04: Cell Cycle Flashcards
what is the only way to make a new cell
to duplicate an existing cell
what is the one part of the cell cycle that is shared in all life
passing genetic info to the next generation of cells
chromosome duplication occurs during which phase
S phase (s for dna synthesis)
how long does s phase take
about half of the cell cycle time
which major events are included within M phase
- nuclear division (mitosis)
- cytoplasmic division (cytokinesis)
when are dna molecules condensed into sister chromatids
during prophase
when does G1 phase happen
between M and S
when does G2 happen
between S and mitosis
what are the gap phases
G1 and G2
what is the point of the gap phases
- time delays to allow cell growth
- provides time for the cell to monitor environments to ensure suitable conditions
what happens if during G1 they determine the conditions aren’t favourable
- delay the process
- sometimes going into G0
“START” is found where
in yeasts
“restriction point” is found where
in mammalian cells
true/false the term “start” can be used for all cells
true
true/false all cells undergo the conventional four-phase cell cycle
false
true/false cell-cycle control is similar in all eukaryotes
true
when are the 3 major checkpoints in the cell cycle
- start checkpoint in G1/S
- G2/M checkpoint
- metaphase/ anaphase checkpoint
what does the G1/S checkpoint look for
is environment favourable?
what does the G2/M checkpoint look for
- is all DNA replicated
- is environment favourable
what does the metaphase/anaphase checkpoint look for
are all chromosomes attached to the spindle
if a cell passes the G1/S checkpoint, what can it do
enter cell cycle and proceed to S phase
if a cell passes the G2/M checkpoint, what can it do
enter mitosis
if a cell passes the metaphase/anaphase checkpoint, what can it do
trigger anaphase and proceed to cytokinesis
true/false cell cycle control system switches are typically binary
- true
- on/off
true/false cell cycle control system switches launch events in a complete, irreversible manner
true
true/false cell cycle control system is robust and reliable
true
true/false cell cycle control system is highly adaptable and can be modified
true
what does Cdks stand for
cyclin-dependent kinases
what are the most important cdk regulators
cyclins
true/false cdks have no protein kinase activity when bound to cyclins
- false
- they only have activity when bound
when do G1/S cyclins activate cdks
in late G1
what do G1/S cyclins do
help trigger progression through Start, resulting in a commitment to cell-cycle entry
when do s cyclins bind to cdks
soon after progression through start
what do S cyclins do
help stimulate chromosome duplication
when do m cyclins activate cdks
end of G2, start of M
what do m cyclins do
stimulate entry into mitosis at the G2/M transition.
true/false cyclin binding alone fully activates the associated cdk
- false
- also requires a separate kinase, the Cdk-activating kinase (CAK)
what does CAK do
- phosphorylates an amino acid near the entrance of the Cdk active site
- this causes a conformational change that greatly increases the activity of the Cdk subunit
true/false CAK levels are constant throughout the cell cycle
true
how is cyclin-cdk complex turned off via phosphorylation
- Wee1 kinase phosphorylates closely spaced sites above the active site for the cyclin
what undoes Wee1 activity
- Cdc25
- it removes the inhibitory phosphates that Wee1 has added
what regulation mechanism is particularly important for generating the rapid activation of M-Cdk activity
Wee1 kinase and Cdc25
The activities of G1/S- and S-Cdks early in the cell cycle are governed by what
Cdk inhibitor proteins (CKI)
what does CKI stand for
Cdk inhibitor proteins
how does CKI work
- wrap around the cyclin–Cdk complex
- promoting a rearrangement in the Cdk active site that renders it inactive
positive feedback is used frequently in cell regulation for what reason
to generate robust, all-or-none regulatory effects
what is a good example of positive feedback in cell regulation
activation of M-Cdk at the G2/M transition
describe the positive feedback in the activation of M-Cdk at the G2/M transition
- accumulation of M-cyclin during G2
- which means an accumulation of M-Cdk complexes as the cell approaches mitosis
- they’re phosphorylated by CAk but Wee1 also acts, so they’re held in an inactive state
- once it reaches G2, there is a stockpile of M-Cdk that is primed and ready to act, yet suppressed
- the phosphatase Cdc25 is activated by M-Cdk
- Cdc25 will remove the phosphatase that inhibits M-Cdk
- positive feedback happens
- rapid and irreversible activation of all M-Cdk, leading to phosphorylation of the many proteins that drive early mitosis
what is the main cause of the positive feedback that happens in the activation of M-Cdk
the ability of M-Cdk to activate its activator (cdc25) and inhibit its inhibitor (Wee1)
what is the expected result of a mutation that would inactivate Wee1
premature mitosis -> small cells
what is the expected result of a mutation that would inactivate CDC25
