Final: Cell Cycle

Question 1

When using microscope to determine mitosis vs interphase without dyes, what are the shapes?

Answer 1

Mitotic cells: round and detached from surrounding cells and bottom of dish
Interphase: more attached and flat

Question 2

G1 phase

Answer 2

growing cells, must clear G0 checkpoint to continue

Question 3

S phase

Answer 3

DNA synthesis, DNA doubles and cells have 2 copies of the genetic code (NOT bundled)
BrdU used to identify S cells
sister chromatid linkage by cohesins

Question 4

G2 phase

Answer 4

centrosomes organize all the stuff that’s about to be divided
centrosome duplication ends here (begins in S phase)

Question 5

M phase

Answer 5

Mitosis

Question 6

Prophase

Answer 6

DNA forms chromosomes by condensing together and forming chromatids (held together by centromeres) and mitotic spindle starts to form and centromeres migrate to opposite sides of the cell
centrosomes migrate to opposite sides of the nucleus
chromosome condensation by condensins begins in prophase (cohesin complexes activated by M-CDK phosphorylation and condensins compact the chromosomes making them short and thick for segregation)
M-CDK activity modifies MT associated proteins to drive the shift in MT dynamics

Question 7

Metaphase

Answer 7

chromosome catching (MTs growing and shrinking at the plus end to catch a chromosome) and cell lines up at the metaphase plate

Question 8

Anaphase

Answer 8

chromosomes pulled to opposite ends of the cell

Question 9

Telophase

Answer 9

nucleus splits

Question 10

What does a flow cytometer measure? What if cdc28 or cdc7 is inhibted?

Answer 10

DNA content per cell
cell cycle arrest causing accumulation in a specific phase (likely G1 for cdc28 and G2/M for cdc7)

Question 11

Cyclin levels ___ during the cell cycle while CDK levels _____

Answer 11

rise and fall
stay constant

Question 12

How do CDKs become active?

Answer 12

bind to cyclins when there are high cyclin levels

Question 13

How do CDKs become inactive?

Answer 13

when cyclin is degraded by proteasome destruction

Question 14

What does cyclin activating kinase (CAK) do?

Answer 14

phosphorylates a site on the CDK which fully opens the active site
CAK pushes CDK to its most active form

Question 15

What do CDK inhibitors (CKIs) do?

Answer 15

bind to CDK/cyclin complex and block activity by wrapping around the complex and blocking the kinase site

Question 16

What do CKI regulators do?

Answer 16

Ubiquitin ligases target CKI’s for degradation
transcription factors turn on CKI gene expression
phosphorylate CKIs

Question 17

What is Wee1 and what does it do?

Answer 17

Kinase
Phosphorylates CDK at inhibitory site and it inhibits CDK activity even if it’s bound to cyclin
acts as a brake preventing premature entry into mitosis

Question 18

What is Cdc25 and what does it do?

Answer 18

Phosphatase
Removes the inhibitory phosphate that was put on by Wee1 and activates the CDK/cyclin complex
removes the brake that Wee1 put on

Question 19

What happens if a mutant doesn’t have Wee1 or Cdc25?

Answer 19

The cell would be abnormally small because Wee1 delays mitosis so it grows before division
No Cdc25 means inhibitory phosphorylation can’t be removed (which doesn’t really matter because Wee1 isn’t working anyway)
CDK activation will occur as soon as there’s enough cyclin so the cells will enter mitosis early

Question 20

What happens if you miss the metaphase checkpoint?

Answer 20

mis-segregation of chromosomes causing aneuploidy

Question 21

What would cause the cell cycle to stop after G2?

Answer 21

if DNA can’t be properly passed on

Question 22

What would cause the cell cycle to stop after M?

Answer 22

if MTs can’t properly line up for chromosome segregation

Question 23

What is Rb and what does it do? What happens if it’s mutated or inactivated?

Answer 23

Rb is a tumor suppressor protein that regulates the G1/S checkpoint
it binds to and sequesters E2F to inhibit transition into S phase
If mutated or inactivated, E2F won’t be inhibited and cells can replicate uncontrollably

Question 24

What is p53 and what does it do?
What happens if it’s mutated or inactive?

