)
2. Then use FISH to probe for these RDNA to see the morphological changes in the cell cycle

Question 7

Condensation in the cell cycle (results from FISH study)

Answer 7

1. Lines/loops in metaphase saying it’s condensed
2. Puff=uncondensed

Question 8

Kinase switch: kinase dependant condensation

Answer 8

1. G1/S Inhibition of condensin by casein kinase 2
2. Ultrasensitive activation of condensin by CDK-cyclin A/B in prophase
3. Maintenance of condensin activity by polo kinase (Cdc5) in anaphase: cdc5 phosphorylates condensin and hyperactivates its DNA supercoiling activity

Question 9

How do we remove condensin from the chromosomes

Answer 9

1. Cdc48/p97/VCP is thought to recognize proteins that can ubiquitinate/SUMOlation