Cell Cycle

Question 1

Basic Goals of Cell Division

Answer 1

• approximate doubling in volume of content
• exact duplication of genome
• exact segregation of duplicated genomes
• division into two new cells, each with approx equal volumes/content and one complete genome copy

Question 2

Phases of cell cycle

Answer 2

G1, S, G2, M

Question 3

G1

Answer 3

gap before DNA synthesis (2N DNA/cell)

Question 4

S

Answer 4

DNA synthesis (gradually increasing DNA/cell)

Question 5

G2

Answer 5

gap after DNA synthesis (4N DNA/cell)

Question 6

M

Answer 6

mitosis (4N DNA/cell until cytokinesis)

Question 7

Alternatives to cell division

Answer 7

polyploidy, multinucleation

Question 8

Regulation of Cell Cycle

Answer 8

Cyclin dependent kinases and checkpoints: Times when the cell monitors specific activities before proceeding to the next cell cycle stage.
Occur at:
G1/S-enter S or Start
G2/M- enter M
Metaphase/Anapahse transistion in M (Exit M)

Question 9

Cyclin-Dependent Kinases

Answer 9

regulators of cell cycle progression. consist of catalytic subunit and regulatory subunit. (CdK +cyclin) they are inactivated until phosphorylated when indicated.

• S-CDk once activated turns on DNA replication machinery
• M-CDk turns on mitosis when activated and when turned off completes mitosis (last checkpoint)

Question 10

G1

Answer 10

Typical “resting” phase
General biosynthesis (growth maintenance)
External signals and internal state combine to make decision to remain in G1 or divide (proceed to S)
External signals: receptor activation
Internal state: size, DNA integrity

Question 11

S Phase

Answer 11

• Histone synthesis
• Synthesis of enzymes required for DNA replication
• Replication of DNA from multiple Replication Origins
• Removal of replication licensing factors from ORC prevents re-replication until next cell cycle. CDC6 is on ORC (signalling it’s ready), becomes phosphorylated, comes off so replication can start, and isnt put back on until next cycle
• Duplication of centrioles (but centrosomes don’t duplicate until M phase)

Question 12

G2

Answer 12

• continued synthesis of proteins required for mitosis and cytokinesis, including the cyclin for the cdk that regulates entry into M phase.
• DNA repair checkpoint activity
• Addition of cohesins to link sister chromatids, Condensins are added in mitosis

Question 13

Mitosis-Prophase

Answer 13

chromosomes condense, spindle forms, kinetochore assemble.
astral MT add stability
Kinesin motors crosslink some MT and aid in elongation

Question 14

Mitosis-Prometaphase

Answer 14

• nuclear envelope breakdown (lamin phosphorylation)
• spindle invades nucleus
• kinetochores capture MT

Question 15

Nuclear Envelope Breakdown (NEB)

Answer 15

Phosphorylation of lamin IF subunits by mitoticcdk leads to breakdown, which is the start of prometaphase

Question 16

Kinectochores and MT

Answer 16

capture the MT and support bidirectional movement of chromosomes.

Question 17

Bipolar attachment

Answer 17

need attachment of chromosomes to spindle on both sides. results in tension on the spindle poles and allow chromosomes to line up at the metaphase plate. cannot continue to anaphase until biopolar attachment on all chromosomes
-balanced forces prevent collapse. kinetochores pull in and overlapping MT push out

Question 18

Metaphase/Anaphase Checkpoint

Answer 18

controlled by kinase activity. tension on kinetochores (by bipolar attachment) inhibits the kinase, turns on phosphatase, dephosphorylates mcdk, and allows anaphase

Question 19

Anaphase Promoting Complex

Answer 19

triggered by the phosphatase- destructs cyclin subunits (keeping in metaphase) and cohesions. chromatids can then separate since mcdk is off-trigger anaphase

Question 20

Metaphase

Answer 20

chromosomes are aligned at the equator of the spindle, kinetochore MT are attached

Question 21

Anaphase

Answer 21

• separation of sister chromatids

- two types of movement- chromosome to pole (kinetochore) and pole/pole separation (kinesins on MT).

Question 22

Telophase

Answer 22

reformation of the nucleus. phosphatase reverses the effects of mcdk during anaphase and telophase
spindle is disassembled, nucleus reassembles
prep for cytokinesis

Question 23

Cytokinesis

Answer 23

• contractile ring of antiparallel microfilaments associated with the cell membrane and non-muscle myosin II constricts the cell
• orientation of mitotic apparatus determines the position of the contractile ring (perpendicular)

Question 24

Mitosis and Cytokinesis

Answer 24

not always linked:
cells lose adhesion and roundup during mitosis and cytokinesis
can become: 
-binucleate
-polyploid
-undergo apoptosis

Question 25

Clinical correlation

Answer 25

drugs that disrupt MT dynamic instability (Vincristine and Taxol) block progression through mitosis

Prolonged block of mitosis in animal cells leads to cell death (apoptosis), so these drugs work as relatively non-selective anti-cancer agents