The cell cycle and mitosis

In a nutshell

Eukaryotic cell division occurs through a highly regulated cycle called the cell cycle. The cell cycle is a series of events which involves periods of cell growth, replication of DNA, mitosis and cell division (cytokinesis). However, the cell cycle can be disrupted and cancerous tumours can develop as a result of this. Prokaryotic cells can also divide, some do so through binary fission. Viruses also divide, however, they require a host.

The cell cycle

Some cells retain the ability to divide and they do so by following a cell cycle. The cell cycle is a series of events which involves periods of cell growth, replication of DNA, mitosis and cell division (cytokinesis). A mammalian cell will typically complete a cell cycle every $24\ hours$ and $90\%$ of this time will be spent in interphase. Interphase is known as the resting phase as no division takes place. It is made up of the G1, S and G2 phases. The cell cycle is described below.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; The cell cycle and mitosis — A. Division, B. Interphase, 1. Mitosis, 2. Cytokinesis

Phase of the celL cycle	Events within the phase
G1 phase (gap 1 phase- also called the growth phase)	During the G1 phase, the cell grows bigger and the organelles duplicate. Transcription occurs during G1 and enzymes are produced that are needed to replicate the DNA in the S phase. The tumour suppressor gene ( $p53$ ) controls this phase.
S phase (synthesis phase)	The DNA replicates in the S phase. Once duplicated, the chromosomes form identical sister chromatids. Note: This phase is short to reduce the chance of spontaneous mutations.
G2 phase (gap 2 phase)	Cellular growth occurs during the G2 phase.
M phase (mitosis phase)	During the M phase, the cell growth stops and mitosis occurs.
Cytokinesis	Cytokinesis is cytoplasmic division. During this phase, the cytoplasm divides to form two daughter cells.

Mitosis

Mitosis is a type of cell division in which a parent cell divides and produces two genetically identical daughter cells. This means that they contain the same DNA as the parent cell. Mitosis is required for growth and repair of multicellular organisms.

Note: In the event of a mutation, the DNA of the daughter cells will not be the same as the parent cell.

Interphase occurs before mitosis, and it is during this stage that the DNA is replicated.

The stages of mitosis are explained below.

Stage	Description	Diagram
Prophase	During prophase, the chromosomes condense (become shorter and thicker) and become visible. The nuclear envelope and nucleolus break down which leaves the chromosomes free in the cytoplasm. The centrioles move to opposite poles of the cell. The spindle fibres then begin to form between the centrioles. *Note:* There are no centrioles in plant cells but the spindle fibres do form.
Metaphase	During metaphase, the chromosomes (made of two sister chromatids that are both identical to the parent DNA) attach to the spindle by their centromeres. This means that the chromosomes line up at the equator of the cell.
Anaphase	During anaphase, the centromeres split which separates the two sister chromatids of each chromosome. This occurs when the spindle fibres contract and pull the chromatids by their centromeres. The chromatids separate and move to opposite poles (ends) of the cell.
Telophase	During telophase, the chromosomes uncoil (become long, thin chromatin again) and the nuclear envelope reforms around each group of chromosomes. This forms two separate nuclei. Nucleoli reform in each nucleus and the spindle breaks down. *Note:* Telophase is the opposite of prophase.
Cytokinesis	During cytokinesis the cytoplasm divides. In animal cells, the cell surface membrane constricts around the centre of the cell which forms two new cells. Plants cells synthesise a new cell plate between the two new cells.

Calculating the length of each phase

You can calculate an estimate of the amount of time spent in each phase of mitosis.

Example

You view a sample of $150$  cells undergoing mitosis. Of these $150$  cells, $15$  cells are in metaphase. One complete cell cycle lasts $24\ hours$ . What is the total time spent in metaphase?

So, $15$ out of $150$ cells are in metaphase at one point in time so metaphase must be $\frac{15}{150}$ of the cell cycle.
The total cell cycle is $24\ hours$ which is the equivalent of $1440\ minutes$ .
This means that cells spend $\frac{15}{150} \times 1440 = 144\space minutes$  in metaphase.

$\underline{Therefore,\ the\ total\ time\ spent\ in\ metaphase\ is\ 144\ minutes.}$

Uncontrolled cell division

The cell cycle is regulated by two types of genes. Damage or mutation to the genes that control the cell cycle and mitosis can result in uncontrolled cell division. This uncontrolled division forms a tumour which keeps dividing. The tumour can become cancerous if it changes from benign to malignant. Malignant tumours divide rapidly and are more likely to be life-threatening, whereas, benign tumours divide more slowly so are less likely to be life-threatening.

Cancer treatments

Cancer treatments often target and disrupt the cell cycle which, in turn, prevent cell division and stop cancer growth. Drugs can be used to prevent DNA replication by inhibiting the synthesis of enzymes needed for the process. They can also interfere with spindle formation to inhibit metaphase.

However, these chemotherapy drugs can not distinguish between the tumour cells and the normal cells and will also kill normal body cells that are dividing. Tumour cells divide more frequently so they are more likely to be killed than normal body cells. A particular exception is the hair-producing cells that divide rapidly and are therefore prone to damage. This often causes hair loss in cancer patients undergoing chemotherapy.

Binary fission

Prokaryotic cells replicate by binary fission which means that the cell replicates its genetic material and then splits into two daughter cells. The circular DNA replicates once and plasmid(s) replicate multiple times.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; The cell cycle and mitosis

1.	Original cell.
2.	The cell gets bigger and the circular DNA moves to opposite poles (opposite ends) of the cell.
3.	The cell membrane grows between the two DNA molecules and the cytoplasm divides.
4.	Two daughter cells are produced that contain one copy of the circular DNA and differing numbers of the plasmid(s).

Viral replication

Viruses are acellular, non-living particles so they can’t undergo cell division. They have nucleic acids (DNA or RNA) as their genetic material, but they can only reproduce inside the cells of other organisms. These cells are known as host cells. The virus has a protein coat called a capsid. The viral capsid has attachment proteins on its surface which allow the virus to attach to the host cell. They will inject their nucleic acid into the host cell and the host cell will then start to produce viral components which can be assembled into new viruses.