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

What are DNA probes?

• 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

Suggest why DNA probes are longer than just a few bases

• a sequence of 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 single strand of DNA

• forming hydrogen bonds / base pairs

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

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 to UV light if a fluorescent marker was used OR use autoradiography (expose to x-ray film) if a radioactive probe was used

What is gel electrophoresis?

• a method used to separate nucleic acid (DNA/RNA) fragments OR proteins

• according to length / mass AND charge (DNA is negatively charged due to phosphate groups and protein charge varies based on amino acid R groups)

Explain how gel electrophoresis can be used to separate DNA fragments

1. DNA samples loaded into wells in a porous 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

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 / faster

Describe examples of the use of labelled DNA probes

• screening patients for heritable condition e.g. 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 e.g. to high blood cholesterol)

Describe the role of a genetic counsellor

1. explain results of genetic screening, including consequences of a disease

2. discuss treatments available for genetic condition

3. discuss lifestyle choices / precaution that might reduce risk of a genetic condition developing e.g. regular screening for tumours or a mastectomy

4. explain probability of condition / alleles being passed down to offspring → enable patients to make informed decisions about having children

What is personalised medicine?

• medicine tailored to an individuals genotype / DNA

• increasing effectiveness of treatment e.g. by identifying the particular mutation / allele causing cancer and treating it with tailored drugs

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

• some people could be heterozygous / carriers

• 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

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

• screening for incurable diseases or diseases that develop later in life (where nothing positive can be done in response) may lead to depression

• may cause undue stress if patient does not develop the 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 - expensive