Research updates
15 - Ethics and the Gene Map page 3
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3. How is gene sequencing done? Link to the Medical Research Council web site
 
Collecting DNA
As with gene mapping, many different approaches are used in gene sequencing. The difficult bit is obtaining the gene. Once you have the gene, there are machines which can quickly work out its chemical structure, i.e. the sequence of nucleotides. The problem is isolating the gene you are interested in. Here's one simple approach.
Figure 1
Figure 3. Work in the sequencing laboratory at the Sanger Centre, Wellcome Trust Genome Campus, Hinxton

Suppose you want to sequence the genes for making a- and b-haemoglobin. Red blood cells do little else but synthesise these two proteins. So if you isolate the RNA from red blood cells, much of it will either be the messenger RNA that when translated gives a-haemoglobin, or the messenger RNA that when translated gives b-haemoglobin. These two types of mRNA can be manipulated to obtain the original DNA sequence.
 
Unfortunately, this approach only works if you can obtain fairly large amounts of relatively pure messenger RNA. More complicated approaches are having to be used to sequence most of the human genome. Indeed, at least 75% of the human genome doesn't even make mRNA! Some of this DNA has other functions, for example, making ribosomal RNA or being involved in gene regulation. However, some biologists argue that much of it may have no function, being so-called selfish DNA, hitchhiking a ride on the back of the functional DNA. The Human Genome Project will help to sort out whether this is the case or not.
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