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DNA Probes & Microarrays

Biology: Cambridge International A Level, topic 19.1.4

DNA Probes

DNA probes are used to identify which alleles are present in a sample of DNA. This can be used in medical diagnosis by identifying if an individual has specific disease-causing alleles.

Probes

Probes

  • A DNA probe is a section of single-stranded DNA.
  • The probe is complementary to the DNA of the target allele (e.g. the allele that causes cystic fibrosis).
Hybridisation

Hybridisation

  • The complementary base sequence in a probe means that if the target allele is present in the DNA that is being sampled, it will bind to the DNA probe.
  • When a base sequence binds to a complementary base sequence, this is called hybridisation.
Diagnosis

Diagnosis

  • DNA probes are labelled with a radioactive phosphate or fluorescent tag.
  • If the disease-causing allele hybridises with the probe, this label will be detected.
  • DNA probes are used in this way to diagnose diseases.

Diagnosis

Multiple labelled DNA probes can be used to diagnose the presence or absence of disease-causing alleles in a DNA sample. This can be done using two different techniques.

Electrophoresis

Electrophoresis

  • Electrophoresis is a technique that separates fragments of DNA according to size. The sections of DNA are then transferred onto a nylon sheet.
  • DNA probes labelled with fluorescent tags are 'washed over' the DNA fragments.
  • The DNA probes hybridise with any fragments that are complementary to the probes.
Microarrays

Microarrays

  • Microarrays use many DNA probes at once.
  • A microarray is a slide with many indents. Each indent contains the DNA probes for a specific gene.
  • The fluorescently labelled DNA sample is 'washed over' the microarray.
  • Any DNA fragments that are complementary to the probes will hybridise.
Identifying DNA probes

Identifying DNA probes

  • DNA probes that have hybridised to the DNA sample in either electrophoresis or a microarray can be identified.
  • If the probes are labelled with fluorescent tags, they will be revealed by UV light.
  • If the probes are labelled with radioactive phosphate, they will be revealed by X-ray.
  • The DNA probes that are identified indicate which alleles are present in a sample of DNA.
  • This allows the identification of expressed genes.

Test yourself on DNA Probes & Microarrays

  • Which of the following statements about DNA probes are true?

  • What is the name of the process when two complementary base sequences bind together?

  • If DNA probes are labelled with fluorescent tags, how can they be identified?

  • What are the two techniques used to diagnose disease using DNA probes?

  • Which of the following statements about microarrays are true?

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