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2.5 – Cell Division

2.5.1 - Outline the stages in the cell cycle, including interphase (G₁, S, G₂), mitosis and

cytokinesis

Interphase is when the DNA replicates.

The cell will also replicate its

centrosome, which is important for

movement of chromosomes. This is split

into three stages:

G₁ is the first stage, and stands for

Gap 1. During this time, the

cytoplasm is still active, and the cell

continues with its normal

functions, such as protein synthesis, mitochondria replication or chloroplast

replication. There is all the activity of a growing cell.

S is the synthesis phase when the DNA is replicated. The mass of the DNA in the cell

doubles. All the chromosomes are copied and form chromatids. These remain

attached until they divide in mitosis.

G₂ is the third stage, standing for Gap 2, when there is more growth of the cell, then

preparation takes place for cell division.

Mitosis then happens. This consists of four stages: prophase, metaphase, anaphase and

telophase. The chromosomes are separated and distributed.

Cytokinesis is the division of the cytoplasm to form two daughter cells. The cell cycle is then

repeated.

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2.5.2 - State that tumours (cancers) are the result of uncontrolled cell division and that

these can occur in any organic tissue

Tumours, or cancers, are cell mass formed as a result of uncontrolled cell division. They can

occur in any tissue.

In a tumour, the normal repressed state of mitosis is disrupted by mutation to the proto-

oncogene. As a result, the cells begin to divide uncontrollably. The proto-oncogene mutates

into the oncogene, resulting in the loss of control of cell division.

The cells form an irregular mass of cells; the tumour. Some cells may break away and form a

secondary tumour elsewhere. Eventually they take over the surrounding, healthy cells,

which leads to malfunction and death.

It is caused by damage to DNA chromosomes. The accumulation of mistakes in DNA causes

cancer, which is why it is more common in older people. Another cause is damage to the

gene that codes for p53, the protein which stops the copying of damaged DNA.

The damage to the DNA can result from ionising radiation (X-rays, gamma rays...), some

chemicals (tar in tobacco smoke) as well as virus infections. Some factors are also inherited.

The development of cancer requires at least two mutations; one of the proto-oncogene;

two of the tumour suppressor.

Cancer exerts its deleterious effect on the body by destroying the adjacent tissues (such as

compressing nerves, eroding blood vessels), replacing normal functioning cells (such as

replacing blood forming cells in the bone marrow or the heart muscles so that the heart

fails).

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2.5.3 - State that interphase is an active period in the life of a cell when many metabolic

reactions occur, including protein synthesis, DNA replication and an increase in the

number of mitochondria and/or chloroplasts

This is always the longest part of the cell cycle. During interphase, the

nucleus undergoes many changes. The chromosomes disperse as chromatin

and become actively involved in protein synthesis. Copies of the information

in particular genes or groups of genes are taken from the chromosomes for

use in the cytoplasm. Proteins are assembled in the ribosomes by combining

amino acids in sequences dictated by the information from the gene.

The synthesis of new organelles takes place in the cytoplasm during interphase. There is

intense biochemical activity in the cytoplasm and the organelles, and there is an

accumulation of stored energy before nuclear division occurs again.

Also, each chromosome replicates into the two identical structures called chromatids. These

remain attached until they divide in mitosis.

In this time, the cell itself will continue to carry out its specialised function. The length of

interphase varies between cell types. After cytokinesis, G1 occurs: when various proteins are

synthesised to allow the cell to specialise. Then, in the S stage, the DNA is replicated. G2 is

the preparation stage for mitosis. Mitochondria (and chloroplasts in plants) are replicated.

2.5.4 - Describe the events that occur in the four phases of mitosis (prophase, metaphase,

anaphase and telophase)

During mitosis, the chromatids are separated and distributed to two daughter nuclei.

Mitosis is a continuous process with no breaks in it, although we divide it into four stages.

Prophase

The chromosomes become visible as long, thin threads. They shorten and thicken through

the process of supercoiling. In supercoiling, DNA is combined with histone proteins and

non-histone proteins to form the readily stainable chromatin. The genes must be left in

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predictable positions and a distinctive overall chromosome shape. The

supercoiling makes the structure so dense that it can be seen with a light

microscope during mitosis.

At the end of prophase, it is possible to see that the two chromatids are held

together at the centromere. During this time, the nucleolus disappears and

the nuclear membrane breaks down.

Metaphase

The centrioles move to opposite ends of the cell. Microtubules in the

cytoplasm start to form a spindle, radiating out from the centrioles.

These attach to the centromeres and are arranged at the equator of

the spindle. In plant cells, the same structure is formed, but without

the presence of centrioles.

Anaphase

The centromeres divide, the spindle fibres shorten, and the chromatids

are pulled by their centromere to opposite poles. Once separated, they

are referred to as chromosomes.

Telophase

The nuclear membrane reforms around both groups of chromosomes

at opposite ends of the cell. They begin to decondense and become

chromatin again. The nucleolus reforms. Interphase then follows the

division of the cytoplasm.

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2.5.5 - Explain how mitosis produces two genetically identical nuclei

Cell division produces genetically identical daughter cells. They have a set of chromosomes

identical to each other and to the parent cell from which they were formed.

An exact copy of each chromosome is made by accurate replication during interphase.

There is conservation of the chromosome number, so the chromosome numbers of the

daughter cells are exactly the same as each other and the parent cell.

The chromatids remain attached by their centromeres during metaphase, when each

becomes attached to a spindle fibre at the equator of the spindle.

Centromeres then divide during anaphase, and one copy of each chromosome moves to

each pole of the spindle. The chromosomes then form two new nuclei when the cell splits

into two, each with an exact copy of the original nucleus.

Hence, the two nuclei from each new cell are genetically identical.

2.5.6 - State that growth, embryonic development, tissue repair and asexual reproduction

involve mitosis

In the growth and development of an embryo, it is very important that all cells carry the

same genetic information as the existing cells from which they are formed, and with which

they share surrounding cells or tissues. Multicellular organisms increase their size through

growth, which involves increasing the number of cells through mitosis. These cells can then

differentiate and specialise their function. In the same way, when repair of damaged or

worn out cells occurs, they are exact copies of the ones they replace. If this was not the

case, different body part would begin working to conflicting blueprints, resulting in chaos.

This form of cell division is also the basis of asexual reproduction, in which the offspring

produced are identical to the parent. In other words, they are clones. This is very common

in nature.