3. Cellular reproduction: the cell cycle Flashcards
in single celled organisms, cellular reproductions ensures ___________ of species.
propagation
multicellular organisms
- 1 cell (zygote) divides to become highly organized complex of many of cells
- cell division continues through life
- adult humans have millions dividing at any moment
- replaces old, damaged cells
cell cycle
the time from one division to the next division
cell cycle is divided into 2 visible phases:
interphase and Mitotic phase (M phase)
Interphase
G1, S, G2
-cell growth and metabolism
-length of interphase can vary depending on cell type
*
M phase
-includes mitosis (nuclear division) and cytokinesis (cell division)
-occurs after interphase
MITOSIS (prophase, prometaphase, metaphase, anaphase, telephase)
CYTOKINESIS
G1
cell grows and carries out normal metabolism; organelles duplicate
S
DNA replication and chromosome duplication
G2
cell grows and prepares for mitosis
Cells can be grouped into 3 broad categories with respect to cell cycle
- cells that do no cycle, but enter a permanent arrest stage (G0)
(ex. neurons, muscles cells, RBCs- once differentiated, they remain in that state until they die) - Cells that normally do not divide, but can be induced
(eg. liver cells, lymphocytes- liver cells reproduce with infection or too much alcohol. the damaged liver cells need to be replaced) - Cells that divide regularly
(eg. epithelial cells that line body cavities and surfaces)
How is cell cycle controlled?
Cytoplasmic factors control cell division
Who did and experiment where they fused mitotic and interphase cells?
Rao and Johnson (1970)
What did Rao and Johnson found?
if interphase cells were in G1, they observed “premature chromosomes” (slightly condensed)
I.e the non-mitotic cell tried to enter M phase
What did Rao and Johnson conclude?
a “factor” present in the mitotic cell directs cells to undergo mitosis
Entry into mitosis requires a protein complex called _____________.
Maturation promoting factor or MPF
Rao and Johnson’s factor
MPF is a complex of two proteins:
- Cyclin b
2. Cdk1 (cyclin dependent kinase)
Cyclin b
- regulates Cdk1 activity
- levels rise and fall throughout cell cycle
Cdkl1
(cyclin dependent kinase )
-a kinase is a protein that adds a phosphate to other proteins; the activity of the target protein is modified.
In what phase does cyclin b increase its concentration?
G2 phase
decreases in M phase
In what phase does MPF increase its concentration?
end of G2 and beginning of M.
MPF phosphorylates and activates _______ of _______.
protein, mitosis
In the ______ phase, cyclin creases enough to bind to _______.
G2, Cdk1
Some MPF functions (3)
- activates lamin proteins- causes nuclear envelope to break up
- alters DNA-bound proteins (histones)- causes DNA to condense
- activates proteins of mitotic spindle
phosphorylate
introduce a phosphate group into a molecule or compound
third tenet of cell theory
new cells originate only from other living cells
wild type
not a mutant
M forms two daughter cells
- sign indicates a _______
mutant
Wee1 and Wee1-
Wee1 protein is an inhibitor of MPF (prevents mitosis)
Wee1- mutant lacks inhibitor- results in early division ( the cell divides prematurely, prematurely creat daughter cells, forming small cells )
Cdc25 and Cdc25-
Cdc25 protein activates MPF
Cdc25- mutant lacks activator, division fails to occur; the cell does not divide but continues to grow resulting in an overly large cell
in the g2 phase cyclin increases enough to bind to cdkl however MPF is not ______ yet because it does not work alone
active
Cdk subunit of MPF requires a multi step activation process
step 1
Page 577
step 1. (interphase G2) Cdk (cdc2 kinase) is inactive, it interacts with a mitotic cyclin but is still inactive because wee1 adds phosphate tyr15- which inhibits activity. a separate kinase CAK transfers a phosphate to another residue (Thr161- which is required later.
