Monday, 6 July 2015


 The early development of genetics

For the first 30 years of its life this new science grew at an astonishing rate. The idea
that genes reside on chromosomes was proposed by W. Sutton in 1903, and received
experimental backing from T.H. Morgan in 1910. Morgan and his colleagues then
developed the techniques for gene mapping, and by 1922 had produced a comprehensive
analysis of the relative positions of over 2000 genes on the 4 chromosomes of the
fruit fly, Drosophila melanogaster.
Despite the brilliance of these classical genetic studies, there was no real understanding
of the molecular nature of the gene until the 1940s. Indeed, it was not until
Gene Cloning and DNA Analysis: An Introduction. 6th edition. By T.A. Brown. Published 2010 by
Blackwell Publishing.
the experiments of Avery, MacLeod, and McCarty in 1944, and of Hershey and Chase
in 1952, that anyone believed that deoxyribonucleic acid (DNA) is the genetic material:
up until then it was widely thought that genes were made of protein. The discovery
of the role of DNA was a tremendous stimulus to genetic research, and many famous
biologists (Delbrück, Chargaff, Crick, and Monod were among the most influential)
contributed to the second great age of genetics. In the 14 years between 1952 and 1966,
the structure of DNA was elucidated, the genetic code cracked, and the processes of

transcription and translation described.

The advent of gene cloning and the polymerase

chain reaction

These years of activity and discovery were followed by a lull, a period of anticlimax
when it seemed to some molecular biologists (as the new generation of geneticists styled
themselves) that there was little of fundamental importance that was not understood.
In truth there was a frustration that the experimental techniques of the late 1960s were
not sophisticated enough to allow the gene to be studied in any greater detail.
Then in the years 1971–1973 genetic research was thrown back into gear by what at
the time was described as a revolution in experimental biology. A whole new methodology
was developed, enabling previously impossible experiments to be planned and
carried out, if not with ease, then at least with success. These methods, referred to as
recombinant DNA technology or genetic engineering, and having at their core the process
of gene cloning, sparked another great age of genetics. They led to rapid and
efficient DNA sequencing techniques that enabled the structures of individual genes to
be determined, reaching a culmination at the turn of the century with the massive
genome sequencing projects, including the human project which was completed in 2000.
They led to procedures for studying the regulation of individual genes, which have
allowed molecular biologists to understand how aberrations in gene activity can result
in human diseases such as cancer. The techniques spawned modern biotechnology,
which puts genes to work in production of proteins and other compounds needed in
medicine and industrial processes.
During the 1980s, when the excitement engendered by the gene cloning revolution
was at its height, it hardly seemed possible that another, equally novel and equally
revolutionary process was just around the corner. According to DNA folklore, Kary
Mullis invented the polymerase chain reaction (PCR) during a drive along the coast
of California one evening in 1985. His brainwave was an exquisitely simple technique
that acts as a perfect complement to gene cloning. PCR has made easier many of the
techniques that were possible but difficult to carry out when gene cloning was used on
its own. It has extended the range of DNA analysis and enabled molecular biology to find
new applications in areas of endeavor outside of its traditional range of medicine, agriculture,
and biotechnology. Archaeogenetics, molecular ecology, and DNA forensics
are just three of the new disciplines that have become possible as a direct consequence
of the invention of PCR, enabling molecular biologists to ask questions about human
evolution and the impact of environmental change on the biosphere, and to bring their
powerful tools to bear in the fight against crime. Forty years have passed since the dawning
of the age of gene cloning, but we are still riding the rollercoaster and there is no
end to the excitement in sight.