21.4 Locating genes, genetic screening, and counselling

Question 1

What has recombinant DNA technology enabled us to do in regards to genetic disorders

Answer 1

Enabled us to diagnose and treat many of these genetic disorders

Question 2

What is a DNA probe

Answer 2

A short, single-stranded length of DNa that has some sort of label attached that makes it easily identifiable.

Question 3

What are the two most commonly used probes

Answer 3

1. Radioactively labelled probes - made up of nucleotides with the isotope 32P. Identified using an X-ray film that is exposed by radioactivity
2. Fluorescently labelled probes - which emit fluoresce under certain conditions, e.g. when the probe has bound to the target DNA sequence

Question 4

How are DNA probes used to identify particular alleles of genes

Answer 4

1. A DNA probe is made that has base sequences that are complementary to part of the base sequence of the DNA that makes up the allele of the gene that we want to find
2. The double-stranded DNA that is being tested is treated to separate its two strands
3. The separated DNA strands are mixed with the probe, which binds to the complementary base sequence on one of the strands. This is DNA hybridisation
4. The site at which the probe binds can be identified by the radioactivity or fluorescence that the probe emits

Question 5

What is DNA hybridisation

Answer 5

DNA hybridisation takes place when a section of DNA or RNA is combined with a single-stranded section of DNA which has complementary bases

This is achieved by heating DNA until its double strand separates into two complementary single strands. When cooled the complementary bases on each strand recombine

Question 6

How can we locate a specific allele of a gene

Answer 6

1. The sequence of nucleotides on the mutated gene is determined by DNA sequencing. Genetic libraries now store the DNA sequences of many genetic diseases
2. Fragment of DNA with complementary bases to the mutant allele is produced
3. DNA probe is formed by fluorescently labelling the DNA fragment
4. PCR techniques are used to produce multiple copies of the DNA probe
5. Probe is added to single-stranded DNA fragments from the person being screened
6. If the donor has the mutated gene, some donor DNA fragments will have a complementary base sequence to the probe and the probe will bind to these bases
7. These DNA fragments will now be labelled with the probe and can be distinguished from the rest of the fragments
8. If complementary fragments are present, the DNA probe will be taken up and the dye will fluoresce - A spacial microscope detects this.