Mitosis & Meiosis Flashcards
What are the 4 stages of the cell cycle & what is the major event in each stage?
- M
- mitosis
- G1
- growth
- S
- DNA synthesis
- G2
- “gap” before mitosis
Why is the G1 phase important?
cells must increase in volume, othewise each division leads to smaller and smaller cells
What at the 3 major checkpoints & what factors are they trying to assess?
- Enter mitosis (G2 into M)
- Is all DNA replicated?
- Is all DNA damage repaired?
- Pull duplicated chromosomes apart (M)
- Are all chromosomes properly attachd to the mitotic spindle?
- Enter S phase (G1 into S)
- Is the environment favorable?
The shift from one phase into another is dependent on what proteins?
They do this through what mechanism?
cyclins
bind to cyclin-dependent kinases (CDK) and activate them, so they can phosphorylate other proteins that perform the functions of that phase
How does the cyclin concentration change throughout the cell cycles?
concentration increases steadily until it reaches a maximum, but does not activate until the right time
once transitioned to the next phase, the concentration drops rapidly for that particular cyclin
Describe the process by which cyclins activate CDKs.
cyclins bind the appropriate partner CDK, which is phosphorylated at the time of binding – forming an inactive complex
The phosphatase Cdc25 removes the phosphate to activate the complex
CDK activation occurs via what type of feedback loop? How is this helpful?
Descrbribe this process.
Positive feedback loop - allows for rapid activation
Cdc25 (phosphatase) must be phosphorylated to be active
It is phosphorylated by a CDK
so, as Cdc25 proteins are phosphorylated, they remove phosphates from moe CDK molecules, which in turn phosphorylates more Cdc25 molecules
What is the name of the molecule responsible for de-phosphorylating the cyclin/CDK complexes?
Cdc25
What is the name of the molecule that add the phosphate to the CDK molecules before they bind to cyclin?
Wee1
In addition to the phosphorylation directed regulation of cyclin-CDK complexes, how else is the entrance to a new phase controlled?
cyclin-dependent kinase inhibitors (CKI)
bind to cyclin-CDK complexes & preven their function – helps to hold cells in a certain stage while concentration of cyclins increases
What are the 3 CKIs?
p27
p21
p16
The rapid loss of cyclins at the end of a stage is due to what process?
protein degradation initiated by the addition of ubiquitin
this directs them to the proteasome for digestion
What molecule is activated in late S-phase to degrade M and S cyclins? How does it function?
Anaphase-Promoting Complex (APC)
adds ubiquiton to M and S phase cyclins, which directs them to a proteosome for degradation
How does the cell block DNA synthesis until it is repaired?
Proteins recognize DNA damage and activate a kinase that acts on p53
phosphorylated p53 (active) binds to the promoter for the gene coding p21 & increases transcription
p21 binds to cyclin-CDK complexes as a CKI & blocks S-phase cyclin
Thus, DNA synthesis does not begin until it is repaired & the inhibition is relieved
What are the 2 major proteins involved with condensing DNA strands so they are able to separate within the dividing cell?
condensin and cohesin
How are the chromosomes arranged in preparation of division?
Loops of DNA are gathered by condensin rings (shortening the length of the chromosome)
The two copies of DNA are attached (1) a the kinetochore and (2) along their length by choesin rings
What is the kinetochore?
It is important in what checkpoint?
protein complex at the centromere
they attach to the spindle (microtubules)
Spindle microtubules attach to both sides of kinetochore & unless both sides are attached to opposing poles of the cell, mitosis will not proceed
What is the name of the short spindle fibers produced once they reach opposite poles?
aster
How do microtubules adjust their length as they attache to the opposite poles of the cell & the kinetochores?
When do the spindles reach their final length?
polymerization makes them longer
depolymerization make them shorter
The spindles reach their final length when the chromosomes align on the metaphase plate
What are interpolar microtubules?
the spindle fibers grow past the chromosomes & associate with the microtubules from the opposite pole
they will provide the motive force for the chromosomes
motor proteins attach to these & move them relative to each other
Describe the steps of removing the cohesin rings.
- addition of ubiquitin by the APC to a protein called securin
- securin is in a complex with separase
- Degradation of securin allows separase to act on cohesin to degrade it
- Releasing the sister chromatids
What is happening with centrsomes in each stage of the cell cycle?
- G1: centrosomes with tubules
- S - G2: replicates
- M: move apart with short microtubule (aster)
- some spindles lengthen & attach to centromere
What is the region of the chromosome where the spindle attaches?
centromere
kinetochore is the protein complex where attachement occurs
Why is movement along the spindle during chromosome separation rapid?
lots of tension in mitotic spindle during metaphase
What are the motor proteins in involved in chromosome separation & what are their individual functions?
- interpolar microtubules push poles apart using kinesin
- dynein cortex pulls aster microtubles and poles away from center
Describe how the chromosomes move toward the periphery of the cell during anaphase.
- Those attached to kinetochores become shorter, bringing the chromosomes closer to the centrosome
- Interpolar tubules push agains each other using kinesin
- In the aster, dyenin attaches to the cortex & pulls the aster and spindle close to the cell membrane
What are the two divisions that happen in meiosis?
-
Reduction Division
- after replication the centromeres remain intact & one of each homologous pair moves to opposite poles
-
Equational Division
- no replication has taken place and the centromeres divide to allow the sister chromatids to separate to different cells
What stage do the sex chromosomes separate in spermatogenesis?
during the reduction division
When does recombination occur & how does it begin?
It starts to occur when the homologous chromosomes line up in Prophase I
Its begins with a double-stranded break in one chromosome & the invasion of the paired DNA by the broken strand
This induces a new break in the opposite member of the pair & ligation of the different strands
What is a Holiday Structure?
An X configuration of a homologous pair of chromosomes that occurs durign recombination
All 4 strands are attached and the DNA must be cut and resealed at the junction to separate them into separate chromosomes again
It can up up the X or across the X, then reseal to linear forms
How is the role of cohesin different in meiosis as compared to mitosis?
-
cohesins bind before replication to bring the homologous pairs together before replication
- they are _not degrade_d in the reduction division so that each homologue will be a unit sent to a pole intact
- Spindles tubules sense tension from cohesin rings rather than kinetochore complex when aligning pairs
What protein complex is responsible for resolving the Holliday junctions?
Synaptonemal Complex at sites of recombination
They surround the homologus pairs & slide along them during anaphase
They are removed by slipping off the ends of the chromosomes
What are the stages in which the oocytes spend prolonged periods of time suspended?
- Prophase I: fetus to when follicle begins to grow
- Metaphase II: from ovulation to fertilization
When does meiosis begin in males & how long does it take to develop a mature spermatozoa?
any time from puberty onward for a cohort of cells
takes ~ 65 days
