Locating genes, genetic screening and counselling

Question 1

What is a DNA probe ?

Answer 1

a short, single-stranded length of DNA that has some sort of label attached that makes it easily identifiable

Question 2

What are the two most commonly used probes ?

Answer 2

radioactively labelled probes- the probe is identified using an X-ray film that is exposed by radioactivity
Fluorescently labelled probes- which emit light under certain conditions, for instance when the probe has bound to the target DNA sequence

Question 3

Explain how a DNA probe can be used to identify particular alleles of genes

Answer 3

a DNA probe is made with a sequence of bases that are complimentary to the part of the base sequence of the DNA that makes up the allele that we want to find.
The double stranded DNA that is being tested is treated to separate the two strands.
The separated DNA strands are mixed with the probe which binds to the complimentary 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.

Question 4

What is DNA hybridisation

Answer 4

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

Question 5

What must happen before DNA hybridisation can take place ?

Answer 5

the two strands of the DNA molecule must be separated. This is achieved by heating DNA until its double strands separates into its two complimentary single strands.
When cooled the complimentary bases on each strand recombine with eachother to reform the original double strand.

Question 6

Outline the process of genetic screening for a disease

Answer 6

determine the order of nucleotides on the mutated gene by DNA sequencing
produce a fragment of DNA that has complimentary bases to the mutated portion of the gene
label the fragments to produce a DNA probe
make multiple copies of the donor DNA using PCR techniques
add the probe to the DNA fragments from the individual being tested. If the donor has the mutated allele the probe will bind to the complimentary bases on the donor DNA
These fragments will now be labelled and can be distinguished from the rest of the DNA.

Question 7

What does Genetic screening do ?

Answer 7

Screening can determine the probabilities of an individual or a couple having offspring with a genetic disorder.

Question 8

What is the benefit of genetic screening of tumour suppressor genes ?

Answer 8

if the mutated tumour suppressor gene is detected by genetic screening, the individuals who are at a greater risk of cancer can then make informed decisions about their life style and future treatment.
They can choose to eat more healthily and avoid mutagens.

Question 9

What is an advantage of genetic screening ?

Answer 9

personalised medicine

Question 10

What is personalised medicine ?

Answer 10

Some peoples 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 a drug which will produce the desired outcome. This can save money and in some cases it avoids medications that could cause harm and raising false hopes.

Question 11

What is genetic counsellor ?

Answer 11

genetic counselling is like a special form of social work where advice and information are given that enable people to make personal decisions about themselves or their offspring.

Question 12

What is the aim of genetic counselling ?

Answer 12

One important aspect of genetic counselling is to research the family history of an inherited disease and to advise parents on the likelihood of it and their children.

Question 13

What can a counsellor inform people of ?

Answer 13

emotional
psychological
medical
social
economic consequences
on the basis of the advice the couple can then choose whether or not to have children.

Question 14

Where is Genetic fingerprinting used?

Answer 14

forensics, plant and animal breeding and medical diagnosis,to determine family relationships, to determine genetic diversity in a population

Question 15

How does genetic fingerprinting work ?

Answer 15

relies on the fact that the genome of most eukaryotic organisms contains many repetitive, non coding bases of DNA. These are known as Variable number tandem repeats (VNTRs)
For every individual the number and length of VNTRS has a unique pattern.
The more closely related two individuals are, the more similar the VNTRS.

Question 16

What is Gel electrophoresis ?

Answer 16

Gel electrophoresis is used to separate DNA fragments according to their size

Question 17

What does VNTRS stand for ?

Answer 17

variable number tandem repeats

Question 18

Explain the making of a genetic fingerprint

Answer 18

DNA is extracted from the sample
restriction endonucleases cut the DNA into fragments
fragments are separated using gel electrophoresis. DNA fragments treated to produce single stranded DNA fragments
DNA fragments are transferred from the gel to nylon membrane
DNA probes are added to label the fragments. These radioactive probes attach to specific fragments
Membrane with radioactively labelled DNA fragments is placed onto an X-ray film.
Development of the X-ray film reveals dark bands where the radioactive DNA probes have attached.

Question 19

Explain the process of Gel electrophoresis

Answer 19

The DNA fragments are separated according to size. The DNA fragments are placed on to an agar gel and a voltage is applied across it. The resistance of the gel means that the larger the fragments, the more slowly they move. Therefore over a fixed period, the smaller the fragments move further than the larger ones. If the DNA fragments are labelled for example with radioactive DNA probes their final positions in the agar gel can be determined by placing a sheet of X-ray film over the agar gel for many hours.

Question 20

Explain how genetic fingerprinting can be used to help resolve questions of paternity

Answer 20

individuals inherit half their genetic material from their mother and half from their father. Therefore each band on a DNA fingerprint of an individual should have a corresponding band in one of the parents DNA finger prints.

Question 21

Explain how genetic fingerprinting can be used to determine genetic variability within a population

Answer 21

the more closely related two individuals are related the closer the resemblance of their genetic fingerprints,

Question 22

What is the genetic diversity like of a population that has very similar genetic fingerprints ?

Answer 22

low genetic diversity

Question 23

Explain how genetic fingerprints can be used in forensic science

Answer 23

DNA is often left at the scene of a crime, for example blood at the scene of a violent crime
Genetic fingerprinting can establish whether a person is likely to have been present at the crime scene.
Even if there is a close match between a suspects DNA and the DNA found at the crime scene, it does not follow that the suspect carried out the crime. For example,
The DNA may of been left on some other innocent occasion
The DNA may belong to a close relative

Question 24

Explain how Genetic fingerprints can be used in medical diagnosis such as diagnosing Huntington’s disease

Answer 24

Genetic fingerprints can help in diagnosing diseases such as Huntington’s.
A sample of DNA from a person with the allele for Huntington’s disease can be cut with restriction endonucleases and a DNA fingerprint prepared. This can then be matched with fingerprints of people with various forms of the disease and those without the disease. In this way the probability of developing symptoms and when can be determined.

Question 25

Explain how Genetic fingerprinting s used in plant and animal breeding

Answer 25

Genetic fingerprinting can be used to prevent undesirable inbreeding during breeding programmes on farms or in zoos.

It can also identify plants or animals that have a particular allele for a desirable gene.
Individuals with this allele can be selected for breeding in order to increase the probability of their offspring having the characteristic it produces.

Another application is the determination of paternity in animals and thus establishing the family tree of an individual.

Question 26

Explain why a patient may choose to receive genetic counselling

Answer 26

to help decided whether or not to undergo genetic screening
to discuss treatments available for a genetic condition
deciding whether or not to have a child
to help a patient learn about a particular condition
to discuss lifestyle choices that might reduce the risk of a genetic condition developing.