5.3 Recombinant dna technology

A number of human diseases are the result of individuals being unable to produce for themselves chemicals which have a metabolic role. Many such chemicals, e.g. insulin and thyroxine, are proteins and therefore the product of a specific portion of DNA. The treatment of such deficiencies previously involved extracting the missing chemical from either an animal or human donor. This has presented problems. While the animal extracts may function effectively, subtle chemical differences in their composition have been detected by the human immune system, which has responded by producing antibodies which destroy the extract. Even chemically compatible extracts from human donors present a risk of infection from other diseases. Whether from animals or humans, the cost of such extracts is considerable.

It follows that there are advantages in producing large quantities of 'pure' chemicals from non-human sources. As a result, methods have been devised for isolating the portion of human DNA responsible for the production of insulin and combining it with bacterial DNA in such a way that the microorganism will continually produce the substance. This DNA, which results from the combination of fragments from two different organisms, is called recombinant DNA.

5.4 Evolution through natural selection (Darwin / Wallace)

Evolution is the process by which new species are formed from pre-existing ones over a period of time. It is not the only explanation of the origins of the many species which exist on earth, but it is the one generally accepted by the scientific world at the present time.

Quite independently, Charles Darwin and Alfred Wallace developed the same theory on the mechanism of evolution. As a result, they jointly presented their findings to the Linnaean Society in 1858. The essential features of the theory Darwin put forward are:

1. Overproduction of offspring

All organisms produce large numbers of offspring which, if they survived, would lead to a geometric increase in the size of any population.

2. Constancy of numbers

Despite the tendency to increase numbers due to overproduction of offspring, most populations actually maintain relatively constant numbers.

3. Struggle for existence

Darwin deduced that members of the species were constantly competing with each other in an effort to survive. In this struggle for existence only a few would live long enough to breed.

4. Variation among offspring

The sexually produced offspring of any species show individual variations so that generally no two offspring are identical.

5. Survival of the fittest by natural selection

Among the offspring there will be some better able to withstand the prevailing conditions. That is, some will be better adapted ('fitter') to survive in the struggle for existence. These types are more likely to survive long enough to breed.

6. Like produces like

Those that survive to breed are likely to produce offspring similar to themselves. The advantageous characteristics that gave them the edge in the struggle for existence are likely to be passed on to the next generation.

7. Formation of new species

Over many generations, the individuals with favourable characteristics will breed, with consequent increase in their numbers. The development of a number of variations in a particular direction over many generations will gradually lead to the evolution of a new species.

The evolutionary theory of Darwin and Wallace is based on the mechanism of natural selection. Let us look more closely at exactly how this process operates.

Selection is the process by which organisms that are better adapted to their environment survive and breed, while those less well adapted fail to do so. The better adapted organisms are more likely to pass their characteristics to succeeding generations. Every organism is therefore subjected to a process of selection, based upon its suitability for survival. The organism's environment exerts a selection pressure. The intensity and direction of this pressure varies in both time and space.

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