Week 5 - Cell Division

Question 1

Mitosis

Answer 1

All cells divide by this mechanism except for germ cells

Germ cells use meiosis* (sperm and egg cells)

Question 2

Phases of cell division

Answer 2

4 segments

M-phase

G1 phase

S - phase

G2 phase

Question 3

G1 Phase

Answer 3

Cells make a decision to divide or not to divide

They dont just stop tho, they dissasemble the replication machinery and go into a G0 phase… separate form cell cycle.

IF they decide to devide they pas the start (or restriction site) they are commited to replicate DNA and divide. will get two genetically similar daughter cells.

Question 4

M - phase

Answer 4

Actually dividing phase

5 states of M-phase

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

These are divided by events that you can see in cell through a microscope.

Question 5

Herkst Stain

Answer 5

Herkst stain - used to bind to DNA and fluorisces it (turns blue under UV light)

Question 6

S -phase

Answer 6

DNA replicates in nucleus of cell

Centrosome also replicates so there are two daughter centrosomes ( a pair of centrosols) need electron microscope to see this

Then it sits like this through end of G2..

Question 7

Prophase

Answer 7

DNA condenses and highly organizes

Chromosome condensation is what its called, formation of much shorter replicated chromosomes..

Daugher centrosomes tart to move apart - microtubule mediated - the microtubuels push the centrosomes apart

Nucleaer envelope starts to break down (caused by phosphorylation of lamins)

Question 8

Prophase cont..

Answer 8

THen centrosomes move on opposite sides of cells, microtubules nucleate out towards the condencsed chromosomes in the middle, the middle is called the kinetocore

Microtubules constantly lenghten and shorten and find chromosomes.. one binds a chromosome, then the other microtuble from other side grab onto it as well.

The centrosomes are now called spindle poles*

Question 9

Prometaphase

Answer 9

The chromosomes are being moved back and forth between spindle poles by the microtubules pulling and pushing.. they start to line up in the center of cell…

Question 10

Metaphase

Answer 10

Metaphase is defined by all of the chromosomes being lined up in center of cell.. Push and pull forces between spindle pole microtubules is equal*

Microtubules act on kinetocore (core that holds two chromatds together)

Energy to move chromosom is caused by depolymerization and polymerization of microtubules (motive force)

Interaction with motor protiens, dimens and kinesins, also a motove force of chromosomes

Thir motive force is actin and myocin interactions

**Cells will not move on in replcation until all chromosomes are aligned. The kinetocore, if NOT bound to microtubule, will put out a signal to inhibit mitotic progression.. protien is bioR1 (somthing). This is called Metaphase arrest.

Question 11

Anaphase

Answer 11

After chromosomes are aligned in center…

then Separase dissolves proteins that hold chromatins together at kinetocore

Securin is the protein that inhibits separase, securin is present in cell untill metaphase, then it is destroyed by protealysis, and separase dissolves the proteins.

Since microtubules already have tension on them, when separase is present the chromatids immediatedly separate toward the spindlpoles..

This is the start of ahaphase** Very important part of cell cycle.

Question 12

Telophase

Answer 12

Cytokenisis - pinchinc of cells into two..

This happens in telophase.

Then the daugher chromatids that have been pulled apart during anaphase start to decondense and the nuclear envelope starts to reform.

Spindlepoles become less active

actin filaments form a contractil ring in the center of the two new cells.. called a cleaveage furl that pinches the two cells apart into two daughter cells.

Now have two daughercells in ealry G1 phase. now make desicion again to replicate or not.

Question 13

S-phase

Answer 13

S-phase replicating centrosome and DNA

Question 14

G2

Answer 14

Preparing to divide

Question 15

G1

Answer 15

Deciding whether to divide of not

Question 16

G1 cell with S phase cell experiment

Answer 16

Used a virus to bind two cells in these phases

The g1 cell immediately started to replicate DNA

Then then did an experiment with G2 phase and S phase (dominant S phase factor)

no re-replication of G2 phase so then they figured out that the cells had a block to re-replication (no dominant S phase factor)

So once a cell replicated its DNA there is a block so they cannot -re replicate .. if this happend we would have really messed up DNA.

Then did with M phase and either G1, S, or G2

it immediatedly made the cells condense chromosomes and move into m-phase (dominant M-phase factor)

Question 17

Wee1 and Cdc25

Answer 17

Kinase and phosphatase can activate or inactivate proteins in cells..

