Probes and screening for disease

Background Genetic disorders are often the result of gene mutations. People with a mutant allele often have a family history of the disease. It is important they are screened for the disease to help determine the likelihood of their offspring inheriting the disease.

Background continued We can use DNA probes to locate specific genes but in order to do this we need to know the sequence of bases in the gene we are screening for (as the probe will be complementary to some of these bases)

Read the middle of page 11 about gene probes, then down to the bottom of the page

DNA probes Are a short single stranded piece of DNA which are ‘labelled’ (they are either radioactive or fluorescent). They are complementary to a portion of the gene and enable the gene to be visualised. There are 2 main types of label used:

Radioactive labels: These are made using nucleotides which contain radioactive phosphorus. They can be identified using a photographic plate.

Do you remember the principles of gel electrophoresis?

Fluorescent labels: These emit light (fluoresce) under certain conditions.

Genetic screening

What the spec says about “medical diagnosis” The use of labelled DNA probes and DNA hybridisation to locate specific genes. Once located, the base sequence of a gene can be determined by restriction mapping DNA sequencing

Many human diseases result from mutated genes or from genes that are useful in one context but not in another, e.g. sickle cell anaemia. (you will find out more about this by doing the application exercise on p 273) DNA sequencing and the PCR are used to produce DNA probes that can be used to screen patients for clinically important genes. The use of this information in genetic counselling, e.g. for parents who are both carriers of defective genes and, in the case of oncogenes, in deciding the best course of treatment for cancers. (This is covered on p16 in your booklet) Candidates should understand the principles of these methods. They should be aware that methods are continuously updated and automated (Eg DNA array, automated sequencing )

Aims for today Review genetic counselling from p16 in booklet Find out a bit more about DNA arrays Find out about DNA fingerprinting and profiles

First of all - Southern Blotting Look at the figure on p17 in your booklet which describes the process. You do not have to learn the steps in detail but should have an overview of what happens. It’s just a way of transferring the patterns form the bands of DNA in a gel to a more robust nylon sheet.

DNA array The different intensities of fluorescence under different colours of laser light indicate different levels of gene expression-enables us to screen lots of samples or one sample with lots of probes

Using gene probes to screen for sickle cell disease Normal allele Sickle allele Darker band where two copies of the sickle allele are found – no band for the normal allele