3.4.8 Gene probes

Question 1

What are gene probes?

Answer 1

• short lengths of single stranded DNA with a complementary base sequence to target allele

Question 2

What are gene probes used for?

Answer 2

Used to locate / identify specific alleles of genes

Question 3

Uses of DNA screening (4)

Answer 3

• Identifying inherited conditions e.g. Huntington’s
• Identifying health risks e.g. risk of developing cancers
• Identifying whether drugs will be effective
• Personalised medicined tailored to individual’s DNA

Question 4

What does a genetic counsellor do? (3)

Answer 4

• Advising patients and relatives about risks of genetic disorders
• Advising about and explaining results of screening
• Gives options of prevention or treatment

Question 5

How and why are DNA probes detected?

Answer 5

• Able to see if probe has attached to gene - makes fragment visible
  1. Fluorescent labels - detected by UV light
  2. Radioactive labels - detected using x-ray film

Question 6

How does gel electrophoresis separate DNA fragments? (2)

Answer 6

1. Separates fragments of DNA using electricity
   - the DNA is slightly negatively charged so is repelled from the negative electrode and attracted toward the positive electrode
2. Fragments separated by size due to the the gel
   - gel provides resistance meaning smaller molecules mover further and faster than heavier ones

Question 7

4 stages of using gene probes

Answer 7

1. Extraction
2. Create probes
3. Split DNA
4. Add probe

Question 8

1. Extraction

Answer 8

• extract DNA from patient cells
• amplify the DNA using PCR
• cut the DNA using restriction endonuclease and separate fragments by gel electrophoresis

Question 9

2. Create probes

Answer 9

• specific complementary gene probes for the target allele created with a gene machine
• these are amplified using PCR

Question 10

3. Split DNA

Answer 10

• patient DNA is split into 2 strands - H bonds between bases broken by heating to 95 c
• this exposes bases for the DNA fragments to attach to

Question 11

4. Add probe

Answer 11

• gene probe added to patient DNA

- if the allele is present the probe will bind to one strand of the DNA - DNA hybridisation

Question 12

What is DNA hybridisation?

Answer 12

2 complementary single stranded DNA molecules combine through base pairing to form a double stranded DNA molecule

Question 13

Why are DNA fragments transferred?

Answer 13

• DNA fragments transferred to a nylon membrane so DNA doesn’t degrade
• then heated to cause DNA strands to separate, incubated at 55 c with a DNA probe allowing probe to bind to target allele
• washed to remove unbound probe