Cell Division Flashcards
1
Q
Cyclins regulate the cell cycle via …
A
… CDKs
2
Q
Cyclins
A
- Cyclins accumulate in different stages of the cell cycle.
- Bind and activate cyclin dependent kinases, CDKs.
- Cyclins are targeted for destruction by ubiquitination.
3
Q
Cyclin-CDK complexes phosphorylate key players in all stages of the cell cycle:
A
- Initiate DNA replication
- Nuclear envelope breakdown
- Chromosome separation
4
Q
Cyclin-CDK activities are further regulated by …
A
… phosphorylation (both positive and negative)
5
Q
Cyclins targeted for destruction by ubiquitination
A
- 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
6
Q
Faithful replication and separation of genome
A
- Replicate all the DNA, one time only: S-phase (synthesis)
- Divide the copies from one nucleus into two: M-phase (mitosis)
7
Q
Origins of replication
A
- “fire” once per S-phase
- Cyclin-cdk targeted for destruction by APC/C
If you remove a protein that activates the APC/C and Emi1, replication origins keep firing, this causes the cells re-replicate DNA but never divide.
8
Q
Homologous chromosomes
A
- One maternal
- One paternal
9
Q
Sister chromatids
A
- Two copies of replicated chromosomes
- Two maternal, two paternal
- Kept together by cohesin rings
10
Q
M-phase
A
- Mitosis
- Polarisation of the microtubules into a bipolar spindle.
- Process begins in the S-phase with the duplication of the centrioloes and centrosome.
11
Q
Centrosome
A
Centrioles + mass of proteins called pericentriolar material
12
Q
Centrioles
A
- Comprised of microtubule array.
- Mother and daughter.
- Duplicated in S-phase
13
Q
Mitosis
A
- Ensures the accurate partitioning of the genome to daughter cells.
- Shortest stage of the cell cycle
- Dynamic, highly ordered process
- Six phases
14
Q
Phases of Mitosis
A
- Prophase: pro- “before”
- Prometaphase: pro- “before” metaphase
- Metaphase: meta- “adjacent”
- Anaphase: ana- “back”
- Telophase: telos- “end”
- Cytokinesis: cyto- “cell” + -kinesis “movement”
15
Q
Prophase
A
- Chromosome condensation begins
- Centrosomes move apart, begin to form mitotic spindle
- At this point, the nuclear envelope is still intact.
16
Q
Cohesin
A
Holds sister chromatids together
17
Q
Condensins
A
- Similar ring structure to cohesins
- Further loop chromatin into tight bundles
18
Q
Prometaphase
A
- Condensed chromosomes attach to microtubules.
- The MTs are growing.
- Nucleus envelope breaks down:
- Phosphorylation of lamins by cyclin B-CDK1
- Leads to disassembly of lamins.
- Nuclear pore complexes (phosphorylated) disassemble.
19
Q
Metaphase
A
- Sister chromatids line up on the metaphase plate.
- Dynamic instability cause the MTs to grow slowly, shrink rapidly.
- Physical force from dynamic MTs orients the mitotic spindle.
- MTs reaching for the central chromasomes.
- Sliding of antiparallel MTs at the equator
20
Q
Metaphase plate
A
Equator between two poles
21
Q
Kinetochore
A
- Protein complex linking chromatin and microtubules.
- Large multimeric complex of structural and signalling proteins.
- Sensor proteins that monitor attachment and sense tension to microtubules.
- Force of microtubule dynamics pushes/pulls chromatids.
- Balance of forces when aligned on equator
22
Q
Spindle Assembly Checkpoint
A
- Mitotic Checkpoint Complex (MCC): inhibits APC/C when kinetochores are exposed.
- When all kinetochores attached to spindle, the APC/C released and activated and targets securin for degradation. Here the separase is free to cleave cohesins
- Negative feedback loop: Just one exposed kinetochore stops APC and the signal amplified through kinases.
23
Q
Anaphase
A
- Cohesins holding sister chromatids together degraded.
- Sister chromatids move to opposite poles.
- Very fast! (for cells)
24
Q
Telophase
A
- Nuclear envelope re-forms and assembles around individual chromosomes.
- Microtubules bundle and push nuclei apart.
- Contractile ring begins to form on midline, and will become the cleavage furrow.
25
Cytokinesis
* 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 of this stage leads to binucleate cells
26
Mitosis produces...
* 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
27
Meiosis produces...
* Haploid cells with genetic material for sexual reproduction
* Haploid (1n) = one copy of each chromosome (haploos, “single”)
* Gamete: sperm or egg (gamos/gamete, “marriage/spouse”)
* Gamete + gamete = zygote (zygon, “yoke, join”)
* Homologous chromosomes segregated
* Mixing of DNA between homologous chromosomes
28
Oogenesis
* One haploid egg cell per meiosis
* 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
29
Homologous chromosome pairing
* Occurs in the first half of meiosis:
* Pair via complementary DNA sequences
* Two pairs of sister chromatids form a four-chromosome bivalent joined by synaptonemal complex.
* Process called synapsis.
* Homologous chromosomes can then swap genetic material.
30
Crossing over
* Swapping of genetic material between maternal and paternal chromosomes
* Only about 10% of the time
* At least one per chromosome
* Somehow cells ensure that crossovers do not occur near one another.
31
Genetic diversification
* Independent assortment of maternal and paternal chromosomes
* Crossing over
* Gene conversion
32
Gene conversion
* Non-crossing over homologous recombination
* Copy maternal sequence into paternal chromosome, or vice versa
33
Non-disjunction
* When chromosome segregation goes wrong.
* Down syndrome – trisomy 21 (very small chromosome)
* Trisomy 18 or 13 - usually mosaic (complete trisomies nonviable)
* Turner syndrome – one X, developmental problems
* Klinefelter syndrome – XXY (infertility)
* XYY – no severe symptoms
