Cambridge International A Level Biology Revision Guide

Chapter 19: Genetic technology
Crispr
Until recently, genetic technology has had to rely on
inaccurate methods of editing DNA. For example,
a modified virus can be used to insert DNA into the
human genome, but we have had no control about
just where the DNA is inserted. It might be inserted
in the middle of another gene, with unpredictable
consequences.
A technique called Crispr is changing this. Crispr
has been developed from a mechanism used by
some bacteria to defend themselves against viruses.
It was first discovered in 1987, but its details were
worked out only in 2012. Since then, it has been used
successfully to edit DNA in a range of organisms,
including human cells. A Crispr-associated (CAS)
enzyme cuts DNA at a point that is determined by
a short strand of RNA. Since the RNA can be made
‘to order’ to match any unique sequence of DNA, the
genetic engineer can dictate precisely where a cut is
made in human DNA to remove a ‘faulty’ allele or
to insert a therapeutic allele as in gene therapy
(Figure 19.1). The genetic engineer now has a tool
kit that can ʻcut and pasteʼ genes with much
greater precision.
Figure 19.1 Crispr makes it possible to insert DNA at a
precisely determined point in a chromosome.
The structure of DNA, and the way in which it codes for
protein synthesis, was worked out during the 1950s and
1960s. Since then, this knowledge has developed so much
that we can change the DNA in a cell, and thereby change
the proteins which that cell synthesises. Not only that, but
we can sequence the nucleotides in DNA and compare
nucleotide sequences in different organisms. It is also
possible to carry out genetic tests to see if people
are carriers of genetic diseases and, in a few cases, use
gene therapy to treat those who have these diseases.
Gene technology has brought huge benefits to many
people, but may have consequences that we cannot foresee.
These technologies raise social, economic and ethical
issues, that we must confront.
Genetic engineering
The aim of genetic engineering is to remove a gene (or
genes) from one organism and transfer it into another so
that the gene is expressed in its new host. The DNA that
has been altered by this process and which now contains
lengths of nucleotides from two different organisms is
called recombinant DNA (rDNA). The organism which now
expresses the new gene or genes is known as a transgenic
organism or a genetically modified organism (GMO).
Recombinant DNA is DNA made by joining pieces from
two or more different sources.
Genetic engineering provides a way of overcoming
barriers to gene transfer between species. Indeed the genes
are often taken from an organism in a different kingdom,
such as a bacterial gene inserted into a plant or a human
gene inserted into a bacterium. Unlike selective breeding,
where whole sets of genes are involved, genetic engineering
often results in the transfer of a single gene.
We will look first at the general principles involved in
genetic engineering and then at some of the techniques in
more detail.
An overview of gene transfer
There are many different ways in which a GMO may be
produced, but these steps are essential.
1 The gene that is required is identified. It may be cut
from a chromosome, made from mRNA by reverse
transcription or synthesised from nucleotides.
2 Multiple copies of the gene are made using the technique
known as the polymerase chain reaction (PCR).
3 The gene is inserted into a vector which delivers the gene
to the cells of the organism. Examples of vectors are
plasmids, viruses and liposomes.
4 The vector takes the gene into the cells.
5 The cells that have the new gene are identified
and cloned.
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