Cambridge International A Level Biology Revision Guide

Chapter 19: Genetic technology
■■
In parts of the world where a lot of genetically modified
crops are grown, there is a danger of losing traditional
varieties with their desirable background genes for
particular localities and their possibly unknown traits
that might be useful in a world where the climate is
changing. This requires a programme of growing and
harvesting traditional varieties and setting up a seed
bank to preserve them.
Despite these concerns, there are now millions of hectares
of genetically modified crops and trees growing across
the world. In the USA in 2011, half the cotton crop and
more than half the maize and soya crops were genetically
modified. Significant areas of China, Brazil and India are
used for these crops, and farmers in developing countries
are adopting the products of gene technology with
enthusiasm. The exception is Europe, with its careful, but
strict, controls. But Europe also has well-organised groups
of protesters. Almost all of the field trials of genetically
modified crops that have taken place in the UK during the
last ten years have been vandalised (Figure 19.25).
But are there any damaging effects on human
societies of genetic technology? Have any of the theoretical
hazards had an actual effect on human societies?
There is little evidence of genes ‘escaping’ into the
wild. No ‘superweed’ has appeared to reduce crop growth.
There are no examples of foods produced from genetically
modified organisms unexpectedly turning out to be toxic
or allergenic. Unless the known effects of genetically
modified crops become much greater than have so far been
measured, the effect on human societies may be said to be
small, but positive. There are, though, possible effects that
cannot yet be measured, such as the future consequences
of any loss of biodiversity from growing genetically
modified crops.
485
Figure 19.25 This genetically modified maize, growing in
Shropshire in the UK, is protected by an electric fence.
Summary
■■
■■
Genetic technology involves using a variety of
techniques to investigate the sequence of nucleotides in
DNA and alter an organism’s DNA. Genetic engineering
involves the extracting of genes from one organism
and placing them into the DNA of another to form
recombinant DNA (rDNA). The gene(s) need to be
inserted in such a way that they will be expressed in the
genetically modified organism (GMO).
Restriction enzymes cut across DNA at specific sites,
known as restriction sites: these can be staggered cuts
that give rise to short lengths of unpaired bases known
as sticky ends or straight cuts to give blunt ends. Pieces
of DNA with sticky ends that are complementary to each
other are able to join together by forming hydrogen
bonds. The enzyme ligase joins the sugar–phosphate
backbones of pieces of DNA.
■■
■■
■■
In genetic engineering, vectors are used to carry pieces
of DNA into cells: typical examples are plasmids, viruses
and liposomes. Plasmids are small circles of doublestranded
DNA; they are useful for genetic engineering
because they can be cut with restriction enzymes
and have promoters and gene markers (e.g. genes for
antibiotics, GFP or GUS) inserted into them alongside
the gene(s) to transform the host cell. A promoter must
be inserted alongside the gene because organisms
will not transcribe and express a gene unless there is a
binding site for RNA polymerase.
Cells that have taken up plasmids with the desired gene
can be identified by detecting fluorescence (GFP) or
appropriate staining (GUS).
Lengths of DNA for genetic modification can be
synthesised directly from mRNA by using the enzyme
reverse transcriptase. Specific lengths of DNA can also
be synthesised from nucleotides using knowledge of the
genetic code.