Every human chromosome carries specific genes in a specific order. Therefore, you might expect that...

• Every human chromosome carries specific genes in a specific order.

• • Therefore, you might expect that we would get the

same pattern of restriction fragments if it was cut with a particular restriction endonuclease.

• However this is not the case……..

• Why???

Key points:• Between the genes there are many repetitive,

non-coding base sequences called RFLPs (restriction fragment length polymorphisms) and SLTs (short tandem repeats).

• The length of these varies from one individual to another so the probability of two individuals having the same repetitive sequences is very low.

• This is the basis of genetic fingerprinting.

Basic idea: As different pieces of DNA have different lengths of repeated base sequences the distance between recognition sites of restriction endonuclease changes. This means pieces of DNA of different length are produced and these can be compared using gel electrophoresis.

Southern Blotting

Read the bottom of page 33 in your booklet describes the process. You do not have to learn the steps in detail but should have an overview of what happens.

• The next slide shows the results of the very first RFLP (repeated fragment length polymorphisms) analysis which led to a criminal receiving the death sentence in the USA.

• A young couple had been murdered whilst they slept in their car. Their bodies were discovered the next day. A post mortem showed they had both died of gunshot wounds and that the woman had been raped. One man was later arrested driving the couple’s stolen car. Under police questioning, he identified a friend who had been with him on the night of the murders.

• DNA from the semen found in the woman’s body and a DNA sample from each suspect was digested using the same restriction enzyme, and the restriction fragments separated using gel electrophoresis.

• Who did it?

• You will see that the DNA fingerprint of these men does not have the same pattern of restriction fragments as suspect 1 but does have the same pattern as suspect 2.

• On the basis of this evidence, suspect 2 was found guilty of rape and murder and given a double death sentence. The jury at the time was told that the chance of an innocent person showing the same pattern of restriction fragments was about 1 in 9 billion. At the time of the trial, the human population of the world was less than 6 billion.

Now complete the Bains family activity:

Mr Bains first settled in England in 1990. Two years later, he applied to bring his wife and four children to join him. The immigration authorities needed to be sure that the children were entitled to come into England and so they carried out a RFLP analysis on the whole family. Mr Bains had been married before. Sadly, his first wife had died giving birth to their son. Some years later, Mr Bains remarried and he and his new wife had two daughters. They also adopted another son.

1. Look at lanes1 and 2. They show the separated restriction fragments of Mr and Mrs Bains. Explain why the two patterns of restriction fragments are different.

2. Lane 6 shows the restriction fragments from Mr and Mrs Bains’ adopted son. Explain why this conclusion is valid.

3. Use the diagram to identify Mrs Bains’ stepson. Explain your answer.

Dynamic learning activities:

• Section 12:

• DNA fingerprinting worksheet.

• Analysis activity Crime investigation