This is what regulates activity of cells in the cell cycle**

Cdks are essentially binary on / off switches (kinases) that launche specific irreversible events

Question 18

Regulation of cell cycle phases

Answer 18

The major transitions in cell cycle is regulating by phosphorylation

G1-Cdk to restriction phosphorylation

G1/S Cdk Phosphorylation

S-Cdk (phosphorylation)

G2/ M -Cdk is by phosphorylation

M-Cdk

Met to anaphase is by proteolysis*

Question 19

Cyclin and Cdk and Wee1 and CAK and Ckis

Answer 19

Two spots for phosphorylation

Cdk (cyclin dependent kinase)

There are multiple cyclins and Cdks but there are specific pairs that regulatie the different phases of the cell cycle. Cycline is the regulatory region.

When cyclin is bound to Cdk it doesnt do much, must be phosphorylated (one spot) to change conformation and activate it.

If the other spot is phosphorylated (the inhibitory site) it becomes inactive - Wee one is the kinase that suppresses Cdk activity.

CAK (cdk activating kinase) adds the first phosphorylation to the activating site.

Cdc25 is the phosphorylase that removes the phosphorylation that Wee1 kinase put on it, this activates Cdk..

Ckis bind to cyclin cdk complex and block the kinase activity of the cdk - this can arrest the cell cycle in G1 or S phase if unfavorable conditions.. (DNA damage)

So cell will have a bunch of these inactive forms of cdc and cyclin bound with both phosphorylations.. then all cell has to do to initiate cell division is used Cdc25 to dephosphorylate the one spot and activate the complec.

Question 20

Cki

Answer 20

cyclin dependent kinase inhibitors

another level of cell cycle regulation..

Question 21

Cyclin

Answer 21

cyclin is another way for cells to regulate division

cyclin is a substrate for in ubiquitin ligase

APC (anaphase promoting factor)

A chain of ubiquinones is a signal for the proteosome to come and destroy cyclin

KNOW what cyclin does at each phase in cell cycle.. Do not neet to know names though.

Question 22

Cdc 25

Answer 22

definately know what this does.

Question 23

Early G1 phase is low cdk activity

Answer 23

It was all destroyed in M-phase

Now have proteins that bind to targets if NOT phosphorylated. So by shutting off cdk activity they are not phosphorylated and able to bind to their targets

Their targets are called origen of replication targes. They are sites on chromosomes.. hundreds of them on human chromosomes.

The proteins like Cdc6 adnd ctd bind to the origen of replication targes when theye are NOT phosphorylated.

Then MCM helocase binds to it. This bunch of bound proteins on ORCs are claled a pre-replication complex.

Then more proteins bind, froming the pre-initiation complex. This complex is ready to replicate.

The final event that triggers the replication of this complex is a phosphorylation by a cdk complex, then because they are phosphlrylated they fall off the ORC

Cdks are now active until the next early G1 phase.. this keeps the ORC active by phosphorylating the proteins that

Question 24

Enzyme linked receptors

Answer 24

Growth factor receptors**

What does RAS do? If bound to GDP it is inactive, but when it is bound to GTP it is active,

MAPK - once it is phosphorylated it moves into the nucleus and interacts with transcription factors that sit on the genes, phosphorylating them, and then the genes are transcribed. The mRNA moves into cytoplasm where the proteins are made.

One protein meade is C-MYC then moves back into the nucleus to initiate transcription factors on other genes that make G1cyclins (and cyclin D)

C-MYC is called an immediate/early gene - first genes expressed when growth factors are activating

The second genes (G1cyclins) are called delayed response genes

now g1cyclins bind to cdk partners and forms the complexes to activating the cell cycle.. has to have one spot phorphorylated and the other spot dephosphorylated.

These cyclin compexes can activate protines like the retoblastoma protein (pRb), pRb keeps the E2F protein by binding to it, this blocks replication. When pPb is phosphorylated it releases E2F (activating it) and E2F moves into nucleus to make DNA replication possible.

E2F not activates the G1-S cyclins and also the S-phase cyclins and also activatines DNA synthesis

Then G1-S cyclins and S-phase Cycins bind to their respective Cdks and can be activated..

So… if cyclin D (G1 cyclins) the cell would always be replicating. .How to turn it off? the G1 cyclin and its mRNA have very short half lifes.. and in adults the growth factor levels are very low, keeping RAS and MAPK activity low and it shuts all activity down to prevent G1 to S phase transition. To turn it on, add growth factor and it takes a while to turn back on. This is why it takes while for the cell to move from G0 back into G1 and S phase…