A Code for All Life
The principle of hereditary transmission is a central tenet of
life on earth: all organisms inherit a structural and functional
organization from their progenitors. What is inherited
by an offspring is not necessarily an exact copy of the parent
but a set of coded instructions that gives rise to a certain
expressed organization. These instructions are in the form of
genes, the fundamental units of inheritance. One of the
great triumphs of modern biology was the discovery in 1953
by James Watson and Francis Crick of the nature of the
coded instructions in genes. This was followed by the discovery
of the way in which the code is translated into the
expression of characteristics. The genetic material (deoxyribonucleic
acid, DNA) is composed of nitrogenous bases
arranged on a backbone of sugar phosphate units. The
genetic code lies in the linear order or sequence of bases in
the DNA strand.
Because the DNA molecules replicate themselves in
their passage from generation to generation, genetic variations
can persist once they have happened. Such molecular
alterations, called mutations, are the ultimate source of biological
variation and the raw material of evolution.
A basic principle of modern evolutionary
theory is that organisms attain their
diversity of form, function, and behavior
through hereditary modifications of
preexisting lines of ancestors. It means
that all known lineages of plants and
animals are related by descent from
common ancestral groups.
Heredity establishes the continuity
of life forms. Although offspring and
parents in a particular generation may
look different, there is nonetheless a
basic sameness that runs from generation
to generation for any species of
plant or animal. In other words, “like
begets like.” Yet children are not precise
replicas of their parents. Some of
their characteristics show resemblances
to one or both parents, but they also
demonstrate many traits not found in
either parent. What is actually inherited
by an offspring from its parents is a
certain type of germinal organization
(genes) that, under the influence
of environmental factors, guides the
orderly sequence of differentiation of a
fertilized egg into a human being, bearing
the unique physical characteristics
as we see them. Each generation hands
on to the next the instructions required
for maintaining continuity of life.
The gene is the unit entity of inheritance,
the germinal basis for every
characteristic that appears in an organism.
The study of what genes are and
how they work is the science of genetics.
It is a science that deals with the
underlying causes of resemblance, as
seen in the remarkable fidelity of reproduction,
and of variation, which is the
working material for organic evolution.
Genetics has shown that all living forms
use the same information storage, transfer,
and translation system, and thus it
has provided an explanation for both
the stability of all life and its probable
descent from a common ancestral form.
This is one of the most important unifying
concepts of biology.