Cell Cycle I Flashcards
Dogma of Life
Must find food
Must not be eaten
Must reproduce (cells come from cells)
Controlled by cell cycle
Cell Cycle
Cycle of duplication and division that produces mor/new cells
Orderly progression of events that results in one cell becoming 2
Events take place in a cell in a specific sequence leading to division (cytokinesis) and duplication (of chromosomes and organelles)
The cell cycle includes the moment ____ until ____.
The three major functional aspects of the cell cycle are:
The cell is born
The moment the cell becomes two
Cell growth/chromosome replication
Chromosome segregation
Cell division
Goal of Cell Cycle
Produce two genetically identical daughter cells
DNA faithfully replication into two copies
Precise replication of all base pairs in Diploid human genome
Challenges of Cell Cycle
Mistakes- about 6 mistakes per cell division
Replicated chromosomes must be accurately distributed in daughter cells (segregation)
Why is the cell cycle important?
Some cells need to be replaced constantly- intestinal cells only live 3-4 days
(Whereas liver cell cycle is one year)
Blood cells need constant replacement as well (120 days). Old and damaged cells are constantly being filled out by the spleen.
When the cell cycle malfunctions, what is the result?
Either too many or not enough cells produced.
Cancer- excess cell proliferation, cells divide uncontrollably. To understand cancer, must understand cell cycle.
How is the cell cycle controlled?
Regulatory proteins!
Make up the cell-cycle control system and governs progression. Biochemical “switches”.
Reg proteins initiate main events of cycle (duplication and segregation) by responding to signals from in or outside the cell.
What molecules coordinate the events of the cell cycle so that they occur at the appropriate time?
Proteins:
Prevent prep for segregation of chromosomes until DNA replication is complete. This control system regulates cell #s in a multi-cellular organism (eg- # RBCs)
What happens when cell cycle control malfunctions?
CANCER
What else must cells do during the cell cycle (besides replicate)?
Growth! Cells must also duplicate contents. This process is coordinated with division to maintain cell size.
Major Chromosomal Events in the Cell Cycle
- Chromosome duplication (S-phase/DNA synthesis phase)
- Chromosome segregation (M phase/Mitosis)
- Cytokinesis (cell division)
4 phases of cell division
- Prophase- chromosome condensation (into sister chromatids) and attach to mitotic spindles
- Metaphase- sister chromatids line up at equator of cell, attached to opposite poles of spindle
- Anaphase- sister chromatids become daughter chromosomes and are pulled to opp poles of spindle
- Telophase- spindle disassembles, chromosomes packaged into separate nuclei, cytokinesis occurs
Four Phases of Cell Cycle
G1, S, G2, M
Interphase=G1, S, G2
S Phase- synthesis of DNA
M Phase- separate chromosomes and divide cells
GAP phases- Growth
G1- between M and S
G2- between S and M
3 Major Transition Checkpoints of Cell Cycle
Start: G1 to S phase (cell commits to cell cycle and chromosome duplication)
G2 to M phase (chromosome alignment on spindle in metaphase)
Anaphase and cytokinesis (trigger sister chromatid separation and cytokinesis)
Immortalized Cell Lines:
Cell lines that grow forever, aren’t restricted to a limited number of cell divisions.
Murine Leukemia Cell’s (MEL)
USeful in studying RBCs cell dev and generation
HEL (Human erythroleukemia cell line)
Cell cycle control system
“timer” that triggers sequence of events but is arrest-able at checkpoints
Blocks progression through start if receive signals to do so (eg- unfavorable conditions)
Cyclin dependent kinases (Cdks)
Biochemical switches used in cell cycle control system
Turn ON various steps of cell cycle
Phosphorylation proteins downstream to activate them nd regulate cell cycle events
Activities of Cdks rise and fall, but levels remain the same.
Cyclins
REGULATE CDKS, ARE REQUIRED
Levels of cyclin vary during cell cycle.
Must bind to Cdks to initiate activity.
Also direct Cdks to specific target
What are the four classes of cyclins?
- G1/S cyclins
- S cyclins
- M cyclins
- G1 cyclins
G1/S cyclins
Start the cell cycle!
- activate Cdks in late G1
- helps trigger progression through start
- commitment made to cell cycle entry
- levels drop in S phase
S cyclins
Duplicate DNA
- bind Cdks after progression through start
- helps stimulate chromosome duplication
- S-cyclin levels remain high until mitosis
M-Cyclins
Mitosis
- activate Cdks that stimulate entry into mitosis at G2/M checkpoint
- M-cyclins removed at mid-mitosis
G1 cyclin
Govern activity of g1/S cyclins (control progression through start checkpoint)
What are the 4 Cdks in vertebrates?
G1/S-Cdk
S-Cdk
M-Cdk
G1-Cdk
Activities of ___ rise and fall during the cell cycle,
While the levels of ___ rise and fall.
Cdks
Cyclins
When cyclin is not bound to Cdk…
The active site is blocked by a region of protein (T loop) and the Cdk is inactive.
The binding of Cyclin to Cdk results in
Activation by moving the T loop out of the active site. Phosphorylation fully activates the enzyme.
What molecule causes the phosphorylation of Cdk?
CAK (Cdk activating Kinase)
Cdk Inhibition:
Cdks are primarily controlled by what?
However, what process ends up inhibiting complex activity?
What kinase does this?
What kind of molecule and what molecule can “undo” this process?
