8.4.2 Exploitation of differences in DNA

• 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 is used to locate specific alleles

  1. Extract DNA + amplify by PCR

  2. Cut DNA at specific base sequences (either side of target gene) using restriction enzymes

  3. Separate DNA fragments (according to length) using gel electrophoresis

  4. Transfer to nylon membrane and treat to form single strands with exposed bases

  5. Add labelled DNA probes which bind with target alleles (wash to remove unbound probe)

  6. To show bound probe, expose membrane to UV light if fluorescently labelled probe used OR use autoradiography (expose to X-ray film) if radioactive probe used
• What is gel electrophoresis?

  ● 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)
• 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 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)
• 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 / 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
• What is personalised medicine?

  ● 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
• suggest some points to argue for the screening of individuals for genetically determined conditions and drug responses

  ✓ 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
• suggest some points against the screening of individuals for genetically determined conditions and drug responses

  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)