Answer 24

p53 is a tumor suppressor that acts as a checkpoint responding to DNA damage
it activates p21 to halt the cycle in G1 and initiate repair or apoptosis
If mutated or inactive, DNA damage checkpoints are bypassed and cells accumulate mutations

Question 25

What is p21 and what does it do? What happens if the pathway is deregulated?

Answer 25

p21 is a CKI that prevents cell cycle progression by binding to cyclin/CDK complex it's activated by p53 in response to DNA damage If the pathway is deregulated there is unchecked cell division

Question 26

What happens to Wee1 activity in cancer?

Answer 26

Wee1 activity is suppressed in cancer which upregulates Cdc25 and causes premature mitosis

Question 27

What would happen to CDK without the positive feedback loop?

Answer 27

CDK activity would rise slowly instead of shooting up fast

Question 28

What key target of the G1/S Cdk allows the expression of S phase CDKs (and sets up a positive feedback loop for the transition)? How is the positive feedback loop created?

Answer 28

Rb (retinoblastoma protein) Rb binds and inhibits E2F in early G1 G1/S CDKs phosphorylate Rb causing Rb to release E2F Free E2F activates transcription of S phase cyclins Positive feedback loop created because more E2F activity leads to more cyclin E and A transcription which leads to more active CDKs which leads to more Rb phosphorylation which leads to even more E2F activation

Question 29

What is the function of MYC?

Answer 29

can induce apoptosis, can turn off Rb, can link external signals to cell cycle entry its dysregulation can contribute to development and progression of cancer

Question 30

What happens if MAPK is always activated?

Answer 30

Activated MAPK promotes transcription of early response genes like MYC which drive Cyclin D expression and push the cell pass G1 checkpoint and into proliferation if it's always active the cell will constantly make cyclin D and keep pushing into the cell even without external signals which leads to uncomfortable proliferation

Question 31

What is the main goal of the APC/C-Cdh1 complex?

Answer 31

keep CDK activity low so the cell doesn't accidently re enter S or M phase before it's ready

Question 32

What causes APC/C-Cdh1 activity to be shut off and what happens once it's shut off?

Answer 32

cyclin E-CDK2 phosphorylates Cdh1 which cannot bind to APC/C when it's phosphorylated, so the APC/C-Cdh1 complex activity is shut off When the complex's activity is shut off the cyclin levels can rise and CDK activities build up allowing the cell to enter S phase

Question 33

Why is the timing of DNA replication linked to CDK activity?

Answer 33

CDKs control the transition from one phase to the next

Question 34

What are origins of replication? How does this relate to G1 and S phase?

Answer 34

Origins of replication are specific DNA sequences where the process of DNA replication begins (starting points for DNA synthesis to ensure the entire genome is accurately duplicated) when DNA replicates it starts at points along the chromosome and origins of replication sit on DNA and helicase helps unwind DNA G1 phase has low CDK activity which prevents origin firing but as it progresses to S phase CDK activity rises which leads to the activation of replication

Question 35

Prometaphase

Answer 35

kinetochore assembly; assemble at each centromere of the condensed sister chromatids nuclear envelope breakdown; M-CDK phosphorylates nuclear lamins causing the nuclear envelope to disassemble which allows spindle MTs to access chromosomes

Question 36

What happens to cohesins after S phase? Before mitosis?

Answer 36

After S phase cohesins are loaded along the entire length of sister chromatids (arms to centromere) Before mitosis cohesins along arms are removed by PIK1 and aurora B kinase activity

Question 37

What protects cohesins at centromeres from being removed like those on the arms? How? What does this result in?

Answer 37

Shugoshin: recruits PP2A to prevent cohesin phosphorylation at centromeres This results in free arms but sister chromatids are tightly linked at centromeres

Question 38

Why do we need free arms but linked centromeres?

Answer 38

Free arms allow separation of sister chromatids without entanglement which ensures each daughter cell gets one copy of each chromosome centromeric linkage prevents premature separation of sister chromatids before the spindle fibers are properly attached