The Cell Cycle: Regulation and Division

CSC 858 3/17/05

Dr. Andrew BieberichDr. Michael Goldman

Reading: Tozeren and Byers Chapter 7For April 7, read Chapter 3

Copyright Andrew Bieberich 2005

Ultimately, cells decide whether to divide or not based uponinput of information from outside the cell.

single cells(yeast, bacteria)

Are there enoughnutrients?

Are toxic wastemolecules tooconcentrated toproceed withoutcell damage?

cells within multi-cell organisms

Is the cellattached to others? Is ittoo crowded?

Are the correctgrowth factorspresent?

Regulation of the multi-celled eukaryotic cell cycle

1. Semi-modular control system.

2. Five major checkpoints that act as switches in the system.

3. Cyclin and Cyclin and Cyclin...

4. Growth factors coordinate cell cycles across multiple cells.

5. G0, when time stands still.

6. Cancer: when switches malfunction.

G1

START

M

S

DNA replicationcheckpoint

DNA damagecheckpoint

G2

metaphasecheckpoint

degredationof metaphasecyclins

Across cell types, the cellcycle may take minutes, months, or arrest indefinitely.

Therefore, we knowthat something sophisticatedmust be controlling it.

A “clock” is not flexible.Phase triggering is only slightlymore so.

The system required is one that uses switchescontrolled by subsystems with feedback.

G1

START

M

S

DNA replicationcheckpoint

DNA damagecheckpoint

G2

metaphasecheckpoint

degredationof metaphasecyclins

G1

Chromosomes becomeuncondensed. Also called ‘G0’ if cell arrestsin this state.

START

M

daughter cells

S

DNA replicationcheckpoint

DNA damagecheckpoint

DNA is replicated.

G2

Chromosomescondense, Topoisomerase IIhelps to untanglethem.

metaphasecheckpoint

degredationof metaphasecyclins (check beforeentering G1)

Growth factors and integrin-mediated singalling pathways both affect production of cyclin D in the nucleus.

cell surface receptor

nuclear membrane

nuclear poregrowth factor

integrin boundto ECM

Rb

E2Fsignal transductionpathways with manyphosphorylation steps

Growth factors and integrin-mediated singalling pathways both affect production of cyclin D in the nucleus.

cell surface receptor

nuclear membrane

nuclear poregrowth factor

integrin boundto ECM

signal transductionpathways with manyphosphorylation steps

Rb

E2F

CDK4

cyclin D

cyclin D geneexpression

RbCDK4

cyclin D

E2F

Turn on genesfor DNA replicationin S phase.

Turn on genefor cyclin E andcyclin A

RbCDK2

cyclin E / A

Progression past START is reached when the feedback loop increases the rate at which Rb is phosphorylated and DNAreplication genes are turned on by E2F.

RbCDK2

cyclin A

Cyclin A-CDK2, in addition to increasing the rate atwhich Rb is phosphorylated, also begins phosphorylating(and thus activating) the DNA Replication Complex.

CDK2

cyclin A

Activated DNA RCbinds to replication fork and recruits DNA Pol III

DNA Pol III

When activated DNA RC begins to bind replicationforks, the G1 - S phase transition is complete.

S phase: all DNA in all chromosomes must bereplicated and checked for damage.

DNA Pol epsilon detectspresence of replication forks-check before exiting S phase.

Many proteins (and thus manygenes) are involved in checkingfor damaged DNA at this point.Apoptosis may occur if damageis too great.

p53

BRCA1

regulatory cross-talk

Apoptosis can be thought of as “programmed cell death.”

A set of molecular machinery kept in reserve for this purposeis turned on, and the cell auto-destructs by digesting all ofits components.

Loss of function of p53, or other genes that encode damagechecking proteins, can be a “pre-cancerous” condition. Cellsthat divide without checking for DNA damage may replicatemutant forms of cell cycle regulating genes. These cells mayin turn become the progenitors of tumors.

ribosomes

mitochondria

histones

centrosomes/centrioles

Cell components other than DNA must be replicated also during S phase.

John Kyrk’s webpage has an illustration of primary DNA packaging with histones, and this serves as a powerful suplement to our discussion of G2 phase.

You may see this by clicking this link:

http://www.johnkyrk.com/chromosomestructure.html

As is mentioned in Tozeren and Byers, not much is known about the molecular signalling that controls timing of G2. We know that S phase ends when the DNA replication andDNA damage checkpoints are passed, and mitosis (M phase)is considered to have begun when chromosomes are fully condensed. During G2, in between S and M, DNAtopoisomerase II works to untangle the uncondensed chromosomes so that they may condense separately.

Mitosis: Tozeren and Byers explain the separate phases of mitosis completely, so it is not necessary to simply repeat what they said. However, you should make sure that you are familiar with what happens during each of these phases:

ProphaseMetaphaseAnaphaseTelophase

You will notice that not everyone describes these in exactly thesame way. (For instance, John Kyrk lists ‘Prometaphase’ as anextra step. I’ve never seen that before.) The basic events thattake place are as described in Chapter 7 of Tozeren and Byers,so you may go by what they have written. To see John Kyrk’smitosis animation: http://www.johnkyrk.com/mitosis.html

centrosome

microtubule

paired sister chromatidsaligned along metaphase plate

There is a checkpoint at metaphase of mitosis.The sensor proteins detect whether tension is exerted on centromeres of all chromatid pairs.

centromere

securin proteins binding centromeres of sister chromatids

centromere

kinetochore

microtubule made ofalpha and beta tubulinsubunits

MT motor

After the metaphase checkpoint sensory system detects perfect alignment, anaphase begins. The MT motors pulleach chromatid along its microtubule, and the microtubuledisassembles as it is pulled toward the kinetochore.

Growing animal cells under artificial culture conditions:

Normal cells will usually not divide more than ~10 timesunder culture conditions, so primary cultures of recently isolated cells do not last long.

Alternatively, cell lines that are immortal may be used- these are often cells containing genetic mutations that cause cancer-like growth, and so are less accurate model systems for “normal” cell activity.