Cell Cycle

Question 1

Cell cycle stages

Answer 1

1. Chromosome
   duplication is in S
   phase (DNA
   synthesis phase)
2. Chromosome
   segregation + cell
   division occur during
   M phase (Mitosis)
3. Cytokinesis – cell
   division

Question 2

Mitosis

Answer 2

This phase is characterized by nuclear
division (mitosis) at the beginning and by
cell division (cytokinesis) at the end.

Question 3

Interphase

Answer 3

• Gap 1 (G1) phase
– RNA and protein synthesis needed for
DNA replication
• DNA synthesis (S) phase
• Gap 2 (G2) phase
– DNA stability is checked

Question 4

G1 checkpoint

Answer 4

Correct any DNA damage before
continuing

Question 5

G2 checkpoint

Answer 5

Verify completeness of complete genomic duplication

Question 6

Metaphase checkpoint

Answer 6

Ensures chromosomes are attached to mitotic spindle

Question 7

Activation of cell cycle

Answer 7

• Myc activation

• Active G1 CDK
– CDK = Cyclin
dependent kinase

• Inhibit Rb
– Phosphorylation

• Release E2F
– Further gene
transcription
potentiates Rb
inactivation

Pushed into S phase.

Question 8

Rb and cyclin activity

Answer 8

• Hypo- and hyperphosphorylated RB modulates the passage of cells
from the G1 to the S phase
• Initiation of S phase protein synthesis requires the E2F transcription
factors.

– Cyclin E and Cyclin A
• Both activate CDK 2 – keep Rb inactive/E2F active

Hyperphosphorylation of RB by successive cyclin–CDK complexes
releases the E2F factors to allow transition of cells past the G1
checkpoint.

Question 9

Binding of cyclins to cyclin-dependent kinases (CDKs) causes partial
activation of their kinase activity

Answer 9

– Full activation requires the action of the CDK-activating kinase (CAK).

Question 10

The CIP/KIP family of CDK inhibitors (CKIs), such as p27

Answer 10

binds to the
cyclin–CDK complex to inactivate the kinase activity of CDK.

Question 11

Without cyclin bound (inactive state), the active site of Cdk is
blocked by a region of the protein called the T loop

Answer 11

– The binding of cyclin causes T-loop to move out of active site
(Cdk partly active)
– Phosphorylation of Cdk at T-loop fully activates enzyme (“cave
site”)
– Phosphorylation of Cdk caused by CAK (Cdk activating kinase)

Question 12

What cyclins at the beginning of G1?

Answer 12

Cyclin D complexes with CDKs 4 and 6.

When the cells transit the restriction point (R) and enter the S phase, cyclin D is rapidly
degraded.

Question 13

What cyclins at G1–S phase transition?

Answer 13

The cyclin E–CDK2 complex is active.

Question 14

What Cyclins are active during the S phase to induce the enzymes necessary for DNA
synthesis.

Answer 14

Cyclin A–CDK2.

Question 15

Which Cyclins initiate Mitosis?

Answer 15

Cyclin A–CDK1 and Cyclin B–CDK1.

Question 16

What are CIP/KIP family of (CKIs)?

Answer 16

Bind to G1 and S phase cyclin–CDK complexes to inactivate the
kinase activity of CDK.
– Eventually degraded

Question 17

What are INK4?

Answer 17

Family of CKIs bind specifically to G1 CDKs
– prevents them from associating with cyclin D.

Question 18

Cyclin-CDK activity

Question 19

What Cyclins belong to what Phase?

Answer 19

Question 20

What Cyclins belong to What stage?

Answer 20

Question 21

What phosphorylates and dephosphorylates the “roof site” of CDK to inhibit and activate them?

Answer 21

Wee1 (Inhibit) and Cdc25 phosphatase (Activate)

Question 22

What is a Cdk Inhibitory
Protein?

Answer 22

• A CKI (e.g. p27)
binds to both Cdk
and cyclin to
inactivate

• Primarily used for
control of G1/SCdks
+ S-Cdks
early in cell cycle

Question 23

Progression through metaphase to anaphase is triggered by protein destruction.

What ubiquitin ligase family of enzymes degrades CDKs?

Answer 23

Key regulator is APC/C: anaphase-promoting complex or
cyclosome

Question 24

What cyclins are major targets of APC/C and protein degradation?