inhibition of mitosis -> large cells
true/false progression through the metaphase-to-anaphase transition is triggered by protein phosphorylation
- false
- by protein destruction
The key regulator of the metaphase-to-anaphase transition is what
anaphase-promoting complex (APC/C)
how is APC/C activated in metaphase
by association with Cdc20
how does APC/C work in metaphase-anaphase
- inactive APC/C associated with Cdc20…
- now becomes active
- with the aid of E1 and E2, APC/C assembles polyubiquitin chains on target protein
- this target is recognized and degraded in a proteasome
what kind of ligase is APC/C
ubiquitin ligase
what are the different mechanisms for controlling the cell cycle
- cyclin binding
- phosphorylation by kinases
- dephosphorylation by phosphatases
- binding of Cdk inhibitors (CKAs)
- controlled proteolysis
- transcriptional regulation
describe how the control of the initiation of DNA replication happens
- replication origin is bound by the origin recognition complex (ORC)
- ORC functions only in late mitosis and early G1 when it associated w Cdc6
- Cdc6 binds to ori
- inactive helicase (Mcm) is bound and associated Mcm proteins
- prereplicative complex forms
- S-Cdk stimulates assembly of initiator proteins on each Mcm helicase (one on each strand)
- DDK (protein kinase) phosphorylates and activates DNA helicase… DNA unwinds
- S-Cdk phosphrylayes ORC
- DNA replication starts
describe Mitogen stimulation of cell- cycle entry
- Myc is a regulatory protein
- it leads to increased G1-Cdk activity
- this triggers the phosphorylation of members of the Rb family of proteins
- the phosphorylation inactivates the Rb
- this frees the gene regulatory protein E2F
- E2F can transcribe the gene for G1/S cyclin (cyclin E) and S-cyclin (cyclin A)
- the resulting Cdks activates further enhanced Rb phosphorylation
- another positive feedback loop
- but more importantly the resulting active S-Cdk leads us to DNA synthesis
what does Myc do
increases the expression of many delayed response genes
what is the expected result of a mutation that would increase Myc activity?
unregulated progression into cell cycle-> uncontrolled cell growth and division
what is the expected result of a mutation that would inactivate Rb activity?
unregulated progression into cell cycle-> uncontrolled cell growth and division
how does DNA damage arrest the cell cycle in G1
- When DNA is damaged, various protein kinases are recruited to the site of damage and initiate a signaling pathway that causes cell-cycle arrest
- The first kinase at the damage site is either ATM or ATR
- other protein kinases (Chk1 and Chk2) are then recruited and activated
- this results in the phosphorylation of the transcription regulatory protein p53
- Mdm2 normally binds to p53 and promotes its ubiquitylation and destruction in proteasomes.
- Phosphorylation of p53 blocks its binding to Mdm2
- sooooo… p53 accumulates to high levels and stimulates transcription of numerous gene
- this includes the gene that encodes the CKI protein p21
- p21 binds and inactivates G1/S-Cdk and S-Cdk complexes, arresting the cell in G1
what Cdk does does Cyclin D partner with
Cdk 4 and 6
what Cdk does does Cyclin E partner with
Cdk 2
what Cdk does does Cyclin A partner with
Cdk 2 and 1
what Cdk does does Cyclin B partner with
Cdk 1
What is the vertebrate equivalent of G1-cyclins
Cyclin D
What is the vertebrate equivalent of G1/S-cyclins
Cyclin E
What is the vertebrate equivalent of S-cyclins
Cyclin A
What is the vertebrate equivalent of M-cyclins
Cyclin B
Cdks are _____ without cyclins bound
inactive
when you phosphorylate M-cyclin/Cdk complex, what happens
its inhibited
when you phosphorylate S-Cdk, what happens
its activated
what does the T-loop normally do when Cdk is in its inactive state
it blocks the active site (where Cdk would normally do its job)
describe how phosphorylation activates S-Cdk
- t-loop is normally blocking the active site
- when cyclin A joins Cdk2, the t-loop moves away from the active site
- Cdk2 is now partially active
- now phosphorylation happens
- CAK adds a phosphate to a threonine residue on the T-loop
- the T-loop will change its shape
- this change makes the enzyme better at its job
- so now Cdk is fully active
what is p27
a protein that acts as an inhibitor for the cyclin-Cdk complex
how does p27 inhibit cyclin-cdk complexes
- binds to both the cyclin and cdk in the complex
- it distorts the active site of the cdk
- also blocks the ATP-binding site, making it harder for Cdk to use the energy it needs to work
what does APC/C stand for
Anaphase Promoting Complex/ Cyclosome
what activates APC/C
cdc20
what is proteolysis
breaking proteins into AA
how does cdc20 work
- helps the APC/C recognize specific proteins, like M-cyclin, that are tagged for destruction
- The APC/C looks for special “signals” in the proteins it needs to target, usually specific amino acid sequences
how does APC/C control proteolysis
- APC/C is like a clean-up crew for the cell. It helps the cell get rid of certain proteins when it’s time to move from one stage of cell division to the next.
