Mitosis Flashcards
Give a basic description of the cell cycle.
Chromosomes replicate to produce sister chromatids. The nucleus breaks apart and replicated chromosomes condense in preparation for mitosis. The sister chromatids separate during mitosis and two cells are formed during cytokinesis.
What are cyclins?
Proteins associated with the cycle of cell division.
What do cyclins do?
They bind and activate cyclin dependent kinases (CDKs).
What do CDKs do?
They phosphorylate key players in the cell cycle - they initiate DNA replication and phosphorylate lamins which cause nuclear envelope breakdown.
How are cyclins regulated?
Destruction - they are targeted by ubiquitination.
What is ubiquitin?
A small molecule that attaches to proteins to inactivate them.
What are the features of mitosis?
It ensures the accurate partitioning of the genome to daughter cells and is the shortest stage of the cell cycle (30 minutes to 2 hours). It is dynamic and highly ordered and has 6 phases.
What is S-phase?
All the DNA is replicated and it occurs only one time.
What is M-phase?
When the copies from one nucleus are divided into two.
What is proliferating cell nuclear antigen? (PCNA)
It is part of the DNA replication complex and has a speckled texture during S-phase.
What are homologous chromosomes?
A pair of chromosomes - one paternal and one maternal.
What are sister chromatids?
Two copies of replicated chromosomes - two paternal and two maternal.
What are cohesin rings?
A protein complex that keeps the replicated sister chromatids together.
Why does replication of DNA occur once and only once within the cell cycle?
The cyclin-CDK complexes are destroyed by anaphase promoting complex/cyclosome (APC/C).
What does polarisation of microtubules form?
A bipolar spindle.
When does polarisation of microtubules being?
S?G2 with the duplication of the centrosome.
What is the other name for the centrosome?
The microtubule organising centre.
What are centrosomes made up?
Centrioles and a mass of proteins called the pericentriolar material. This pericentriolar material surrounds the centrioles.
What are centrioles?
Complex molecular scaffolds that form rings that act as nucleators of the microtubules.
When are centrioles replicated?
S/G2.
What are the 6 phases of mitosis?
Prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis.
What happens in prophase?
Chromosome condension - cohesin holds the chromatids together and condensins (similar structure to cohesin) that further loop chromatin into tight bundles. The centrosomes move apart and form the mitotic spindle.
What happens in prometaphase?
The condensed chromosomes are attached to microtubules and the nuclear envelope breaks down.
How does the nuclear envelope breakdown?
There is phosphorlyation of lamins by cyclin B-CDK1. Fragments form vesicles that contain lamin B and the nuclear pore complexes disassemble.
What happens in metaphase?
Sister chromatids line up on the metaphase plate.
What is dynamic instability of microtubules?
They grow slowly and shrink rapidly.
What orients the mitotic spindle?
The physical force from the dynamic microtubules.
What happens if orientation of the mitotic spindle goes wrong?
Cancer may result.
What is kinetochore?
A protein complex linking chromatin and microtubules.
What force pushes/pulls chromatids?
The force of microtubule dynamics.
What do sensor proteins do?
They monitor whether microtubules are attached or not.
What is the spindle assembly checkpoint?
The last checkpoint in which cells commit before dividing.
What does the mitotic checkpoint complex do?
It inhibits anaphase promoting complex when kinetochores are exposed.
What happens when all of the kinetochores are attached to spindle?
APC is released and activated which targets securin for degradation. Separate is free to cleave cohesion.
How is the kinetochore system an example of negative feedback?
One exposed kinetochore stops APC and the signal is amplified through kinases.
What happens in anaphase?
Cohesin is degraded and the sister chromatids move to opposite poles. This process is very fast.
What happens in telophase?
The nuclear envelopes reform and assemble around individual chromosomes. Microtubules bundle and push the nuclei apart and contractile rings begin to form of the midline which will become the cleavage furrow.
What happens in cytokinesis?
Contractile rings cinches and pinches and the actin-myosin fibres slide against one each other. The midbody forms at the scission point.
What happens if there is failure of cytokinesis?
Binucleate cells.
What destroys cyclin CDK complexes after the DNA is replicated?
APC/C
What happens if Emi1 is removed from APC/C?
This protein activates the APC/C, so replication origins continue firing and the cell never enters M phase.
