Evolution and Genetic Engineering - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 709

Another method of transfer, which is far more specific with respect to the genes that are
transferred, is integration transduction. Here, the phage incorporates into specific sites in
the chromosome, where the phage's DNA is integrated into the host's DNA. The frequency
of transduction of a gene is related to its distance away from the site of incorporation. The
process is summarized by the upper process shown in Fig. 14.4 . A lysogenic cell is one
carrying a prophage or phage DNA incorporated into chromosomal DNA. Phage is an

example of such a temperate phage (a phage that can either lyse a cell or become incorporated
into the chromosome). Such phages almost invariably insert at a specific site in the chromo-
some. The conversion of a prophage (the phage DNA in the chromosome) into the lytic cycle
is normally a rare event (10 4 per cell division), but it can be induced in almost the whole
culture upon exposure to UV light or other agents that interfere with DNA replication.

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14.3.4. Episomes and Conjugation
A third type of gene transfer is analogous to reproduction sextually as contact of two intact
cells is the key. The gene transfer involves another genetic element: episome , which is a DNA
molecule that may exist either integrated into the chromosome or separate from it. When
episome exists separately from the chromosome (extrachromosomally), it is essentially
a plasmid. A well-known episome is the F or fertility factor. Such factors are responsible
for the process known as conjugation .
Most experiments with conjugation are conducted with the F factor, which is present in
low copy number. Direct cell-to-cell contact is required. This DNA molecule encodes at least
13 genes involved in its self-transfer from one cell to another.
In a population of E. coli , there are frequently some cells with the F plasmid, which are
termed F þ (male). Other cells are F (female). F þ cells encode proteins to make a sex pilus .
When F þ and F cells are mixed together, the sex pilus connects an F þ to an F cell. The
sex pilus may act as a conduit for the transfer of a copy of the F plasmid.
This process is normal and does not involve transfer of chromosomal genes or recombina-
tion. A rarer event is when the F plasmid has been integrated into the chromosome itself to
form a single, large, circular molecule. Thirteen sites for integration are known. Such cells are
termed Hfr (for high-frequency recombination).
When transfer is initiated, the F plasmid moves not only itself, but also the attached chro-
mosome, to the recipient cell. The time required to transfer a whole E. coli chromosome is
100 min. If contact between the two cells is broken during the transfer process, only a propor-
tional amount of the chromosome can be transferred. Since the transfer starts at a known
point, Hfr cells can be used to map the location of genes on the chromosome. This technique
for gene mapping is being replaced by methods for directly sequencing nucleotide sequences
in DNA. If F þ and F cells differ in properties (e.g. the ability to make lysine), conjugation can
be used to alter the properties of the F cell.
Conjugation, transduction, and transformation all represent forms of gene transfer from
one cell to another. However, gene transfer occurs within a cell.

14.3.5. Transposons: Internal Gene Transfer
We discussed the presence of insertion elements on the chromosome, e.g. integration trans-
duction. A closely related phenomenon is a transposon , which refers to a gene or genes that