The Cell Cycle

A eukaryotic cell cannot divide into two, the two into four, etc. unless two processes alternate:

doubling of its genome (DNA) in S phase (synthesis phase) of the cell cycle
halving of that genome during mitosis (M phase)

The period between M and S is called G₁; that between S and M is G₂.

So, the cell cycle consists of:

G₁ = growth and preparation of the chromosomes for replication
S = synthesis of DNA (and centrioles) [see DNA Replication]
G₂ = preparation for
M = mitosis

When a cell is in any phase of the cell cycle other than mitosis, it is often said to be in interphase.

Control of the Cell Cycle

The passage of a cell through the cell cycle is controlled by proteins in the cytoplasm. Among the main players in animal cells are:

Cyclins There are 3 groups:
- G₁ cyclins
- S-phase cyclins
- M-phase cyclins
Their levels in the cell rise and fall with the stages of the cell cycle.
Cyclin-dependent kinases (CDKs) Again, there are 3 groups:
- G₁ CDKs
- S-phase CDKs
- M-phase CDKs
Their levels in the cell remain fairly stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated. They add phosphate groups to a variety of protein substrates that control processes in the cell cycle.
The anaphase-promoting complex (APC) and other proteolytic enzymes. The APC
- triggers the events leading to destruction of the cohesins and thus allowing the sister chromatids to separate.
- degrades the mitotic (M-phase) cyclins

Steps in the cycle

a rising level of G₁ cyclins signals the cell to prepare the chromosomes for replication
a rising level of S-phase promoting factor (SPF) prepares the cell to enter S phase and duplicate its DNA (and its centrioles)
as DNA replication continues, one of the cyclins shared by G₁ and S-phase CDKs (cyclin E) is destroyed and the level of mitotic cyclins begins to rise (in G₂)
M-phase promoting factor (the complex of mitotic cyclins with M-phase CDK) initiates

assembly of the mitotic spindle
breakdown of the nuclear envelope
condensation of the chromosomes
these events take the cell to metaphase of mitosis
at this point, the M-phase promoting factor activates the anaphase promoting complex (APC) which
- allows the sister chromatids at the metaphase plate to separate and move to the poles (= anaphase), completing mitosis
- destroys the M-phase cyclins. It does this by conjugating them with the protein ubiquitin which targets them for destruction by proteasomes.
- turns on synthesis of G₁ cyclins for the next turn of the cycle
- degrades geminin, a protein that has kept the freshly-synthesized DNA in S phase from being re-replicated before mitosis.

Meiosis and the Cell Cycle

The special behavior of the chromosomes in meiosis I requires some special controls. Nonetheless, passage through the cell cycle in meiosis I (as well as meiosis II, which is essentially a mitotic division) uses many of the same players, e.g., MPF and APC. (In fact, MPF is also called maturation-promoting factor for its role in meiosis I and II of developing oocytes.

Checkpoints: Quality Control of the Cell Cycle

The cell has several systems for interrupting the cell cycle if something goes wrong.

A check on completion of S phase. The cell seems to monitor the presence of the Okazaki fragments on the lagging strand during DNA replication. The cell is not permitted to proceed in the cell cycle until these have disappeared.
DNA damage checkpoints. These sense DNA damage
- before the cell enters S phase (a G₁ checkpoint);
- during S phase, and
- after DNA replication (a G₂ checkpoint).
spindle checkpoints. Some of these that have been discovered
- detect any failure of spindle fibers to attach to kinetochores and arrest the cell in metaphase (M checkpoint - example)
- detect improper alignment of the spindle itself and block cytokinesis
- trigger apoptosis if the damage is irreparable.

All the checkpoints examined require the services of a complex of proteins. Mutations in the genes encoding some of these have been associated with cancer; that is, they are oncogenes. This should not be surprising since checkpoint failures allow the cell to continue dividing despite damage to its integrity.

Examples

p53
The p53 protein senses DNA damage and can halt progression of the cell cycle in both G₁ and G₂. Both copies of the p53 gene must be mutated for this to fail so mutations in p53 are recessive, and p53 qualifies as a tumor suppressor gene.

The p53 protein is also a key player in apoptosis, forcing "bad" cells to commit suicide. So if the cell has only mutant versions of the protein, it can live on - perhaps developing into a cancer. More than half of all human cancers do, in fact, harbor p53 mutations and have no functioning p53 protein.

A genetically-engineered adenovirus, called ONYX-015, can only replicate in human cells lacking p53. Thus it infects, replicates, and ultimately kills many types of cancer cells in vitro. Clinical trials are now proceeding to see if injections of ONYX-015 can shrink a variety of types of cancers in human patients.

In some way, p53 seems to evaluate the extent of damage to DNA, at least for damage by radiation.

At low levels of radiation, producing damage that can be repaired, p53 triggers arrest of the cell cycle until the damage is repaired.
At high levels of radiation, producing hopelessly damaged DNA, p53 triggers apoptosis.

ATM

ATM (="ataxia telangiectasia mutated") gets its name from a human disease of that name, whose patients - among other things - are at increased risk of cancer. The ATM protein is involved in

detecting DNA damage, especially double-strand breaks;
interrupting (with the aid of p53) the cell cycle when damage is found;
maintaining normal telomere length.

MAD

MAD (="mitotic arrest deficient") encodes a protein that binds to each kinetochore until a spindle fiber (one microtubule will do) attaches to it. If there is any failure to attach, MAD remains and blocks entry into anaphase.

Mutations in MAD produce a defective protein and failure of the checkpoint. The cell finishes mitosis but produces daughter cells with too many or too few chromosomes (aneuploidy). Aneuploidy is one of the hallmarks of cancer cells suggesting that failure of the spindle checkpoint is a major step in the conversion of a normal cell into a cancerous one.

Infection with the human T cell leukemia virus-1 (HTLV-1) leads to a cancer (ATL = "adult T cell leukemia") in about 5% of its victims. HTLV-1 encodes a protein, called Tax, that binds to the MAD protein causing failure of the spindle checkpoint. The leukemic cells in these patients show many chromosome abnormalities including aneuploidy.

A kinesin that moves the kinetochore to the end of the spindle fiber also seems to be involved in the spindle checkpoint.

G₀

Many times a cell will leave the cell cycle, temporarily or permanently. It exits the cycle at G₁ and enters a stage designated G₀ (G zero). A G₀ cell is often called "quiescent", but that is probably more a reflection of the interests of the scientists studying the cell cycle than the cell itself. Many G₀ cells are anything but quiescent. They are busy carrying out their functions in the organism. e.g., secretion, conducting nerve impulses, attacking pathogens.

Often G₀ cells are terminally differentiated: they will never reenter the cell cycle but instead will carry out their function in the organism until they die.

For other cells, G₀ can be followed by reentry into the cell cycle. Most of the lymphocytes in human blood are in G₀. However, with proper stimulation, such as encountering the appropriate antigen, they can be stimulated to reenter the cell cycle (at G₁) and proceed on to new rounds of alternating S phases and mitosis.

G₀ represents not simply the absence of signals for mitosis but an active repression of the genes needed for mitosis. Cancer cells cannot enter G₀ and are destined to repeat the cell cycle indefinitely.

4 March 2003