Cell cycle Flashcards
normal cell replication depends on…
- Mitogens (growth factors)
- anchors (must be anchored to ECM)
- Contact w/ other cells (contact-inhibited, won’t divide if too crowded)
- # of previous divisions (cells = mortal!)
2 basic functions of cell cycle
- make 2 copies of all genetic info (DNA duplication)
2. get entire genome to each daughter cell (chromosomal segregation)
S phase of cell cycle
replication of DNA/chromosomes
Long time (almost half of whole cell cycle), *part of interphase
M phase of cell cycle
mitosis (nucleus divides) and cytokinesis (cell contents separate)
- lasts ~ 1 hour out of 24 hr cycle!
“Restriction Point”
the checkpoint at end of G1 phase.
where cell determines whether has enough GF and size to replicate
(if not, stays in phase –> “Go”)
Major cell cycle checkpoints
(4)
- restriction point (end of G1): to divide or not to divide?
- S phase checkpoint: is DNA is damaged, and not fixable?
- G2/M checkpoint: is replication completed?
- Spindle assembly checkpoint (M): are chromatids properly arranged on spindles?
Cyclins (types)
Cdk-activating proteins, no separate activity.
4 types:
(needed to pass through checkpoints in each phase of cell cycle)
A = S-cyclins, B = M-cyclins, D = G1-cyclins, E = G1S cyclins
regulation of Cdk activity
- degrade the cyclin (or the Cdk in some cases)
- (de)phosphorylate
- Cdk-interfering proteins (CKIs, CIPs, INKs)
- transcriptional regulation (of cyclins, CKI, etc.)
Activation of M-Cdk
- Mitotic cyclin and CDK bind/complex
- Wee-1 – adds P to complex
- CAK – adds 2nd P to complex
- Cdc25 activates the complex (removes 1 P, opens substrate binding site)
CKI
Inhibitory molec for Cdks, binds to Cdk-cyclin complex;
wraps around so cannot f(x),
(regardless of what other molecs = present)
- counteracted by SCF proteosome
cyclin D-Cdk
needed by ALL cells, EXCEPT for Embryonic Stem Cells (“ES”),
to pass restriction point.
–> concern: stem cells = naturally tumorigenic (!)
needed for cell cycle “clock”
initiation and progression
outside signals:
- Tyrosine Kinase Rs
- GPCRs
- TGF-B Rs
- NRs AND: nutrient status (will not divide if cell = starving)
G1 checkpoint decision (“regulation point”)
- -> divide (onto S phase) or not divide (to Go phase)
1. external mitogenic signals –> “we need more cells”
2. DNA damage??
3. cell has grown enough?
Mitogens that stimulate D-Cyclins (–> cell proliferation)
- Growth factor (via Ras or HERneu…)
- Wnt pathway
- cytkines
- NRs
- Hedgehog
SCF ubiquitin ligase
cell cycle reg. enzyme –> marks CKI for degradation
- SCF indirectly promotes cell cycle progression **
1. phosphorylates CKI
2. CKI = poly-ubiquitinated (by ubiquitin)
- SCF indirectly promotes cell cycle progression **
- -> CKI = degraded
TGF-Beta as tumor suppressor
pathways to inhibiting cell prolif.
3 ways to inhibit cell proliferation: (at restriction point)
- increase expression of CKIs
- Block phosphorylation of Rb
- Prevent Myc expression
Rb protein
regulates cell cycle (inhibits progression) by inhibiting EF2
(EF2 = gene enhancer —> + cell prolif.)
* need >1 phosphorylations of Rb to release from EF2
(phosphorylated by Myc)
Contribution of Myc to cell cycle regulation
- increases phosphorylation of Rb (–> frees EF2)
2. promotes expression of EF2
what’s different in Embryonic stem cells for G1 checkpoint?
- Rb = hyperphosphorylated (–> INactive)
- no need for Mitogens
- no DNA damage check
- Cyclin E = constantly present (NOT degraded)
- –> can blow through checkpoint w/o actual check!
check against mitogen over-stimulation
- feedback from mitogen stimulation –> Arf (chaperone protein)
- Arf sequesters MDM2
*Arf = “14-3-3” (also = Raf inhibitor!)
