3.7 Mitosis

Cell division can take place either by mitosis or meiosis:

-          Mitosis produces two daughter cells that have the same number of chromosomes as the parent call and each other.

-          Meiosis produces four daughter cells, each with half the number of chromosomes of the parent cell.

Mitosis is division of a cell that results in each of the daughter cells having an exact copy of the DNA of the parent cell. Except in the rare event of a mutation, the genetic makeup of the two daughter nuclei is also identical to that of the parent nucleus. Mitosis is also preceded by a period during which the cell is not dividing. This period is called interphase. It is a period od considerable cellular activity that includes a very important event, the replication of DNA. The two copies of DNA after replication remains joined at a place called the centromere. Although mitosis is a continuous process, it can be divided into four stages.

In prophase, the chromosomes first become visible, initially as long thin threads, which later shorten and thicken. Animal cells contain two cylindrical organelles called centrioles, each of which moves to opposite ends (called poles) of the cell. From each of the centrioles, spindle fibres develop, which span the cell from pole to pole. Collectively these spindle fibres are called the spindle apparatus. As plant cells lack centrioles but do develop a spindle apparatus, centrioles are clearly not essential to spindle fibre formation. The nucleolus disappears and the nuclear envelope breaks down, leaving the chromosomes free in the cytoplasm of the cell. These chromosomes are drawn towards the equator of the cell by the spindle fibres attached to the centromere.

By metaphase the chromosomes are seen to be made up of two chromatids. Each chromatid is an identical copy of DNA from the parent cell. The chromatids are joined by the centromere. It is to this centromere that some that some microtubules from the poles are attached, and the chromosomes are pulled along the spindle apparatus and arrange themselves across the equator of the cell.

In anaphase, the centromeres divide into two and the spindle fibres pull the individual chromatids making up the chromosome apart. The chromatids move rapidly to their respective, opposite poles of the cell and we now refer to them as chromosomes. The energy for the process is provided by the mitochondria, which gather around the spindle fibres. If cells are treated with chemicals that destroy the spindle, the chromosomes remain at the equator, unable to reach the poles.

In this stage, the chromosomes reach their respective poles and become longer and thinner, finally disappearing altogether, leaving only wide spread chromatin. The spindle fibres disintegrate and the nuclear envelope and nucleolus reform. Finally the cytoplasm divides in a process called cytokinesis.

Cell division in prokaryotic cells takes place by a process called binary fission. :

-          The circular DNA molecule replicates and both copies attach to the cell membrane

-          The plasmids also replicate

-          The cell membrane begins to grow between two DNA molecules and begins to pinch inward, dividing the cytoplasm into two

-          A new cell wall forms between the two molecules of DNA, dividing the original cell into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids

As viruses are non-living, they cannot undergo cell division. Instead they replicate by attaching to their host cell with the attachment proteins on their surface. They then inject their nucleic acid into the host cell. The genetic information on the injected viral nucleic acid then provides the ‘instructions’ for the host cell’s metabolic processes to start producing the viral components, nucleic acid, enzymes and structural proteins, which are then assembled into new viruses.

Mitosis is important in organisms as it produces daughter cells that are genetically identical to the parent cells. There are three reasons why it is essential to make exact copies of existing cells:

-          Growth. When two haploid cells fuse together to form a diploid cell, it has all the genetic information needed to form the new organism. If the new organism is to resemble its parents, all the cells that grow from this original cell must be genetically identical, mitosis ensures that this happens.

-          Repair. If cells are damaged or die it is important that the new cells produced have an identical structure and function to the ones that have been lost.

-          Reproduction. Single celled organisms divide by mitosis to give two new organisms. Each new organism is genetically identical to the parent organism.