Control of the Eukaryotic Cell Cycle 3 & 4 Flashcards
What is the most important role of the G2/M checkpoint?
To ensure that all of the chromosomes have been accurately replicated without any mistakes/damage
What checkpoint comes before the late stages of the M phase?
SAC or Metaphase/Anaphase checkpoint
What are the phases the cell is able to enter if it passes the G2/M phase?
Early mitosis - prophase, prometaphase, metaphase
What does the SAC/Metaphase-Anaphase checkpoint check?
Checks chromosomes are correctly aligned and attached to the spindle microtubules
Where do M-phase cyclin-cdk complexes form?
During G2 but are held in an inactive form
When are M-phase cyclin-cdk complexes activated?
At the end of the G2 phase
What do M-phase cyclin-cdk complexes do?
Trigger entry into M phase at the G2/M transition
What happens when M-phase cyclin-cdk complexes levels fall at the Metaphase-Anaphase checkpoint?
Permit chromosome segregation and mitotic exit
Levels of what cyclin rise during the G2 phase?
Cyclin B
What phase(s) is CDK1 inactive during and why?
G1 and S phase, due to the inhibitory enzyme, Wee1
Where does the protein phosphate, Cdc25 become activated?
the G2/M transition checkpoint
What does Cdc25 do to CDK1 during the transition of G2 to M phase?
Cdc25 de-phosphorylates CDK1
What drives mitosis through phosphorylation of mitotic regulators?
Activation of cyclin B and CDK1
What is the importance of mitosis being driven through phosphorylation of mitotic regulators?
It is important for nuclear envelope breakdown and setting up a bipolar mitotic spindle; protein destruction feedback; positive feedback loops
What do feedback pathways enhance?
Cyclin B and CDK1 activity, as well as early events of mitosis
What is the SCF complex?
It is a multi-protein E3 ubiquitin ligase complex
What does the SCF complex control?
The G1/S and the G2/M transitions by degrading Cyclin D, Cyclin E, p27 and Wee1
What does the mitotic spindle do?
Used to move segregated sister chromatids to opposite ends of the dividing cell in Anaphase
What is the mitotic spindle composed of?
Microtubules (dynamic polymers of tubulin)
What do the microtubules within the mitotic spindle do?
Interact with sister chromatids via kinetochores and interact with the spindle poles at the centrosome
What occurs when proteins are phosphorylated?
Assembly of the mitotic spindle; attachment to sister chromatids to the mitotic spindle; chromosome condensation; nuclear envelope breakdown
At the end of mitosis, what does each new daughter cell inherit?
A single pair of disengaged centrioles
What stage(s) do cells enter when they form a cilium?
Either progress into G1 or enter G2
What stage do the centrioles duplicate in?
S phase
What stage(s) do centrioles elongate in?
S and G2 phase
At what point do the centrioles begin to separate from one another?
G2/M transition
What is a kinetochore?
A protein structure that forms on a chromatid during cell division and allow it to attach to a spindle fibre on a chromosome
What is the purpose of the kinetochore?
To pull chromatids apart and help during cell division by making sure that each new cell has one chromatid
What is the centromere?
A chromosomal locus
What does the centromere?
Ensures delivery of one copy of each chromosome to each daughter cell
How do chromosomes move towards the mitotic spindle poles?
Microtubules depolymerisation
How does microtubule depolymerisation work?
Chromosomes actively depolymerise attached microtubule plus-ends while being reeled into spindle poles by the flow of tubulin suunits driven by minus-end depolymerisation
What prevents premature chromosome separation before anaphase?
Adhesion of sister chromatids to each other
What is essential to pass the SAC?
Correct bi-orientation of sister chromatids to the mitotic spindle
What blocks sister chromatid separation at the SAC?
Unattached or mis-attached chromosomes
Unequal tension results in what?
A signal that blocks APC/C-Cdc20 activity which leads to mitotic arrest
What complex initiates chromatid separation after the SAC?
Anaphase promoting complex
Describe the chromosome structure after the S phase
Each chromosome is duplicated; consists of two identical double-stranded DNA chains; sister chromatids attached together at the centromere, sister chromatids are held together by a large protein complex (cohesin); cohesin complexes are deposited at many locations along the length of the sister chromatids
Explain how the APC/C separates sister chromatids
Active M-phase cyclin B/CDK1 complexes initiates the assembly of the mitotic spindle and attachment of sister chromatids via their kineotchore; this then phosphorylates and activates the APC/C; active APC/C induces the ubiquintination and destructuon of securin, this allows separase to cleave the remaining cohesin complexes that hold the chromatids together, the pulling forces of the mitotic spindles then pull the chromatids apart
What else does the APC/C destroy?
Cyclin B (end of M phase)
How does APC/C take part in the transition from Metaphase to Anaphase?
By degrading cyclin A, cyclin B and securin
In sexual reproduction, haploid gametes fuse together to form what?
A diploid individual
How is genetic variation achieved in sexual reproduction?
Crossing over and independent assortment
What is gametogenesis?
It is the process in which cells undergo meiosis to form gametes
What is spermatogensis?
It is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis.
How many gametes are produced in a single round of meiosis in oogenesis?
1
How many spermatids are produced from the spermatocyte in spermatogenesis?
4
Where does genetic diversity occur?
Homologous recombination in prophase 1 and random segregation of homologs in anaphase 1 and of sister chromatids in anaphase 2
What is the orientation of paternal and maternal homologs on the first meiotic spindle?
Bi-orientated
When homologous chromosomes recognise each other in meiosis 1, what do they form?
A four-chromatid structure (bivalent)
How is the process of homologous chromosomes forming a bivalent regulated?
Via specific mechanisms involving the expression/function of meiosis 1 specific proteins
What does every meiotic recombination event start with?
Programmed formation of a DNA double-stranded break