Types of functional mutations in genes to cause cancer
- bypass need for mitogens
(act: RTKs, Ras, Cyclin D, PI-3K; INact: PTEN, p53, TFG-Beta) - targeting G1 checkpoints
(inhibit Rb, p53, CKIs; increase Myc/AP-1) - suppress apoptosis (act: PI-3K; INact: PTEN, p53)
Dominant vs. Recessive mutations
Dominant: only need mutation in 1 allele
Recessive: must have same mutation in both alleles to exhibit mutation in f(x)
Main ways to convert proto-oncogene to oncogene
- Deletion in DNA – coding gene OR regulatory sequence
- mut. transcr. machinery –> change gene expression
- Chromosomal rearrangement
- new reg. sequence
- fusion to active transcr. gene –> hyperexpression!
Most common causes of conversion of proto-oncogene to oncogene
- Val –> glut (missense mut): no need for mitogens!
- single deletion alters transmembrane domain of protein
- -> R dimerizes w/o ligand!
Philadelphia chromosome
translocation of tips of chromosomes 9 and 22
–> BCR-ABL fusion protein
(constitutively active tyrosine kinase)
** causes CML (chronic myelogenous leukemia) if in bone marrow
Imatinib/gleevac
a drug used to treat CML (chronic myelogenous leukemia),
- specifically targets cancer cells
- -> no side effects!
ways to have normal allele eliminated in heterozygote,
resulting in oncogene expression
- usually = LOH **
1. non-disjunction 4. chromosome loss and duplication
2. Mitotic recombination 5. gene conversion
3. deletion 6. point mutation
- usually = LOH **
genetic characteristics of oncogenes
Dominant mutations,
gain of function,
* in somatic cells – NOT inherited (fatal in utero)
less tissue preference.
genetic characteristics of tumor suppressor genes
Recessive mutations (BOTH alleles must be affected),
loss of function,
in germ cell OR somatic cells —> CAN be inherited
*strong tissue preference
why not more cancer w/ all cell divisions over lifetime?
- need 2+ genetic events for cancer (1 mut. is not enough)
2. usually* arise from 1 cell w/ multiple mutations (clonal in origin)
Multi-Hit model of cancer induction
ALL cells in a tumor should have at least some COMMON genetic alterations.
– evidence: 1. cells from female tumors all have same X-chrom. inactivated; 2. cancer increases w/ age; 3. hard to induce cancer in animals w/ single oncogene; 3. successive muts in pathways
Rb regulates which checkpoint type?
Rb:
regulates “Regulatory Point” (checkpoint)
p53 regulates which checkpoint type?
p53:
regulates DNA Damage checkpoints
(aka: “guardian of the genome”)
dominant negative mutation
a genetic mutation in a single allele which inhibits the function of the other (wild type) allele
- -> acts as dominant mutation.
ie: Li-Fraumeni syndrome (affects p53 function)
meiosis
1 round of DNA synthesis (replication),
2 successive rounds of cell division
(w/ chromosomal segregation)
major gametogenesis trigger
RA (retinoic acid),
binds to nuclear receptors.
responsible for regulating the timing of cells entering mitosis
Splitting cell organelles in mitosis
- Key: want to split evenly, helped by cytoskeleton.
- Mitochondria: double number, (so ubiquitous that this is enough)
- All others (Golgi, ER): fragment into small pieces then regrow
(Increases chance to split evenly btwn new cells)
Cdk (cyclin dependent kinases)
Category of enzymes, for cell cycle regulation.
Activated by binding cyclins,
Active: phosphorylates cell cycle proteins
G1 cyclins
w/ Cdk –> regulate activity of G1S cyclins;
- activated by mitogens
D cyclin
G1S cyclins
In late G1, bind to Cdks to help pass restriction point;
(Promote commitment into cell cycle)
E cyclin
S cyclins
Right after restriction point,
Bind to Cdks to stimulate chromosome duplication.
(Control early Mitotic events)
A cyclins
M cyclins
Bind to Cdks,
–> stimulate entry into mitosis (M phase)
B cyclins