- Cdc20 is like the boss that tells the clean-up crew when to start working. It shows up during metaphase
- APC/C is looking for certain target proteins, like M-cyclin, that need to be destroyed
- APC/C tags the target proteins with a special tag called ubiquitin. Imagine this tag as a “garbage sticker” that says, “Take this to the trash.”
- E1 and E2 are helpers that help APC/C attach the “garbage sticker” (ubiquitin) to the trash proteins
- the proteasome sees the sticker at breaks it down
true/false ORC is always at the replication origin at the entire cell cycle
true
what does ORC stand for
origin recognition complex
describe positive feedback in the activation of M-Cdk
- Cdk1 (a protein) teams up with M-cyclin (another protein) as the levels of M-cyclin slowly start to rise. When they join together, they form a complex called M-Cdk
- the complex is initially inactive because it has two inhibitory phosphates that were added by a protein called Wee1
- But there’s also an activating phosphate added by Cdk-activating kinase (CAK)
- At the end of G2, an important protein called Cdc25 steps in. Cdc25 removes the inhibitory phosphates that Wee1 put on the M-Cdk complex
- M-Cdk doesn’t just get activated by Cdc25, it also helps activate more Cdc25 in a positive feedback loop. So once M-Cdk is active, it starts turning on even more Cdc25, which helps activate even more M-Cdk
- M-Cdk also has the ability to shut down Wee1, the protein that originally added the inhibitory phosphates. By turning off Wee1, M-Cdk makes sure it stays activated and can continue driving the cell cycle forward
- PP2A-B55 is another phosphatase that can undo the phosphorylation of both Cdc25 and Wee1, but it doesn’t get to do this unless M-Cdk gets activated first
- M-Cdk itself actually inactivates the PP2A-B55 phosphatase, making sure it can keep turning itself on. So, once M-Cdk is active, it sets up a feedback loop where it helps keep itself active and the cell can move into mitosis
how do mitogens work
- bind to cell-surface receptors (like a key fitting into a lock)
- remember that mitogens are signals that tell cells to start dividing
how does a cell get told to divide
- mitogens bind to cell-surface receptors, which start a series of events within the cell
- One of the first things that happens is the activation of a small protein called Ras.
- Ras starts a MAP kinase cascade (which is a chain reaction of protein activations). This cascade sends signals to the cell’s nucleus, telling it to start turning on certain genes
- One of the genes turned on by Ras is Myc (a transcription factor). Myc tells the cell to start expressing delayed-response genes that help the cell get ready for division.
- One of the delayed-response genes makes cyclin D, which partners with Cdk4 to form a complex. This complex is like a green light that tells the cell to start progressing through the cell cycle.
- Cyclin D–Cdk4 then goes on to phosphorylate (add a phosphate group to) a protein called Rb (Retinoblastoma protein)
- Rb normally acts like a brake on the cell cycle by binding to and blocking another protein called E2F, which is needed to turn on genes for the next phase of the cycle
- Rb normally acts like a brake on the cell cycle by binding to and blocking another protein called E2F, which is needed to turn on genes for the next phase of the cycle.
- When Rb is phosphorylated, it inactivates, which means it lets go of E2F
- Now that Rb is out of the way, E2F is free to activate the transcription of genes needed for the next phase (S phase) of the cell cycle.
- These include the genes for cyclin E and cyclin A
- the complexes formed by these cyclins phosphorylate Rb even more, keeping it inactive
how does a cell get told to divide (very briefly)
- Mitogens tell the cell to divide.
- This activates Myc, which makes cyclin D.
- Cyclin D turns off Rb, which lets E2F go.
- E2F turns on genes to keep the cell moving toward division and makes more E2F to keep things going.
what happens when DNA damage occurs
- once DNA damage happens, the cell cycle will immediately stop. here’s how it stops…
- ATM or ATR Kinases Are First Responders remember that kinases add phosphates to proteins
- Chk1 and Chk2 Kinases Are Next, and they add phosphates to a protein called p53
- Mdm2 usually binds to p53 and tells the cell to destroy p53, keeping its levels low. But when p53 is phosphorylated (by the kinases), it can’t bind to Mdm2 anymore. This allows p53 to build up in the cell
- With high levels of p53, it can turn on the expression of various genes that help the cell fix the damage and stop cell division. One of the important genes that p53 turns on is the gene for a protein called p21.
- p21 is a Cdk inhibitor (CKI) that binds to and inactivates the G1/S-Cdk and S-Cdk complexes. These complexes are responsible for driving the cell through the G1 and S phases of the cell cycle. By inhibiting them, p21 stops the cell from dividing
