CELL DIVISION THE TRANSFERENCE OF GENS

The transference of genes

This section discusses how genetic information is transferred from cells to new cells, and also
from parents to children. The first thing to do is to look at how cells pass on genetic information
to new cells.
In order for the body to grow, and also for the replacement of body cells that have died, our cells
must be able to reproduce themselves, but in order for genetic information not to be lost,
they must be able to reproduce themselves accurately. They do this by cloning themselves.
In some prokaryotic organisms this occurs by binary fission, whereby the nucleus in a single
cell becomes elongated and then divides to form two nuclei in the same cell, each of which
carries identical genetic information. The cytoplasm then divides in the middle between the
two nuclei, and so two identical daughter cells result, each with its own nucleus and other
essential organelles.
However, humans, being much more complicated, have eukaryotic cells, which divide by
means of cell division, whereby the division of the nucleus occurs first of all, after which the division
of cytoplasm (known as cytokinesis) takes place. After this division, the new cells will grow
until they reach a stage when the process can be repeated.
Within this process of cell division, the process of transference of genes (or reproduction of
cells carrying genetic information) is divided into two stages: mitosis and meiosis.

Mitosis

This section commences by looking at the way that cells reproduce, particularly how they reproduce
their genetic material.
In humans, cell reproduction takes place using a complex process called mitosis, in which
the number of chromosomes in the daughter cells has to be the same as in the original
parent cell.
In the figures below, only a few of the chromosomes are depicted in order to improve the
clarity of the figures.
Mitosis can be divided into four stages:
•• prophase
•• metaphase
•• anaphase
•• telophase.
Before and after it has divided, the cell enters a stage known as interphase until the time comes
for the next cell reproduction.

Interphase

Mitosis begins with interphase. This was often thought to be a resting period for the cell, but
we now know that the cell is actually very busy during this period getting ready for replication.
If we look at the cell cycle and suppose that one full cycle represents 24 hours, then the actual
process of replication (mitosis) would only last for about one of those 24 hours (Figure 3.9).
During the rest of the time the cell is undertaking DNA synthesis (i.e. producing DNA). During
this period of interphase the cell has to produce two of everything, not just DNA, but all the
other organelles in the cell (see Chapter 2), such as the mitochondria. In addition, the cell has to
go through the process of obtaining and digesting nutrition so that it has the raw materials for
this duplication and also for the energy that will power the various functions of the cell

interphase, the chromosomes in the nucleus are very difficult to see because they are

in the form of long threads. They need to be in this state to make it easier for them to be duplicated.
During the process of duplication, the cells have to ensure that there will be sufficient and
accurate genetic material for each of the two ‘daughter cells’. The strands of DNA separate and
reattach to new strands of DNA. Because of the selectivity of the bases as to which other base
they are able to join in this process, an exact replication of the DNA occurs (Figure 3.5).
In addition, extra cell organelles are manufactured or produced by the replication of existing
organelles. Also during interphase, the cell builds up a store of energy, which is required for the
process of division.

Prophase

The first stage after interphase is prophase. During prophase, the chromosomes become
shorter, fatter and more visible. Each chromosome now consists of two chromatids, each containing
the same genetic information (in other words, the DNA has exactly replicated itself during
interphase). These two chromatids are joined together at an area known as the centromere.
The two centrosomes move to opposite ends of the cell (the poles) and are joined together by
the nuclear spindle, which stretches from end to end (or pole to pole) of the cell. The centre of
the cell is now called the equator. Finally, the nucleolus and nuclear membrane disappear,
leaving the chromosomes within the cytoplasm.

Metaphase

During metaphase, the 46 chromosomes (two of each of the 23 chromosomes) each consisting
of two chromatids move to the equator of the nuclear spindle, and here they become attached
to the spindle fibres.

Anaphase

During anaphase, the chromatids in each chromosome are separated, and one chromatid
from each chromosome then moves towards each pole of the spindle.

Telophase

There are now 46 chromatids at each pole, and these will form the chromosomes of the daughter
cells. The cell membrane constricts in the centre of the cell, dividing it into two cells. The nuclear
spindle disappears, and a nuclear membrane forms around the chromosomes in each of the
daughter cells. The chromosomes become long and thread-like again.

Cell division

Cell division is now complete (Figure 3.10) and the daughter cells themselves enter the interphase
stage in order to prepare for their replication and division.
This process of cell division explains how we grow by producing new cells as well as replacing
old, damaged and dead cells.

Meiosis

Whereas mitosis is concerned with the reproduction of individual cells, meiosis is concerned
with the development of whole organisms (e.g. human beings).
The reproduction of a human being depends upon the fusion of reproductive cells (known as
gametes) from each of the parents. These gametes are:
•• spermatozoa (sperm) from the male;
•• ova (eggs) from the female.
Each cell of the human body contains 23 pairs of chromosomes (i.e. 46 in total). It is very
important that during the process of human reproduction the cell formed when the gametes
fuse has the correct number of chromosomes for a human being (23 pairs). Therefore, each
gamete must possess only 23 single chromosomes, because when gametes fuse during
reproduction all their chromosomes remain intact in the new life form. If each gamete had a
full complement of 46 chromosomes, then the resulting fused cell would possess 92 chromosomes
– or four copies of each chromosome rather than the two that a human cell should possess.
From then on, each succeeding generation would have double the number of chromosomes,
so that after several generations humans would have cells that possess millions and millions
of copies of the 23 chromosomes. To stop this happening, the gametes only possess one
copy of each chromosome, so that the resulting fused cell has 46 chromosomes, like
the parents.
Now you have two new terms to learn and understand: diploid and haploid cells.
•• Diploid cell: a cell with a full complement of 46 chromosomes (i.e. 23 pairs).
•• Haploid cell: a cell with only half that number of chromosomes (i.e. 23 single
chromosomes).
Gametes are therefore haploid cells, because they only possess one copy of each chromosome,
while all other cells of the body are diploid cells.
Gametes actually develop from cells with 46 chromosomes, and it is through the process
of meiosis that they end up with just