Cdk subunit of MPF requires a multi-step activation process step 2
When the cell reaches a critical size, and enzyme called Cdc25 phosphatase is activated which removes the inhibitory phosphate on the Tyr15 residue. The resulting activation of the cdc2 kinase drives the cell to mitosis.
what does Cdc25 do?
is a phosphatase which removes P from Tyr15.
Cdk is now active
Cdk subunit of MPF requires a multi-step activation process
step 3
Cdk is active
by the end of mitosis another phosphatase removes a phosphate group from Thr161, cyclin detaches, and is degraded and the cell begins another cycle and is again degraded
MPF activation and the cell cycle recap
slide 12
- cyclin levels increase during G2
- when cyclin levels high enough Cyclin b and Cdk1 bind together (inactive MPF)
- CAK (activiates) and Wee1 (inactivates) end of G2 (inactive MPF)
- beginning of M phase cdc25 activates (MPF activates)
How is the cyclin action turned back off?
by activating ubiquitin ligase, which terminates MPF activity by causing proteolysis of cyclins
Process of cyclin action turned off
- active MPF in M phase
- MPF phosphorylates (lamins, histones, microtubules, ubiquitin ligase)
- G1 phase, Cdk1 is inactive again and cyclins are low again
or Cdk1 can destroy the cyclin which target for proteolysis by proteasome
Why does MPF cycle work this way?
Because a number of things all have to be properly coordinated before mitosis starts
-is DNA replicated
-is Cell big enough?
-does it have enough stuff?
If wee1 isn’t working the cells go on too soon and end up small (goes from G1-G0)
Cell checkpoints (4)
- end of g1 cell decides whether to continue (START or RESTRICTION- is cell big enough? is environment okay?)
- G2 begining. (Is DNA intact?)
- End of G2 MPF checkpoint (is DNA replicated? is the cell big enough? does is have enough stuff?)
- end of M phase. Metaphase checkpoint (are chromosomes aligned?)
Cyclins and cyclin dependent kinases appear at different _____ of the cell cycle
stages
slide 16
DNA damage checkpoint is inoperative inpatients with ataxia telangiectasia (AT)
- neurodegenerative disease (ataxia=loss of motor control), but affects many other tissues
- telangiectasia=tiny red spider veins (often starting in eyes)
- affected individuals susceptible to cancer
* ionized radiation (ATM protein) -Breaks DNA
* ultraviolet radiation (ATR protein)
DNA Damage checkpoint : cell cycle stops if there is DNA damage (6 steps)
- ATM protein detects a break in DNA strand in G1
- ATM is a kinase. It phosphorylates another kinase Chk2, thus activating it
- Chk2 is a kinase. its phosphorylates the protein p53 (stabilizes it)
- when phosphorylated, p53 turns on p21 gene- p21 is synthesized (produces)
- p21 protein binds Cdk and inhibits it
- Cell is prevented from entering S phase until DNA repaired
What can defects in cell cycle genes cause?
cancer
-uncontrolled cell divisions result in cancer
50% of human tumour cells have mutations in which gene?
p53
1% of human genes implicated in cancer which is approx. _________.
3000
what does the cancer genome project aim to identify?
all gene defects associated with cancers
62% of human disease genes are also found in the _______
fruitfully, drosophila
Components of a checkpoint
- sensor
- transducers
- effector
Sensor
is DNA damaged?
-ATM/ATR proteins. one break stops the whole cell cycle
Transducers
create a signal
-Cdc25, p53
-The MPF proteins
this is one reason that the cell works in a multistep fashion; different control systems interact
Effector
change key enzyme action/protein
-the phosphorylated histones, lamins, ubiquitin and other proteins whose activity drives the process of mitosis
Checkpoints in the cell cycle (3)
- checkpoints from DNA damage (G1 and G2)
- cell cycle halts, allows time for repair - progression from G2 to M
- by maturations promoting factor MPF - the spindle checkpoint during M phase