Rise and fall of cyclin levels
Different phosphorylation of Cdk at the roof site
Wee1 Kinase
Phosphatase, Cdc25
CKI Proteins
Causes cyclin-Cdk complex inhibition
Binds to both the Cdk and the cyclin to cause inactivation
Primarily used for the control of G1/S-Cdks + Cdks early in the cell cycle
Familial hereditary melanoma (skin cancer)
High incidence of melanoma in certain families
Genetic search: INK4A- Cdk inhibitor
-involved in G1 phase
Mutation causes loss of activity/loss of inhibition
Cells cannot control cell cycle and grow uncontrollably
P53
Tumor suppressor
Influences gene expression
P53 up regulates p21 (ie 21 is a target)
If p53 fails, there is less p21 expression and CKI cells divide uncontrollably (Cancer)
P21
A gene up-regulated (promoted) by p53.
Is a CKI to stop cell division (Cdk inhibitory protein)
Target of P53, without p53 cells divide uncontrollably
Proteolysis
Control of cyclin-Cdk activity at S-phase
It’s effect on CKI can turn on S-Cdks. Removes CKI inhibition by destroying CKIs.
Uses a protein called SCF-ubiquitous ligase to add ubiquitin to CKI
(*ubiquitin marks proteins for destruction)
Transfers ubiquitin to target proteins for destruction by proteasomes
What controls mitosis?
M-Cdk
The inactive form gets phosphorylated by CAK but Wee1 kinase maintains the inhibition.
By the end of G2 there are lots of primed M-Cdks around. When these are activated, mitosis commences.
Cdc25, a protein phosphatase, removes the inhibitory phosphates
Cdc25
Protein phosphatase that removes the inhibitory phosphates from M-Cdk complex allowing for mitosis to occur
Double-Circuit Positive Feedback
- Positive feedback - active M-Cdk complex activates Cc25 phosphatase to remove phosphate from roof site (release inhibition)
- Wee1 Kinase is inhibited so roof site is not phosphorylated again
Fast M-Cdk Activation!
Mitotic Progression from metaphase to anaphase
Sister chromatids become daughter chromosomes
Triggered by protein destruction NOT protein phosphorylation
Regulated by APC/C (anaphase promoting complex)
-ubiquitin ligase enzyme
APC/C
Catalyze addition of ubiquitin to proteins to cause destruction
2 major proteins affected: cohesion+securin
- Cohesin complex- sister chromatids glued together along length by protein
- Securin- protects cohesin protein linkages that hold sis tids together in early mitosis
Cohesin
- sister chromatids are glued together along length by protein complex
- members of the SMC proteins (structural maintainance chromosomes)
- form rings around sis tids
Securin
Supports cohesin by inhibiting separase
Separase
Separates/unglues the chromatids
Cleaves cohesin
Necessary when sister chromatids become daughter chromosomes
Describe the process of Mitosis in regards to APC/C
APC/C levels rise in mid-mitosis -> APC/C adds ubiquitin to destroy proteins -> securin is destructed, which was inhibiting separase -> separase is active -> cohesin is cleaved by separase (metaphase to anaphase transition) -> sister chromatids come apart (anaphase begins)
Describe the APC/C Destruction of Cyclins
APC/C levels rise in mid-mitosis -> APC/C adds ubiquitin on targets to destroy them -> APC/C destroys cyclins of Cdk-cyclin complexes -> no cyclins or complexes left -> cell moves into anaphase (no longer replicates DNA)
What are APC/Cs major targets?
What activates inactive APC/C? What happens next?
S-cyclins and M-cyclins
binding to Cdc20
Addition of polyubiquitin to M-cyclin in M-Cdk complex, cyclins destroyed and Cdks dephosphorylated, shift to anaphase
Summarize the Cell Cycle Control System
- Signal causes activation of G1-Cdk
- G1-Cdk stimulates genes making G1/S-cyclin + S-cyclin -> go through START checkpoint
- G1S-Cdk activity induces S-Cdk activity causing DNA replication
- M-Ck drives Expression through G2/M checkpoint
- APC/C+Cdc20 triggers destruction of securin and cyclins at metaphase to anaphase transition
- If anaphase occurs, cell cycle completes
Ordered system of biochemical switches
Summarize the Regulation of Cyclin-Cdk Activites
- Phosphorylation of the Cyclin-Cdk complex
- Binding of a CKI
- Proteolysis of Cyclins
- Ubiquitination of Proteins
Mitosis occurs in two parts
1) increase of M-Cdk activity at G2/M triggers prophase, prometaphase and metaphase
- assembly of mitotic spindle
- Attachment to sister chromatids
2) Metaphase-to-anaphase transition: APC/C triggers destruction of securin, allowing cohesin cleavage
- APC/C also triggers destruction of cyclins and dephosphorylation of Cdk targets
What problems must cells solve with DNA replication?
1) conduct replication of DNA with complete accuracy to prevent mutations
2) every nucleotide is copied once to prevent amplification
**if multiple points of duplication occur on new replicated DNA genes are overexpressed and can cause cancer
What kinase initiates DNA replication? And how many times does it do so?
S-Cdk initiates DNA rep ONCE per cycle
At origin of rep
Begins w initiator proteins
Elongation at rep forks
DNA rep involves two distinct steps, allowing the strict control
Control of Chromosome Duplication (steps)
- G1 phase- prereplicative complex (PRE-RC) assembles at origins of rep (**CRITICAL STEP)
- S phase- rep forks created at sites of DNA rep
- chromosomes duped - MITOSIS/chromosome segregation (absolutely no PRE-RCs made during this step, not again until next cell’s G1)
PRE-RC is inhibited by Cdk activity
