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AN OVERVIEW OF GENETICS
Dr Gideon Louw Biologist
Historical Overview
Should you ever visit the attractive university town of
Cambridge, make sure to enjoy some refreshment at a small
pub known as The Eagle, for it was in this pub that two
brilliant young scientists, James Watson and Francis Crick,
discovered the molecular structure of DNA nearly 50 years
ago. This discovery has proved to be one of the most profound
advances in biology and medicine, as it has led to a far
deeper understanding of how the trillions of cells that
make up our bodies function. Before looking more closely
at DNA we should first examine the chromosomes, which carry
this precious cargo of DNA.
Chromosomes
With very few exceptions, every cell in our bodies contains
a nucleus in which two sets of chromosomes are embedded;
one set donated by each parent, providing 24 distinct chromosomes
in the human. Human females have a pair of X sex-determining
chromosomes, while males have one X and one Y pair of sex
chromosomes. The latter pair often being blamed for the
more "adventurous" behaviour of males. Chromosomes are relatively
large structures and can be examined under an ordinary light
microscope. In this way some hereditary defects, such as
Down's syndrome, can be detected. The cells of a Down's
patient contain a third copy of chromosome 21. Diagnosis
of most hereditary diseases, however, requires far more
sophisticated analysis, including studying DNA sequences
within genes.
DNA
DNA (deoxyribonucleic acid) is a large molecule constructed
in the shape of a double helix, held together by smaller
nitrogenous bases. The analogy of a twisted ladder is often
used to explain this structure. Imagine a ladder that has
been twisted to form a spiral staircase. The sides of the
spiral consist of alternating sugar and phosphate molecules,
while the rungs of the ladder consist of the nitrogenous
bases adenine, thymine, cytosine and guanine. The sequence
of these bases provides the genetic code within our genes
that control the development, growth and functioning of
all the cells in our bodies.
Genes
A gene consists of a specific sequence of nitrogen bases
within a DNA molecule and there are about 30,000 genes in
the complete human genome. The usual way in which genes
control the functioning of our cells is by instructing cells
to make new molecules, usually proteins which in turn alter
cell function in a programmed way. This usually occurs after
a gene has been switched on, either by another gene, or
by a new molecule entering the cell.
The DNA of the activated gene uncoils, exposing the base
code for the synthesis of a new messenger molecule that
incorporates the identical code of the exposed DNA. The
messenger molecule (messenger RNA) then passes out of the
nucleus to enter the body of the cell, where it controls
the synthesis of a new molecule based on the code it is
carrying.
It is difficult to imagine, for example, how many of these
intricate and exquisitely timed reactions must take place
in a human female to ensure successful ovulation, fertilization,
implantation of the fertilized ovum and the development
of a fully formed child from a single cell. It is also no
surprise that sometimes mistakes occur, including mutations,
often with tragic consequences.
The Human Genome Project
On Monday, February 12, 2001, two major research groups
published the results of their respective human genome projects
- two atlases of all the human genes. These results, known
also as The Book of Life, will revolutionize future medical
research because they will allow researches to unravel the
fundamental genetic bases of diseases such as cancer, heart
disease, cystic fibrosis, diabetes and many others.
Drugs can now be targeted to alter specific gene function
instead of the hit and miss techniques currently employed.
The time required to develop new drugs will also be greatly
reduced, possibly from an average of 14 to four years. There
is, however, still much to be done - although we now know
all the letters in this exciting book of life, we still
don't understand the words.
Other interesting facts to emerge from these historical
studies are that our genes are almost identical to those
of all other mammals and that in the case of chimps, we
share 99.9% of their genes. We even share 10% of our genes
with the lowly roundworms and have only twice as many genes
as a fruit fly. The genetic differences between human races
are so small that the very concept of race can be questioned.
These results give greater credence to the theory of evolution
and will advance the philosophy of genetic determinism.
They will also stimulate adventurous experiments in cloning
which are sure to raise many ethical dilemmas.
The huge amount of data included in the American human
genome report is freely available without cost on the website
of the U.S. National Institute of Health.
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