Leture 2??? - Cell division Flashcards
What do cyclins do
Bind and activate to Cyclin Dependent Kinases (CDKs)
Cyclin-CDK complexes phosphorylate key players in the cell cycle
Initiate DNA replication
Nuclear envelope breakdown
Chromosomeseparation
Cyclin-CDK activities further regulated by phosphorylation(both positive and negative)
What does ubiquitin do
Cyclins are targeted for destruction by ubiquitination: point of no return from one stage to the next
APC/C targets proteins for degradation
Adding the small molecule ubiquitin to a protein is the signal for it to be destroyed by the proteosome
What does PCNA do
Proliferating Cell Nuclear Antigen
part of the DNA replication complex (sliding clamp)
Speckled appearance during S-phase
Effects of Emi1
Origins of replication “fire” once per S-phase
Cyclin-cdk complexes targeted for destruction by APC/C
- Remove a protein that activates the APC/C, Emi1
Replication origins keep firing
Cells re-replicate DNA but never divide
What do cohesin rings do
keep replicated sister chromatids together… until it is time for them to be separated
What is the first thing that occurs in Mitosis
Polarisation of the microtubules into a bipolar spindle
Actin and myosin filaments form a contractile ring
Process begins in S-phase with duplication of the centrioles and centrosome
What are centrosomes
Centrioles + mass of proteinscalled pericentriolar material
What are centrioles
Comprised of microtubule array
‘Mother’ and ’daughter
Duplicated in S-phase
What do condensins do
similar ring structure to cohesion, further loop chromatin into tight bundles
What occurs in prophase
Chromosome condensation begins
Centrosomes move apart, begin to form mitotic spindle
Nuclear envelope still intact
What happens in Prometaphase
Condensed chromosomes attach to microtubules
Nucleus envelope breaks down
Phosphorylation of lamins by cyclin B-CDK1
Nuclear pore complexes (phosphorylated) disassemble
What happens in Metaphase
Sister chromatids line up on the metaphase plate
Equator between two poles
Dynamic instability MTs grow slowly, shrink rapidly
Physical force from dynamic MTs orients the mitotic spindle
+ Sliding of antiparallel MTs at the equator
What is a Kinetochore
protein complex linking chromatin and microtubules
Large multimeric complex of structural and signalling proteins
Sensor proteins that monitor attachment to microtubules
Force of microtubule dynamics pushes/pulls chromatids
Balance of forces when aligned on equator (metaphase plate)
Sensor proteins also sense tension on the microtubules
What is the MCC
Mitotic Checkpoint Complex
inhibits APC/C when kinetochores are exposed
When all kinetochores attached to spindle:
APC/C released and activated —> targets securin for degradation
Separase free to cleave cohesins
Negative feedback loop:
Just one exposed kinetochore stops APC
Signal amplified through kinases
What happens in Anaphase
Cohesins holding sister chromatids together degraded
Sister chromatids move to opposite poles
Very fast! (for cells)
What happens in Telophase
Nuclear envelope re-forms
Assembles around individual chromosomes
Microtubules bundle and push nuclei apart
Contractile ring begins to form on midline
Will become the cleavage furrow
How does Cytokinesis work
Contractile ring cinches and pinches
Actin-myosin fibres slide against one another
Midbody forms at scission point
Can take a long time to for cells to completely detach
Failure = binucleate cells
What does mitosis produce
diploid cells with identical genetic material to the original cell
Diploid (2n) = two copies of every chromosome, maternal and paternal
DNA replicated once and only once in S-phase
Chromosomes segregated equally
What does Meiosis produce
haploid cells with genetic material for sexual reproduction
Homologous chromosomes segregated
Mixing of DNA between homologous chromosomes
What is spermatogenesis
Formation of 4 haploid mature sperm cells per meiosis run
(SLIDE 28)
What is Oogenesis
Formation of one haploid egg per meiosis cycle
Half of chromosomes disposed of in polar bodies after metaphase I and II
Oocyte meiosis arrests in metaphase II until fertilisation
Never completes if unfertilised
When does Homologous chromosome pairing occur
First half of mitosis
What is synapsis
Two pairs of sister chromatids form a four-chromosome bivalent joined by synaptonemal complex
What is a chiasma
swapping of genetic material between maternal and paternal chromosomes
