Biomedical Engineering Reference
In-Depth Information
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 them-
selves, 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 fragment and so a copy of this transposon is transferred leaving the origi-
nal behind. Transposition is widespread and occurs in virtually all organisms for
which evidence 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 (ISs), which are short pieces of DNA of a defined
and limited range of sequences. They are often found at either end of a trans-
posable element. Their presence enables various DNA rearrangements to take
place leading to moderation of gene expression. Taking together the reorganisa-
tion of DNA within all types of organisms attributable to transposable elements
and IS, with transfer of DNA between organisms by plasmids and transforma-
tion, in the case of prokaryotes, 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
of the genome and therefore operating under different parameters affecting gene
regulation. Transfer of genes across wide taxonomic gaps is made possible by
the mobile nature of ECEs, many of which may cross species barriers often
resulting in the insertion of all or part of the ECE into the recipient genome.
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