Evolution and Genetic Engineering - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 710

have the ability to “jump” from one piece of DNA to another or to another position indepen-
dently of any homology with the recipient piece of DNA. Transposons differ from insertion
sequences in that they code for proteins. Transposons appear to arise when a gene becomes
bounded on both sides by insertion sequences. Many of the transposons encode antibiotic
resistance.
Transposons are important because (1) they can induce mutations when they insert into
the middle of a gene, (2) they can bring once separate genes together, and (3) in combina-
tion with plasmid- or vital-mediated gene transfer, they can mediate the movement of
genes between unrelated bacteria (e.g. multiple antibiotic resistances on newly formed
plasmids). Transposon mutagenesis can be a very powerful tool in altering cellular
properties.
1 4.4. TECHNIQUES OF GENETIC ENGINEERING
Genetic engineering is also known as recombinant DNA technology. Genetic recombination
is a process of combining genetic elements from two different genomes to form a new genotype
in the absence of mutations, which also occur naturally as discussed in the previous section.
In this section, we will focus on the techniques that are used by the genetic engineers to
manipulate the genetic material for academic, research, and industrial purposes:
14.4.1. Gene Synthesis
The key to genetic engineering is finding the desired gene to be incorporated into the host
to obtain the desired property. It is possible to chemically synthesize a gene in the lab by labo-
riously joining nucleotides together in the correct order. Automated machines can now make
this much easier, but only up to a limit of about 30 bp, so very few real genes could be made
this way (anyway it is usually much easier to make complimentary DNA [cDNA]). The genes
for the two insulin chains and for the hormone somatostatin (42 bp) have been synthesized
this way. It is very useful for making gene probes.
14.4.2. Complimentary DNA
cDNA is a DNA made from mRNA. This makes use of the enzyme reverse transcriptase,
which does the reverse of transcription: it synthesizes DNA from an mRNA template. It is
produced naturally by a group of viruses called the retroviruses (which include HIV), and
it helps them to invade cells.
In genetic engineering, reverse transcriptase is used to make an artificial gene of cDNA
( Fig. 14.5 ). cDNA has helped to solve different problems in genetic engineering. It makes
genes much easier to find. There are some 70,000 genes in the human genome, and finding
one gene out of this many is a very difficult (though not impossible) task. However, a given
cell only expresses a few genes, so only makes a few different kinds of mRNAmolecule. For
example, the B cells of the pancreas make insulin, so make lots of mRNA molecules coding
for insulin. This mRNA can be isolated from these cells and used to make cDNA of the
insulin gene.