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
explain the steps prior to highlighting why proteolysis is important
At the start of the cell cycle, G1/S-Cdk is important for movement through START
it induces S cyclin production. and S-cyclin is produced as the phase goes form G1 into S phase.
S-cyclin-Cdk complex is important to START DNA REPLICATION
but, S-cyclin-Cdk complexes are generated AND inhibited by binding to CKI; so the complex is already there, it is just inhibited by the binding of a CKI (p27). How do you release this? proteolysis
explain proteolysis involvement in cell-cycle control
proteolysis of CKI (p27) can activate S-Cdk’s
this is accomplished by using a protein called SCF-ubiquitin ligase to add ubiquitin to CKI’s (remember: ubiquitin system stimulates destruction of proteins)
when the ubiquitin is transferred to the target proteins, they’re marked for destruction by PROTEASOMES
Explain the proteolysis process done on CKI
SCF protein adds ubiquitin to phosphorylated CKI (Cdk inhibitory protein targeting it for destruction)
SCF activity depends on F-Box subunit (help SCF recognize target proteins)
Adds ubiquitin to CKI proteins in G1 and so activates S-Cdks (and DNA replication) - restores activity of S-Cdk complex
what controls mitosis?
M-Cdk
what holds M-Cdk complex inactive?
Wee1
what happens by the end of G2?
there’s a large stockpile of primed M-Cdk complexes around, but they’re inactive
what activated the primed M-Cdk? what ensues?
Activation of primed M-Cdk is accomplished by Cdc25 protein phosphatase - removes inhibitory phosphates from M-Cdk – activation of M-Cdk complex for mitosis
explain the double-circuit positive feedback in mitosis
1) active M-Cdk complex will activate Cdc25 phosphatase to remove phosphate from roof site - release inhibition
2) also, Wee1 kinase is inhibited so that roof site is not phosphorylated
* activates M-Cdk fast
how is mitosis finished/ accomplished?
mitosis: the progression from metaphase to anaphase, is triggered not by protein phosphorylation but by protein destruction
what is the key regulator for that this step?
APC/C: anaphase-promoting complex (or cyclosome)
what is APC/C and what does it do?
APC/C is a member of ubiquitin ligase family enzymes
it catalyzes addition of ubiquitin to proteins to cause destruction
what are two major proteins affected in this system,? what do they do?
cohesin: sister chromatids are glued together along their length by this protein
securin: this protein protects cohesin protein linkages that hold sister chromatid pairs together in early mitosis
outline all that happens with these proteins
sister chromatids are glued together along their length by cohesin
securin protects cohesins by binding to separase, thereby inhibiting separase. separase is an enzyme that cleaves cohesin; it undoes the glue holding chromatids together.
this is necessary for the sister chromatids to become daughter chromos
so how does Mitosis and APC/C go hand in hand?
APC/C levels right MID-MITOSIS
APC/C adds ubiquitin on targets to destroy proteins;
also, APC/C leads to the destruction of securin (inhibitor of separase, a protease); securin cleaves cohesin = sister chromatids come apart
explains all that happens with APC/C and securin; when?
destruction of inhibitor securin by APC/C
occurs at metaphase-to-anaphase transition
indirectly activates protease (called separase) that separates sister-chromatids and anaphase begins
what else does APC/C target?
S-cyclins and M-cyclins
in this case, how is APC/C activated? what does it lead to?
inactive APC/C is activated by binding to Cdc20
leads to addition of polyubiquitin to M-cyclin in M-Cdk complex
cyclins are destroyed
Cdks are dephosphorylated
why does this happen?
you’re moving into anaphase- you must get ride of S-cyclin and M-cyclin since you don’t want any more DNA and chromosomes
review of the cell cycle control system
1) signals cause activation of G1-Cdk
2) G1-Cdk stimulates genes making G1/S-cyclin + S-cyclin
* go through START checkpoint
3) G1S-Cdk activity induces S-Cdk activity causing DNA replication
4) M-Cdk drives expression through G2/M checkpoint
5) APC/C + Cdc20 triggers destruction of securin and cyclins at metaphase-to-anaphase transition
- anaphase can occur - completion of mitosis
- the cell-cycle control system is an ordered series of biochemical switches
for mitosis cell cycle, regulation occurs in how many parts? elaborate on them
1) increase of M-Cdk activity at G2/M triggers prophase, pro metaphase and metaphase
- assembly of mitotic spindle
- attachment to sister chromatids
2) metaphase-to-anaphase transition: APC/C (anaphase promoting complex) triggers destruction of securin - this is an inhibitor of a protease (separase) that cleaves cohesin
APC/C also triggers destruction of cyclins - leads to Cdk inactivation and dephosphorylation of Cdk targets
what are some problems that can arise with DNA replication?
cell must solve 2 problems:
1) conduct replication of DNA with complete accuracy to prevent mutations
2) every nucleotide is copied once to prevent amplification
what happens if there’s multiple points of duplication on new replicated DNA?
over expression of genes, leading to cancer
what initiates DNA replication? how many times? how?
S-Cdk initiates DNA replication ONCE per cycle
- dna rep begins at origins of replication
- initiation begins with initiator proteins
- elongation occurs form replication forks
- DNA replication in cell cycle involves 2 distinct steps which prevents DNA replication occurring more than once
what are the two steps in DNA replication?
1) At G1 phase in cell cycle PREREPLICATIVE COMPLEX or PRE-RC assembles at origins of replication **
2) at S phase of cell cycle, replication forks are created: sites of DNA replication
chromos duplicated
is any new PRE-RC formed between G1 and the next G1?
no - once is enough; during mitosis no new PRE-RCs are made
what directs this? how?
the cell cycle control system
what inhibits assembly of PRE-RC?
Cdk activity
**while S-Cdk and M-Cdk are highly abundant during S and M stage, no PRE-RC is formed
