Cell Cycle

Question 1

What is the cell cycle?

Answer 1

Time from one division to the next division.

Question 2

What are the 4 phases of the cell cycle in order?

Answer 2

M phase: Includes mitosis and cytokinesis (takes about an hour in mammalian cells)

Interphase: Cell growth and metabolism (includes S and G 1 and 2 phases)

G1 phase: Cell growth and monitoring that falls between the end of cytokinesis and beginning of DNA synthesis

S phase: cell replicates DNA

G2 phase: Cell growth and monitoring that falls between the end of DNA synthesis and beginning of mitosis

Question 3

What are the three main transition points of the cell cycle control system?

Answer 3

1. G1 checkpoint: confirmation of environment favourable for proliferation before replicating DNA; regulated by outside signals
2. G2 checkpoint: confirmation that DNA is undamaged and fully replicated before undergoing mitosis
3. M checkpoint: confirmation that duplicated chromosomes are attached to mitotic spindle before segregation.

Question 4

What are the 3 categories of cells wrt cell cycle?

Answer 4

1. Cells that do not cycle, but enter a permanent arrest stage or G0. (e.g. neurons, muscle cells, RBC)
2. Cells that normally do not divide, but can be induced (E.g. liver cells)
3. Cells that divide regularly (e.g. epithelial cells)

Question 5

Why is controlling the cell cycle important?

Answer 5

Ensures sequential occurrence of DNA replication, mitosis, and cytokinesis by employing checkpoints at transition points.

Question 6

What is the maturation promoting factor composed of?

Answer 6

M cyclin and cyclin dependent kinase

Question 7

What are cyclins?

Answer 7

Non-enzymatic regulatory proteins whose concentration rises and falls at specific times to bind to Cdk’s and control progression of stages in the cell cycle.

Question 8

What are cyclin dependent kinases?

Answer 8

Enzyme that when complexed with a specific cyclin can trigger various events in in the cell division cycle by phosphorylating different proteins.

Question 9

What is M Cyclin?

Answer 9

Proteins with no enzymatic activity that bind to cell-cycle kinases to activate them. Cyclin levels rise and fall in a cyclical fashion throughout the cell cycle.

Question 10

What are the different types of cyclin?

Answer 10

M cyclin: triggers entry into M phase from G2 by forming M-Cdk complex

G1/S cyclin: form G1/S-Cdk complex and launches S phase

S cyclin: form S-Cdk to trigger DNA synthesis

G1 cyclin: form G1-Cdk to drive cell through first gap until S phase

Question 11

When do the different cyclins concentration peaks begin and end?

Answer 11

G1/S cyclin: begins in G1 and ends at S phase

S cyclin: begins in G1 and ends in M phase

M cyclin: begins in G2 and ends after M phase.

Question 12

What is APC/C?

Answer 12

aka “anaphase promoting complex or cyclosome”, complex tags M and S cyclins, as well as securins with a chain of ubiquitin.

Question 13

What does ubiquitin do?

Answer 13

Ubiquitin acts as a flag for proteasomes which degrade cyclins and leave the Cdk intact and inactivated, or degrade securin to activate separase to destroy cohesin linkages.

Question 14

What increases and decreases cyclin?

Answer 14

Increase: gradual via transcription of cyclin genes and synthesis of cyclin proteins

Decrease: Cyclin tagged with Ubiquitin by APC/C which makes it a target for destruction via proteasome.

Question 15

What triggers the abrupt activation of cyclin Cdk complexes?

Answer 15

Inhibitory kinase (Wee 1) phosphorylate the complex which inhibits activation as it forms. Phosphatase (Cdc25) removes the phosphates which activates the complexes.

Question 16

What do activates Cyclin-Cdk’s do?

Answer 16

Catalyze phosphorylation and activation of target proteins, shuts down phosphatase that opposes its activity.

Question 17

What are the pauses in the cell cycle, and why are they caused?

Answer 17

G1 to S: Cdk inhibitors bind to cyclin-Cdk complexes to inactivate them and allow more time for the cell to grow.

G2 to M: Activation of M-Cdk inhibited by inhibiting phosphatases that activate the complex

Exit from mitosis: Inhibiting APC/C and preventing degradation of M cyclin to allow mitotic spindles to grasp chromosomes properly before segregating.

