3.8.4.2 Differences in DNA between individuals of the same species can be exploited for identification & diagnosis of heritable conditions

Question 1

What are DNA probes?

Answer 1

● Short, single stranded pieces of DNA
● With a base sequence complementary to bases on part of a target allele / region
● Usually labelled with a fluorescent or radioactive tag for identification

Question 2

Suggest why DNA probes are longer than just a few bases

Answer 2

● A sequence of a few bases would occur at many places throughout the genome
● Longer sequences are only likely to occur in target allele

Question 3

What is DNA hybridisation?

Answer 3

● Binding of a single stranded DNA probe
to a complementary single strand of DNA
● Forming hydrogen bonds / base pairs

Question 4

Explain how genetic screening can be used to locate specific alleles of genes

Answer 4

1. Extract DNA and amplify by PCR
2. Cut DNA at specific base sequences (either side of target gene) using restriction enzymes
3. Separate DNA fragments / alleles (according to length) using gel electrophoresis
4. Transfer to a nylon membrane and treat to form single strands with exposed bases
5. Add labelled DNA probes which hybridise / bind with target alleles (& wash to remove unbound probe)
6. To show bound probe, expose membrane to UV light if a fluorescently labelled probe was used
   OR use autoradiography (expose to X-ray film) if a radioactive probe was used

Question 5

What is gel electrophoresis?

Answer 5

● A method used to separate nucleic acid (DNA / RNA) fragments OR proteins

● According to length / mass (number of bases / amino acids) AND charge (DNA is negatively charged due to phosphate groups and protein charge varies based on amino acid R groups - shorter lengths are more negatively charged and travel faster)

Question 6

Explain how gel electrophoresis can be used to separate DNA fragments

Answer 6

1. DNA samples loaded into wells in a porous (agarose) gel and
   covered in buffer solution (which conducts electricity)
2. Electrical current passed through → DNA is negatively
   charged so moves towards positive electrode
3. Shorter DNA fragments travel faster so travel further

Question 7

How can data showing results of gel electrophoresis be interpreted?

Answer 7

● 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 / faster

Question 8

Describe examples of the use of labelled DNA probes

Answer 8

● 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 cholesterol)

Question 9

Describe the role of a genetic counsellor

Answer 9

1. Explain results of genetic screening, including consequences of a disease
2. Discuss treatments available for genetic condition
3. Discuss lifestyle choices / precautions that might reduce risk of a genetic
   condition developing eg. regular screening for tumours or a mastectomy
4. Explain probability of condition / alleles being passed onto offspring →
   enable patients to make informed decisions about having children

Question 10

What is personalised medicine?

Answer 10

● Medicine tailored to an individual’s
genotype / DNA
● Increasing effectiveness of treatment
eg. by identifying the particular
mutation / allele causing cancer and
treating it with tailored drugs

Question 11

Evaluate the screening of individuals for genetically determined conditions
and drug responses

Answer 11

• For:
  ✓ Some people could be heterozygous / carriers (eg. in families with a history of a disease)
  ✓ Can enable these people to make lifestyle choices to reduce chances of diseases developing, to prevent suffering / death
  ✓ Allows people to make informed decisions about having their own biological children
  ✓ Allows use of personalised medicines, increasing effectiveness of treatment
• Against:
  X Screening for incurable diseases or diseases that develop later in life (where nothing positive can be done in response) may lead to depression
  X May cause undue stress if patient does not develop the disease
  X Could lead to discrimination by insurance companies / employers
  X Many diseases are rare
  X Many are caused by many genes so would need too many probes (expensive)