Cell Cylce I Flashcards
What is the cell cycle
the cycle of duplication and division
its an orderly progression of events that results in one cell becoming 2 cells; it takes place in a specific sequence
what is the goal of the cell cycle?
- produce two genetically identical daughter cells
- dna in each chromo must be faithfully replicated into two pieces
- precise réplication of 6.4 x 10^9 base pairs in diploid human genome
- about 6 mistakes occur in one cell division
what are the three major chromosomal events in the cell cycle?
1) chromosome duplication - this occurs in S phase (DNA synthesis phase)
2) chromosome segregation - as well as cell division occur during M phase of cell cycle (mitosis)
3) cytokinesis - cell division
what are the 4 phases of the cell cycle?
- S phase - synthesis of DNA
- M phase - separate chromosomes and divide cells
- GAP phases: cells have extra gap phases to allow more time for growth
- G1 phase between M and S
- G2 phase between S and M
Whats the actual order of these 4 phases? which is interphase?
G1, S, G2, (interphase)
M
what are the cell cycle control systems? what are the 3 major transition checkpoints and check for?
1) checkpoint I: start: G1 to S - cell commits to cell cycle entry and chromosome duplication (also called restriction point); checks to see if the environment is favorable.
2) checkpoint II: G2 to M - chromosome alignment on spindle in metaphase; checks to see if environment is favorable and if all DNA is replicated.
3) in M phase: anaphase and cytokinesis - metaphase - to - anaphase transition - trigger sister chromatic separation and cytokinesis; checks to see if all chromosomes are attached to spindle
what biochemical switches does the cell cycle control system use?
cyclin dependent kinases
Cdks phosphorylate proteins downstream to activate them and regulate cell cycle events
speak on Cdks
heart of the cell-cycle control system
the ACTIVITIES of Cdks rise and fall during cell cycle
causes cyclical changes in phosphorylation of substrates downstream that regulate cell cycle events
the cell cycle is governed by the Cdks
what proteins regulate Cdk’s
cyclins
the LEVEL of cyclins vary and cycles during the cell cycle
Cdks are dependent on cyclins- must be bound to cyclin to have protein kinase activity; without cyclin, Cdk is inactive
if cyclin is around, then cyclin-Cdk complexes are formed and triggers cell cycle events
cyclin expression controls what STEP cell is in; it also directs Cdks to their specific target
speak on levels of cyclins and Cdks
cyclins vary according to the point of time in cell cycle
Cdk levels are constant (activity rises and falls with levels of cyclins)
what are the 4 classes of cyclins?
1) G1/S cyclins
2) S cyclins
3) M-cyclins
4) G1 cyclins
elaborate on G1/S cyclins
they start the cell cycle
activates Cdks in late G1
Help trigger progression thru START
commitment made to cell cycle entry
LEVELS DROP IN S PHASE
elaborate on S cyclins
duplicate DNA
- bind Cdks after progression thru START
- helps stimulate chromosomes duplication
- S-CYCLIN LEVELS REMAIN HIGH UNTIL MITOSIS
elaborate on M-cyclins
Mitosis
- activate Cdks that stimulate entry into mitosis at G2/M checkpoint
- M-CYCLIN REMOVED AT MID-MITOSIS
elaborate G1 cyclins
govern activity of G1/S cyclins (control progression through start checkpoint)
*don’t stress this one
what are the 4 Cdks in vertebrates?
1) G1/S- Cdk
2) S-Cdk
3) M-Cdk
4) ) G1-Cdk
what governs the full activity of a Cdk-cyclin complex?
CAK (Cdk Activating Kinase)
- without cyclin bound, the active sit of Cdk is blocked by a region of the protein called the T loop
- binding of cyclin causes T-loop to move out of the active site (Cdk partly active)
- phosphorylation of Cdk at T-loop fully activates enzyme (“cave site”)
** Phosphorylation of Cdk caused by CAK
what other controls govern cyclin-Cdk activity?
inhibition and proteolysis
what is one way Cdk activity is inhibited?
via Wee1 kinase
Cdk activity is determined primarily by rise and fall of cyclin levels, but a different phosphorylation of Cdk inhibits activity of Cdk complex via Wee1 kinase
Wee1 kinase inhibits Cdk activity by phosphorylating the rood “site”
what reverses inhibition by Wee1 kinase?
Cdc25
Cdc25 is a phosphatase that dephosphorylates “roof” site to increase Cdk activity
What is a CKI? give an example
CKI protein is a Cdk Inhibitor protein
a CKI, like p27, binds to BOTH Cdk AND cyclin to inactivate (so it binds to the Cdk-cyclin complex)
primarily used for control of G1/S-Cdks and S-Cdks early in cell cycle
give an example that ties CKIs and disease together
Familial hereditary melanoma (skin cancer); high incidence of melanoma in families
INK4A is a CKI (so it inhibits Cdk); its involved in the G1 phase of cell cycle
mutation occurs in INK4A gene leading to loss of activity (loss of inhibition)
**cannot control cell cycle and grows uncontrollably and you get melanoma
whats another example of CKI and disease?
p53 - this is a major tumor suppressor; it influences the expression of many genes
p21 (a CKI which stops cell cycle) is one of those genes - p53 up regulates p21 (p21 transcription is a target of p53)
if p53 fails to function properly, there will be lower p21 expression and without this CKI the cell divides uncontrollably
Aside from CKI’s, whats another way to control cyclin-Cdk activity at S phase?
Proteolysis