Answer 24

•S-cyclins and M-cyclins are a
major target of APC/C
•Inactive APC/C activated by
binding to Cdc20
•Leads to addition of
polyubiquitin to M-cyclin in MCdk
complex
•Cyclins destroyed
•Inactivates most Cdks
•Cdks dephosphorylated
•Moving into anaphase
–must get rid of S-cyclin and Mcyclin

Question 25

What is p53 and it’s role in the cell cycle?

Answer 25

• DNA damage (chemical or physical agents such as X-rays) activates
protein kinases
– phosphorylate and stabilize the p53 protein.
• Activation of p53 leads to increased transcription of p21, a CKI.
• Binding and inactivation of cyclin–CDK complexes by p21 causes cell
cycle arrest.
• MDM2 keeps p53 inactive

Question 26

Two Apoptosis pathways

Answer 26

•Intrinsic pathway and extrinsic
pathway depend on factors that
induce apoptosis

•Internal stimuli: e.g. abnormalities in
DNA

•External stimuli: e.g. removal of
survival factors and proteins of tumor
necrosis factor family

Question 27

–BAX/BCL-2 key regulators

Answer 27

•Intrinsic pathway is mitochondrial
dependent

–In response to injury, DNA
damage and lack of oxygen, nutrients,
or extracellular survival signals
(mitochondrial dysfunction)

Question 28

What is caspases and how do they work?

Answer 28

• Apoptosis is an intracellular
proteolytic cascade
– mediated by proteases called
caspases
– Activation of caspases is key
event in apoptosis

• Caspases synthesized first as
an inactive precursor –
procaspase

– Becomes activated by
protease cleavage

• Procaspases cleaved at
specific sites

– form a large and small subunit
which form a heterodimer

Question 29

What are the 2 major classes of Caspases?

Answer 29

• Initiator caspases:
– initiates apoptosis
– includes caspase-8
and caspase-9

• Executioner
caspases:
– destroys actual
targets
– executes apoptosis

• includes caspase-3

Question 30

What is the Extrinsic Pathway for apoptosis?

Answer 30

Fas binds to Fas Death receptor – ligand form homotrimers as well
– Then adaptor proteins recruited: FADD adaptor (Fas associated death
domain) & procaspase-8 with death effector domain
– Also form trimers – bring death domains together
– Activate caspase-8 or -10 (forms DISC: Death Inducing Signal
Complex)
– Activates downstream executioner caspases – caspase-3

Question 31

What is the Intrinsic Pathway?

Answer 31

• When cytochrome c is released from mitochondria, it binds to a
procaspase-activating adaptor protein called Apaf1 (apoptotic
protease activating factor-1)
• Apaf1 forms apoptosome which activates caspase-9
• Caspase-9 activates downstream executioner caspases – caspase-
3 (common to both pathways)

Question 32

BAX/BCL-2

Answer 32

BAX (BH123) protein in apoptosis (intrinsic
pathway)
– BAX proteins become activated, form aggregation in
mitochondrial outer membrane and induce release of
cytochrome c – then apoptosome formed by binding to
Apaf1

Question 33

What are Anti-apoptotic BCL proteins?

Answer 33

• Mainly located on cytosolic surface of outer mitochondrial membrane
• These proteins prevent apoptosis by binding to pro-apoptotic proteins
(e.g. BAX/BH123) and prevent aggregation into active form

Question 34

How does Retinoblastoma regulate Cell Cycle?

Answer 34

Hypophosphorylated version stops cell from progressing to G1/S phase.

Hyperphosphorylate version is inactivated and E2F goes on to make transcription genes and Cyclin E and A.

Question 35

What is a tumor suppressor?

Answer 35

RB mutations great
example of tumor
suppressors

– Genes that loose
functional control of
proliferation

Question 36

What is a proto-oncogene?

Answer 36

Proto-oncogenes,
which encode
proteins that promote
cell growth and
division, are
converted to
oncogenes via gain
of function mutations.

• Oncoproteins
– The gene product
that causes cell
proliferation

Question 37

What is Viral Oncogenesis?

Answer 37

• A transducing retrovirus infects
a host cell and integrates into
the host genome.
• During replication, the virus may
accidentally integrate a part of
the host DNA containing a
proto-oncogene into its
genome.
• The proto-oncogene may be
converted into an oncogene by
a subsequent mutation, thus
generating a species of
retrovirus capable of
transforming its host cells.
• Src correlation box