Mitosis & Meiosis

Question 1

What are the 4 stages of the cell cycle & what is the major event in each stage?

Answer 1

1. M
   
   • mitosis
2. G₁
   
   • growth
3. S
   
   • DNA synthesis
4. G₂
   
   • “gap” before mitosis

Question 2

Why is the G₁ phase important?

Answer 2

cells must increase in volume, othewise each division leads to smaller and smaller cells

Question 3

What at the 3 major checkpoints & what factors are they trying to assess?

Answer 3

1. Enter mitosis (G₂ into M)
   
   • Is all DNA replicated?
   • Is all DNA damage repaired?
2. Pull duplicated chromosomes apart (M)
   
   • Are all chromosomes properly attachd to the mitotic spindle?
3. Enter S phase (G₁ into S)
   
   • Is the environment favorable?

Question 4

The shift from one phase into another is dependent on what proteins?

They do this through what mechanism?

Answer 4

cyclins

bind to cyclin-dependent kinases (CDK) and activate them, so they can phosphorylate other proteins that perform the functions of that phase

Question 5

How does the cyclin concentration change throughout the cell cycles?

Answer 5

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

Question 6

Describe the process by which cyclins activate CDKs.

Answer 6

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

Question 7

CDK activation occurs via what type of feedback loop? How is this helpful?

Descrbribe this process.

Answer 7

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

Question 8

What is the name of the molecule responsible for de-phosphorylating the cyclin/CDK complexes?

Answer 8

Cdc25

Question 9

What is the name of the molecule that add the phosphate to the CDK molecules before they bind to cyclin?

Answer 9

Wee1

Question 10

In addition to the phosphorylation directed regulation of cyclin-CDK complexes, how else is the entrance to a new phase controlled?

Answer 10

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

Question 11

What are the 3 CKIs?

Answer 11

p27

p21

p16

Question 12

The rapid loss of cyclins at the end of a stage is due to what process?

Answer 12

protein degradation initiated by the addition of ubiquitin

this directs them to the proteasome for digestion

Question 13

What molecule is activated in late S-phase to degrade M and S cyclins? How does it function?

Answer 13

Anaphase-Promoting Complex (APC)

adds ubiquiton to M and S phase cyclins, which directs them to a proteosome for degradation

Question 14

How does the cell block DNA synthesis until it is repaired?

Answer 14

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

Question 15

What are the 2 major proteins involved with condensing DNA strands so they are able to separate within the dividing cell?

Answer 15

condensin and cohesin

Question 16

How are the chromosomes arranged in preparation of division?

Answer 16

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

Question 17

What is the kinetochore?

It is important in what checkpoint?

Answer 17

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

Question 18

What is the name of the short spindle fibers produced once they reach opposite poles?

Answer 18

aster

Question 19

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?

Answer 19

polymerization makes them longer

depolymerization make them shorter

The spindles reach their final length when the chromosomes align on the metaphase plate

Question 20

What are interpolar microtubules?

Answer 20

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

Question 21

Describe the steps of removing the cohesin rings.

Answer 21

1. addition of ubiquitin by the APC to a protein called securin
   
   • securin is in a complex with separase
2. Degradation of securin allows separase to act on cohesin to degrade it
3. Releasing the sister chromatids

Question 22

What is happening with centrsomes in each stage of the cell cycle?

Answer 22

• G₁: centrosomes with tubules
• S - G₂: replicates
• M: move apart with short microtubule (aster)
  
  • some spindles lengthen & attach to centromere

Question 23

What is the region of the chromosome where the spindle attaches?

Answer 23

centromere

kinetochore is the protein complex where attachement occurs

Question 24

Why is movement along the spindle during chromosome separation rapid?

Answer 24

lots of tension in mitotic spindle during metaphase

Question 25

What are the motor proteins in involved in chromosome separation & what are their individual functions?

Answer 25

• interpolar microtubules push poles apart using kinesin
• dynein cortex pulls aster microtubles and poles away from center

Question 26

Describe how the chromosomes move toward the periphery of the cell during anaphase.

Answer 26

• 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

Question 27

What are the two divisions that happen in meiosis?

Answer 27

• 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

Question 28

What stage do the sex chromosomes separate in spermatogenesis?

Answer 28

during the reduction division

Question 29

When does recombination occur & how does it begin?

Answer 29

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

Question 30

What is a Holiday Structure?

Answer 30

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

Question 31

How is the role of cohesin different in meiosis as compared to mitosis?

Answer 31

• 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

Question 32

What protein complex is responsible for resolving the Holliday junctions?

Answer 32

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

Question 33

What are the stages in which the oocytes spend prolonged periods of time suspended?

Answer 33

• Prophase I: fetus to when follicle begins to grow
• Metaphase II: from ovulation to fertilization

Question 34

When does meiosis begin in males & how long does it take to develop a mature spermatozoa?

Answer 34

any time from puberty onward for a cohort of cells

takes ~ 65 days