Genetics in three easy lessons
1. What is a gene?
Our bodies are
made up of cells, each containing a nucleus of DNA (Deoxyribonucleic acid). This
huge molecule resembles a sort of spiral ladder, a double helix whose parts twist
and overlap. It is divided between 23 pairs of complementary chromosomes–one inherited
from the father and one from the mother in each pair.
This DNA “spiral” contains about three billion “bar codes”, which consist of four
different bases–adenine (A in red), thymine (T in blue), cytosine (C in green) and
guanine (G in yellow)–which are always linked with each other in the same way as
base pairs (A with T and C with G).
About 95 per cent of the DNA in the nucleus has no known function, while the remaining
5 per cent contains some 100,000 genes. Pieces of DNA, which are so minute that they
cannot be seen by a microscope, are composed of several thousands of “bar codes”.
The way the four base-pairs are strung together is a sort of coded message: by interpreting
this code, and switching particular genes on and off, the cells manufacture the proteins
which make us what we are.
2. The making of Dolly
There are several methods
of cloning. But until the birth of the cloned sheep, Dolly, in July 1996, it was
necessary to use test-tube embryos derived from an encounter between an egg and a
spermatozoid. The embryos were divided in two and each half implanted inside a surrogate
mother to obtain two clones. The creation of Dolly was revolutionary because it did
not require using a “normal” embryo (one made by an egg and a sperm). The famous
sheep of the 1990s was born from the “marriage” of an egg cell from which the nucleus
had been removed and an adult cell taken from the sheep to be cloned.
3. Correcting a genetic anomaly
More than 4,000 genetic
diseases are responsible for a third of all infant deaths in the developed countries.
When “defective” genes are spotted, an attempt to “repair” them can be made using
genetic engineering. Still in its infancy, this technique involves injecting “healthy”
genes into diseased cells. But because of the minute size of cells and genes, it
is impossible to do this “by hand” like in a normal surgical operation. So scientists
use vectors–deactivated viruses or retroviruses (a special kind of virus). These
carriers of “good” genes can penetrate the targeted cells of the patient by themselves.
But however promising these techniques may be for the future, none is effective in
treating disease at present.
