8.4.2 differences in DNA for identification and diagnosis

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

  extract DNA and amplify by PCR

  cut DNA at specific base sequences (either side of target gene) using restriction enzymes

  separate DNA fragments / alleles (according to length) using gel electrophoresis

  transfer to a nylon membrane and treat to form single strands with exposed bases

  add labelled DNA probes which hybridise / bind with target alleles (& wash to remove unbound probe)

  to show bound probe, expose membrane to UV light if a fluorescently labelled probe was used or use autoradiography 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 (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)
• how gel electrophoresis can be used to separate DNA fragments

  DNA samples loaded into wells in a porous gel and covered in buffer solution (which conducts electricity)

  electrical current passed through → DNA is negatively charged so moves towards positive electrode

  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

  explain results of genetic screening, including consequences of a disease

  discuss treatments available for genetic condition

  discuss lifestyle choices / precautions that might reduce risk of a genetic condition developing eg regular screening for tumours or a mastectomy

  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
• for -> the screening of individuals for genetically determined conditions and drug responses

  can enable people to make lifestyle choices to reduce chances of diseases developing

  allows people to make informed decisions about having their own biological children

  allows use of personalised medicines, increasing effectiveness of treatment
• against -> the screening of individuals for genetically determined conditions and drug responses

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

  could lead to discrimination by insurance companies / employers

  may cause undue stress if patient does not develop the disease