14.3 Cellular Reproduction Flashcards
Anaphase Control
- role of 2 complex APC and SCF
- Anaphase promoting complex. Its job it so add ubiquitin, ubiquitin tagged portions for proteosomal digestion. Targets the proteosome. Proteolysis is critical to anaphase. There’s 2 adaptors: cdc20 and cdh1. The work to change the substate specificity of the anaphase promoting complex. The target for the ubiquination changes depending on what adaptor it’s associating with. Cdc20 labels securing in Met-An.
APC with the 2 adaptors that specify the target.
•Cdc20 targets securin and will be degraded. IF you degrade securin, make seperase which cleaves cohesion, which hold sis chromo together. Then sis chromatids separate and you go Met to Anaphase. •Eventually the cell will have to enter G1. Promotion to G1. To do that APC works with Cdh1.. The target is mitotic cyclins, these are degraded these promote the cell to enter into mitosis. If degraded, cell will start to enter G1.
•The 2 adaptors change their expression depending on where you are in the mitotic cycle
Remember the targets for these 2 adaptors the role of APC (proetin that ubiqyutanates proteins)
Importance of Proteolysis
experiment that illustrates the NB of proteolysis. Have single cell, arrested in mitosis by incubating with proteasome inhibitor. Add cdk1 inhibitor. Cdk1 promotes cell to centre into mitosks. Starts initiating cytokinesis. Take cdk1 inhibitor out so cdk1 is active again . Set the cell back into the beginning of the mitotic phase and the cell will reenter mitosis. Cdk1 and Cyclin B promote cell to enter mitosis. cdK1 normally not active at end of mitosis. NB of proteolytic digestion to promote cell to go from M phase to G1.
- Time 0 Cdk1 inhibitor added
- Cell completes mitosis and initiates cytokinesis
- Cell washed free of Cdk1 inhibitor
- Cell reenters mitosis since cyclin B (mitotic cyclin) still present- progressed to metaphase
The 2 parts of Anaphase
Split in into 2 phases A and B.
A is when anaphase first begins nd the chromatids first separate (looks a lot like metaphase).
Cell starts to distend and move apart, becomes more elongated. Driving the elongating is the polar mt start elongating due to addition of dimers (also a motor protein that sliding and pushing them farther apart) to the end and start pushing the chromo to the poles. This is B.
Monomers added to plus
Ends of polar microtubules
-Pushes poles apart
Microtubule Depolymerization
- As get pulled to the poles, there’s actually a shortening of the mt. What drives this shortening is actually the overall loss of tubulin dimer. •Experiment that shows you can move a chromo by shortening the mt. Basal body (base for flagella) they force the catastrophic disasbly of the mt. mt from basal body to chromo. dilute so lower [ ] of mt. It disassembles and shortens and pulls in the mt closer.
- Ncd80 teathering protein that teachers the kinetochore/chromo to the mt. They walk along the mt. as it depolymerases behind the. Depol at both ends but it’s greater at the + end.
Spindle (Assembly) Checkpoint
- Anaphase will not proceed until absolutely all are lined up on metaphase plate. The lazy chromo is taking it’s time. The cell will wait till their all lined up. Does not prematurely enter anaphase.
- Protein called spindle checkpoint protein Mad2. Mad2 is pink. Mad2 binds to cdc20 and cdc20 is respncible for activating the seperaze enzyme which separates the chromoatids. As long as Mad2 is present it will not promote the separation of the chromatids. Not understood why.
- Chromo might not be on metaphase plate because momoattached. Only attached to one spindle fiber. Once it gets double attachment it’s good.
Importance of Mechanical Tension
If not equal tension on both sizes . If there’s not then you have aurora b kinase activity which responds to lack of tension and phosphate the substate. That will cause distatchment of mt and allow chromo to reattach to 2 mt. start the cycle again.
Experiment where with the right amount of tension you can get cell to enter into anaphase even if it’s not double attatched. If manages to do this mono attached it pulls he whole chromo to one side.
Activity of Motor Proteins
(Prometaphase)
(Metaphase)
(Anaphase)
Prometaphase:
- Plus end directed motor proteins cause polar microtubules to slide past each other
- Minus end directed motor protein at kinetochore
- Plus end directed motor protein associated with kinetochore and chromosomes
Metaphase
- Plus end directed motor activity on polar microtubules
- Balanced activity of motor proteins at kinetochore
Anaphase
-Kinetochore motors move chromosomes and/or anchor chromosomes to
depolymerizing microtubules.
-Motor proteins separate poles
What proteins are required in Cytokinesis
- Involves actin protein and myosin 2 (conventional myosin, same as in muscles)
- High [ ] on actin by cleavage furrow.
- Abscission is right before seperation has that bridge between them.
- Lineup of actin filaments and bipolar myosin filaments between them. Slide actin filaments and pulling them together forming contraction that pinches off the cells.
Role of Myosin in Cytokinesis
- Starfish egg incubated with myosin antibody which inhibits cytokinesis
- Mitosis continues in absence of cytokinesis
- Experiment illustrates how you can separate mitosis from cytokinesis. Starfish egg has multiple mitotic events. Done by inhibiting myosin II. won’t progress to cytokines just keeps going through mitotic events.
How does the cell know to divide exactly in half?
How does the cell know to divide exactly in half? Pipette tip, they’ve draw in the cell into the pipette tip. Changed the shape of the cell from round to oblong. In the tip, the cells are not equal. What is the same though is the distance between the centrosomes. Dark dot is centrosome. Narrow cell divides quicker. Maybe position of centrosome plays a role. Make one larger and one smaller. signal is sent from the centrosome along the astral mt.
Position of Spindle Pole
- Clevage plane from where the astral mt overlap.
- Distance from poles to cleavage plane is smaller in cylindrical cell
- Shortens time it takes the cleavage signal to reach the surface (cortex)
Cytokinesis in Plant Cells
Plant cells don’t have centrosomes. has to do with mt organization. In interphase mt are very strucured, In mitosis you has mt concentrated in the centre, then they become the spindle apparatus. As cell goes into cytokinesis, the mt organize into a phragomoplast in the centre of the cell.
1) Golgi derived vesicles organized on cell plate
2) “Fingers” extended that fuse with neighbouring vesicles
3) Membranes fuse with original plasma membrane
4) Contents of vesicles form cell plate
