locating genes, genetic screening and counselling Flashcards
many human diseases have a genetic origin,often the result of gene mutation. Recombinant DNA technology has allowed us to diagnose and treat many of these genetic disorders. In doing so, it is often necessary to know where a particular DNA sequence i.e. gene is located. To achieve this we use:
- labelled DNA probes
- DNA hybridisation
what is a DNA probe?
short, single-stranded length of DNA that has some kind of label attached to it that makes it identifiable
what are the 2 most commonly used probes?
- radioactively labelled probes (which are made up of nucleotides with isotope ^32 probe identified using x-ray film that is exposed by radioactivity)
- florescently labelled probes (emit light under certain conditions, for instance when the probe has bound to the target DNA sequence)
DNA probes are used to identify particular alleles of a gene in the following way:
- DNA probe is made that has base sequences complementary to part of the base sequence of the DNA that makes up the allele of the gene we want to find
- double-stranded DNA that is being tested is treated to separate its two strands
- separated DNA strands are mixed with the probe, which binds to the complementary base sequence on one of the strands = DNA HYBRIDISATION
- the site at which the probe binds can be identified by the radioactivity or fluorescence that the probe emits
what needs to be done before specific probe is made?
we need to know the base sequence in the particular allele that we are trying to locate
before hybridisation can take place
- the two strands of DNA molecule must be separated
- this is achieved by heating DNA until its double strands separate into its two complementary single strands (denaturation)
- when cooled the complementary bases on each strand combine (anneal) with each other to reform the original double strand
using DNA probes and hybridisation, it is possible to locate a specific allele of a gene e.g. we may wish to determine whether someone possesses a mutant allele that causes a particular genetic disorder. The process is as follows:
- sequence of nucleotides on the mutated gene is determined by DNA sequencing (genetic libraries now store the DNA sequences of many of the genes responsible for common genetic diseases)
- fragment of DNA with complementary bases to the mutant allele of the gene is produced
- DNA probe is formed by fluorescently labelling the DNA fragment
- PCR techniques are used to produce multiple copies of the DNA probe
- probe is added to the single stranded DNA fragments of the person being screened
- if the donor has the mutated gene, some donor DNA fragments will have a base sequence that is complementary to the probe and the probe will bind to its complementary bases on the donor DNA
- these DNA fragments will now be labelled with the probe and can be distinguished from the rest of the DNA fragments
- if complementary fragments are present, the new DNA probe will be taken up and this dye will fluoresce, detected by a special microscope. If complementary fragments are not present, the DNA probe will not fluoresce
how is it possible to test simultaneously for many different genetic disorders?
- possible to fix hundreds of different DNA probes in an array (pattern) on a glass slide
- by adding a sample of DNA to the array, any complementary DNA sequences in the donor DNA will bind to one or more probes
- so it is possible to test simultaneously for many different genetic disorders by detecting fluorescence that occurs when binding takes place
another area where genetic screening can be valuable is in the detection of oncogenes, responsible for cancer, explain how
- cancers may develop as a result of mutations that prevent tumour suppressor genes inhibiting cell division
- mutations of both alleles must be present to inactivate the tumour suppressor gene and to initiate the development of a tumour
- some people inherit one mutated tumour suppressor gene; these individuals are at greater risk of developing cancer
if a mutated gene is detected by genetic screening , individuals who are at a greater risk of cancer can then make informed decisions about their lifestyle and future treatment. For example
- they can choose to give up smoking, lose weight, eat more healthily and avoid mutagens as far as possible
- they can also check themselves more regularly for early signs of cancer, which can lead to an early diagnosis and a better chance of successful treatment
one of the advantages of genetic screening is personalised medicine- explain how
- allows doctors to provide advice and health care based on an individual’s genotype
- some people’s genes can mean that a particular drug may be either more or less effective in treating a condition
- by genetically screening patients, doctors and pharmacists can determine more exactly, the dose of the dose of a drug which will produce the desired outcome
- this can save money on otherwise overprescribing drugs
- on example is the prescribing of painkillers
- to function effectively, many medications need a specific enzyme to activate them
- about half the pop have genes that alter the function of this enzyme
- screening for the presence of these genes allows the dosage to be adjusted to compensate for the ways in which the genes affect an individual’s metabolism of the painkiller
- ensuring their use is both effective and safe
what is genetic counselling?
- like a special type of social work where advice and information are given that enable people to make personal decisions about themselves or their offspring
- one important aspect of genetic counselling is to research the family history e.g. sickle-cell anaemia of an inherited disease and to advise the parents on the likelihood of it arising in their children, or the risk of them being carriers
- genetic counselling closely linked to genetic screening as screening results provides the genetic counsellor with a basis of informed discussion
