Cell Cycle Flashcards
Cell cycle stages
- Chromosome
duplication is in S
phase (DNA
synthesis phase) - Chromosome
segregation + cell
division occur during
M phase (Mitosis) - Cytokinesis – cell
division
Mitosis
This phase is characterized by nuclear division (mitosis) at the beginning and by cell division (cytokinesis) at the end.
Interphase
• Gap 1 (G1) phase
– RNA and protein synthesis needed for
DNA replication
• DNA synthesis (S) phase
• Gap 2 (G2) phase
– DNA stability is checked
G1 checkpoint
Correct any DNA damage before
continuing
G2 checkpoint
Verify completeness of complete genomic duplication
Metaphase checkpoint
Ensures chromosomes are attached to mitotic spindle
Activation of cell cycle
• Myc activation
• Active G1 CDK
– CDK = Cyclin
dependent kinase
• Inhibit Rb
– Phosphorylation
• Release E2F
– Further gene
transcription
potentiates Rb
inactivation
Pushed into S phase.
Rb and cyclin activity
• 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.
Binding of cyclins to cyclin-dependent kinases (CDKs) causes partial
activation of their kinase activity
– Full activation requires the action of the CDK-activating kinase (CAK).
The CIP/KIP family of CDK inhibitors (CKIs), such as p27
binds to the
cyclin–CDK complex to inactivate the kinase activity of CDK.
Without cyclin bound (inactive state), the active site of Cdk is
blocked by a region of the protein called the T loop
– 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)
What cyclins at the beginning of G1?
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.
What cyclins at G1–S phase transition?
The cyclin E–CDK2 complex is active.
What Cyclins are active during the S phase to induce the enzymes necessary for DNA
synthesis.
Cyclin A–CDK2.
Which Cyclins initiate Mitosis?
Cyclin A–CDK1 and Cyclin B–CDK1.
What are CIP/KIP family of (CKIs)?
Bind to G1 and S phase cyclin–CDK complexes to inactivate the
kinase activity of CDK.
– Eventually degraded
What are INK4?
Family of CKIs bind specifically to G1 CDKs
– prevents them from associating with cyclin D.
Cyclin-CDK activity
What Cyclins belong to what Phase?
What Cyclins belong to What stage?
What phosphorylates and dephosphorylates the “roof site” of CDK to inhibit and activate them?
Wee1 (Inhibit) and Cdc25 phosphatase (Activate)
What is a Cdk Inhibitory
Protein?
• 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
Progression through metaphase to anaphase is triggered by protein destruction.
What ubiquitin ligase family of enzymes degrades CDKs?
Key regulator is APC/C: anaphase-promoting complex or
cyclosome
What cyclins are major targets of APC/C and protein degradation?
•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
What is p53 and it’s role in the cell cycle?
• 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
Two Apoptosis pathways
•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
–BAX/BCL-2 key regulators
•Intrinsic pathway is mitochondrial
dependent
–In response to injury, DNA
damage and lack of oxygen, nutrients,
or extracellular survival signals
(mitochondrial dysfunction)
What is caspases and how do they work?
• 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
What are the 2 major classes of Caspases?
• Initiator caspases:
– initiates apoptosis
– includes caspase-8
and caspase-9
• Executioner
caspases:
– destroys actual
targets
– executes apoptosis
• includes caspase-3
What is the Extrinsic Pathway for apoptosis?
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
What is the Intrinsic Pathway?
• 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)
BAX/BCL-2
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
What are Anti-apoptotic BCL proteins?
• 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
How does Retinoblastoma regulate Cell Cycle?
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.
What is a tumor suppressor?
RB mutations great
example of tumor
suppressors
– Genes that loose
functional control of
proliferation
What is a proto-oncogene?
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
What is Viral Oncogenesis?
• 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