Question 18

What happens when DNA is damaged in G1?

Answer 18

Damage increases concentration and activity of p53 which activates genes encoding p21 (Cdk inhibitor). p21 binds to G1/S-Cdk and S-Cdk, preventing them from driving the cell into S phase to allow time to repair the DNA before replication.

Question 19

What occurs before M phase?

Answer 19

Cells increase in size, DNA of chromosomes is replicated, centrosome is duplicated, then M-Cdk is activated.

Question 20

What are the stages of the M phase?

Answer 20

1. Prophase: duplicated chromosomes condense, centrosome forms mitotic spindles
2. Prometaphse: nuclear envelope dissolves, centrosomes move to poles
3. Metaphase: chromosomes aline at midway plane
4. anaphase: kinetochore microtubules shorten and pull sister chromatids apart
5. telophase: contractile ring begins to form as nuclear envelope begins to reassemble
6. Cytokinesis: contractile ring creates cleavage furrow

Question 21

What is a centrosome?

Answer 21

Principle microtubule-organizing center near the nucleus of animal cells that duplicate to form the two poles of the mitotic spindle.

Question 22

What is an aster?

Answer 22

Star shaped array of microtubules from a centrosome or a pole in the mitotic spindle.

Question 23

What is dynamic instability? What happens to it in M phase?

Answer 23

Process in which individual microtubules alternate between polymerizing and depolymerizing with tubulin subunits. It rises in mitosis due to M-Cdk phosphorylation.

Question 24

What are the different types of microtubules?

Answer 24

Astral, kinetochore, non-kinetochore (interpolar)

Question 25

What are astral microtubules?

Answer 25

Radiate from centrosome to position mitotic apparatus and determine cleavage plane

Question 26

What are kinetochore microtubules?

Answer 26

Radiate from centrosome to attach to protein complex (kinetochore) on centromeres to pull chromosomes to different poles

Question 27

What are non-kinetochore (interpolar) microtubules?

Answer 27

Interdigitate with opposing pair to support framework and push centrosomes apart to opposite poles (via motor proteins)

Question 28

What are the two ends of microtubules?

Answer 28

Minus end: point toward centrosome and away from kinetochore

Plus end: point away from centrosome and towards the kinetochore; aka growing end

Question 29

What are Dynein and Kinesin?

Answer 29

Dynein: motor protein that puls astral microtubles to cell cortex

Kinesin: motor protein that cross-links non-kinetochore microtubules to push centrosomes apart

Question 30

How are chromosomes captured?

Answer 30

1. K-microtubules extent and retract to the centrosome in order to make contact with chromosomes until nuclear envelope breaks down
2. Nuclear envelope breaks down and spindles begin to make connections with chromosomes
3. After initial unstable attachments, they eventually capture the kinetochore
4. chromosomes are tugged back and forth until they become equidistant from the poles, forming the metaphase plate.

Question 31

What are cohesins? How are they broken?

Answer 31

Protein complexes that hold together chromosomes.

They are broken by the protease called separase when its inhibitor securin is tagged and degraded by APC/C.

Question 32

What is the difference between anaphase A and anaphase B?

Answer 32

Anaphase A: Initial poleward movement of chromosomes via shortening of kinetochore microtubules.

Anaphase B: Separation of spindle poles themselves

Question 33

What is the spindle assembly checkpoint?

Answer 33

kinetochores of unattached chromosomes send a stop signal to the cell-cycle control system which blocks activation of APC/C which keeps chromosomes attached via cohesins but delays mitosis by keeping Cdk’s active

Question 34

What is the result of phosphorylating/dephosphorylating nuclear pore proteins and lamins?

Answer 34

Phosphorylation: triggers disassembly of nuclear envelope at prometaphase

Dephosphorylation: assembles nuclear envelope in telophase and completes mitosis.

Question 35

What is the contractile ring in animals and plants?

Answer 35

Animal: contractile ring formed by actin and myosin pinches the plasma membrane during anaphase.

Plant: New cell wall assembles inside equator of old spindle

Question 36

what is the difference between mitosis and meiosis?

Answer 36

Mitosis: duplicates all of its chromosomes and produces genetically identical diploid cells to produce non-gamete cells.

Meiosis: 2 sequential cell divisions without a phase of DNA replication to produce gametes.