60 Environmental Biotechnology
between the genetic material of organisms themselves. Plasmids may be trans-
ferred between bacteria by conjugation, of which there are several types, but
all of which require direct cell to cell contact. Not only are genes transferred
between bacteria on plasmids, but bacteriophages (bacterial viruses) are also
vectors for intercellular transmission. Similarly, eukaryotic viruses are able to
transfer genetic material between susceptible cells. In addition, bacterial cells
may pick up DNA free in the environment under conditions where their cell wall
has become ‘leaky’ to fragments of this macromolecule, a process called trans-
formation. There is also considerable rearrangement of genomic material within
an organism stimulated by the presence of transposons. There are many classes of
transposable elements which are short pieces of DNA, able to excise themselves,
or be excised, out of a genome. Often they take with them neighbouring pieces
of DNA, and then reinsert themselves, sometimes with the assistance of other
genes, into a second site distinct from the original location on the same genome.
Insertion may be into specific sites or random, depending on the nature of the
transposon. Transposition normally requires replication of the original DNA frag-
ment and so a copy of this transposon is transferred leaving the original behind.
Transposition is widespread and occurs in virtually all organisms for which evi-
dence of this process has been sought, both prokaryotic and eukaryotic. The term
‘transposable element’, was first coined by Barbara MacClintock, who discovered
them in maize, publishing her data in the early 1950s. However, it was not until
many years later that the full significance of her work was being recognised, with
similar elements being discovered in bacteria. Transposable elements are known
to promote the fusion of plasmids within a bacterial cell, where more than one
type of plasmid is present. The fusion is stimulated by the presence of insertion
sequences (IS), which are short pieces of DNA of a defined and limited range of
sequences. They are often found at either end of a transposable element. Their
presence enables various DNA rearrangements to take place leading to moder-
ation of gene expression. Taking together the reorganisation of DNA within all
types of organisms attributable to transposable elements and IS, with transfer of
DNA between organisms by plasmids and transformation, in the case of prokary-
otes, and viruses in the case of both prokaryotes and eukaryotes, the potential
for DNA rearrangement within and between organisms is enormous.
It has been proposed (Reanney 1976), that such transfer is far more univer-
sal than had previously been voiced. Transfer of genes by extra chromosomal
elements (ECEs), which is the all-embracing name given to include plasmids
and viruses, models the means by which molecular evolution takes place in the
environment. The proposal is that the evolutionary process occurs principally by
insertions and deletions of the genome such as those caused by the activities
of ECEs and transposable elements and not by point mutations more frequently
observed in isolated cultures such as those maintained in laboratory conditions.
It is further suggested that much of the phenotypic novelty seen in evolution is
the result of rearrangement of existing structural genes into a different region