what are DNA probes?
short, single stranded pieces of DNA with a base sequence that is complementary to bases on part of the target allele/ region, which are usually labelled with a fluorescent or radioactive tag for identification
why are DNA probes longer than just a few bases?
a sequence of just a few bases would occur at many places throughout the genome, longer sequences are only likely to occur in target allele
what is DNA hybridisation?
binding of a single stranded DNA probe to a complementary strand of DNA, forming hydrogen bonds/ base pairs
what is the first step of genetic screening?
extract small sample of DNA from patient and amplify using PCR
what is done to the amplified DNA during genetic screening?
DNA is cut at specific base sequences (either side of the target gene) using restriction endonucleases to produce different sized fragments
how are different DNA fragments separated during genetic screening?
they are separated according to length using gel electrophoresis
explain;
-sample of DNA fragments is placed into well of an agar gel
-the agar gel is placed into a tank containing ionic salt solution. electrodes are attached and an electrical field is generated
-the DNA fragments move through the gel towards the positive electrode (due to -ve phosphate group)
-shorter ones travel further
after gel electrophoresis, what happens to the DNA fragments?
the gel is removed and alkali (sodium hydroxide) is added which breaks H bonds and separates DNA into single strands with exposed bases
nitrocellulose membrane is pressed on top of the gel and the single strands are picked up in corresponding positions to the gel
what is added to single stranded DNA fragments in genetic screening?
specific radioactively labelled DNA probes which has a complementary DNA base sequence to that of the disease causing allele of the gene (target allele)
if target allele is present then the probe will complementary base pair/ hybridise with the target allele, excess unbound probes washed away
how do we show bound probes in genetic screening?
a piece of photographic or X-ray film is placed next to the membrane (autoradiography),
if the probe is present then the radioactivity will cause fogging of the film and a band appears (the film has been exposed)
therefore if no band is present this tells us the individual does not carry the disease allele
how can data showing results of gel electrophoresis be interpreted?
-run a standard with DNA fragments/ proteins of known lengths under the same conditions
-compare to position of unknown DNA fragments/ proteins to estimate their size
-shorter DNA fragments/ proteins travel further
what are 3 examples of the use of labelled DNA probes?
- screening patients for heritable conditions (eg cystic fibrosis)
- screening patients for drug responses (some alleles code for enzymes involved in drug metabolism that enable better responses to certain drugs)
- screening patients for health risks (some alleles predispose patients eg to high blood cholestrol)
describe the role of a genetic counsellor (4)?
- explain results of genetic screening, including consequences of a disease
- discuss treatments available for genetic condition
- discuss lifestyle choices/ precautions that might reduce risk of genetic condition developing eg regular screening for tumours
- enable probability of condition/ alleles being passed onto offspring- enables patients to make informed decisions about having children
what is personalised medicine?
medicine tailored to an individuals genotype/ DNA
increases effectiveness of treatment eg by identifying the particular mutation/ allele causing cancer and treating it with tailored drugs
outline points to support genetic screening?
- some people could be heterozygous/ carriers and these people could be enabled to make lifestyle choices to reduce chances of diseases developing, to prevent suffering/death
- allows people to make informed decisions about having biological children
- allows use of personalised medicine which increases effectiveness of treatment
outline points against genetic screening?
- screening for incurable diseases or diseases that develop later in life (where nothing can be done) can lead to distress/ depression
- may cause undue distress if patient doesn’t develop disease
- could lead to discrimination by insurance companies/ employers
- many diseases are rare
- many are caused by many genes so would need too many probes (too expensive)
what are 3 similarities between genetic fingerprinting and genetic screening?
- both use PCR to amplify DNA sample
- both use gel electrophoresis to separate DNA fragments
- both use labelled DNA probes to visualise specific DNA fragments
what is the main difference between genetic fingerprinting and genetic screening?
genetic fingerprinting analyses VNTRs whereas genetic screening analyses specific alleles of a gene
