Scissor chops pieces

Press, 5 March 1988, Page 21

Scissor chops pieces

DNA is a molecular chain made up of four different molecules which appear in repetitive sequences along the chain. The location of the .repeated sequences on the DNA is specific to each individual, which is why DNA is likened to fingerprints.

Testing for DNA ‘/fingerprints” can be done on any cellular material, such as blood, tissue or semen. In the test the cellular material is subjected to enzymes which free the DNA. - I Another enzyme, called a .“molecular scissor," is added and chops the DNA into p eces

wherever it finds a particular repetitive sequence..!

The pieces are then graded into sizes, using a gel which has an electric current running through it, and stuck' to a nylon membrane ,to retain; their relative positions.

A radioactive “probe” is added — the probe is another piece of human DNA which detects and sticks to sequences .which look like itself.

Dr Francois Iris ! likens the probe to a magnetic letter A which can only pick up other

letter As — "you can put it in a bag of 25,000 letters and it will pick only As."

Because the probe' is radioactive. the bonded sequences can then be picked up on X-ray film.

The radioactive probe used in the Christchurch laboratory was developed by an English scientist, Dr Alec Jeffreys in 1983. Dr Iris says it is the most specific for picking out one particular kind of repetitive sequence in DNA.

He is concerned at; comments that DNA tests can be done on

bloodi up to four years old. "It as been claimed that using probes it is possible to use blood as old as four years. In my expedience with blood samples where the DNA is' slightly degraded, I got results that were totally unreadable, j

“Based on that, I would say that ' these claimsj are a bit unrealistic.

“There are other probes available, but their specificity is not the same as the Jeffreys probe. Most'probes are subject to very stringent limitations."