Unit 12

Question 1

G1 checkpoint

Answer 1

• Regulates transition from G1-to-S phase
• Monitors environment before allowing cell cycle to progress

Question 2

G2 checkpoint

Answer 2

• Regulates transition from G2-to-M phase
• Monitors DNA for proper replication & environment

Question 3

M-phase checkpoint

Answer 3

• Spindle assembly checkpoint (SAC)
• Regulates transition between metaphase & anaphase
• Monitors chromosomes for proper attachment at metaphase plate

Question 4

Mitogens

Answer 4

• Stimulate cell to initiate cell division
• Sets off signal transduction pathway letting cell know to start process of growth & division

Question 5

Rb

Answer 5

• Suppress expression of genes whose proteins are needed for S phase
• Rb-dependent suppression of gene expression must be relieved for cells to commit to entering S phase & rest of cell cycle
• Once these proteins are synthesised the cell is committed to progressing to S phase & growth factors no longer needed

Question 6

ATM & ATR

Answer 6

• Activated by DNA damage
• Phosphorylate downstream kinases that phosphorylate p53 & stabilise it

Question 7

p53

Answer 7

• Phosphorylated by ATM & ATR
• When phosphorylated, can induce expression of proteins p21 & Puma
• Inactivated by many different types of cancer

Question 8

p21

Answer 8

• Expression induced by phosphorylated p53
• Binds to & inhibits complexes of cyclin-Cdk. In G1 this prevents inactivation of Rb & the cell remains in G1

Question 9

Puma

Answer 9

• Expression induced by phosphorylated p53
• Inhibits Bcl2: A key protein required for inhibition of cell death. This prolonged & irreperable DNA damage induces cell death

Question 10

Geminin

Answer 10

• Ensures cell only undergoes one cycle of DNA replication
• Expression of Geminin begins at S phase
• Binds to Cdt1 & prevents it from forming new pre-replication complexes
• Geminin degraded during anaphase, allowing Cdt1 to participate in another round of DNA replication

Question 11

Cdt1 & MCM

Answer 11

• Required for DNA replication
• Help form pre-replication complex

Question 12

Cdc20

Answer 12

• Associates with anaphase promoting complex / cyclosome (APC / C) to form functional ubiquitin ligase

Question 13

APC / C

Answer 13

• Anaphase promoting complex / cyclosome
• Associates with Cdc20 to form functional ubiquitin ligase
• One target is cyclin B, deactivating it

Question 14

Ubiquitin ligase

Answer 14

• Adds small protein (76 amino acids) called ubiquitin to proteins & targets them for proteolysis (degradation)

Question 15

Securin

Answer 15

• Target of APC / C, degrading it
• Binds to separase enzyme that degrades cohesin
• When bound, separase is inactive & sister chromatids remain attached

Question 16

Separase

Answer 16

• Binds to securin
• When bound, separase is inactive & sister chromatids remain attached

Question 17

Mad2

Answer 17

• Can exist in either closed or open form
• In closed form it can bind to & inhibit Cdc20
• Converts into closed form by associating with kinetochore-bound Mad1
• Even 1 unattached kinetochore sufficient to produce enough closed Mad2 to inhibit all cellular Cdc20 & prevent anaphase