A DNA probe is a short length of single-stranded DNA with a base sequence that is complementary to part of a target gene or allele. Because of complementary base pairing, the probe binds only to DNA that contains that sequence.A probe is labelled so that it can be located once it has bound:
• a radioactive label (often phosphorus-32), detected by exposing the sample to X-ray film, a technique called autoradiography
• a fluorescent label, which glows under ultraviolet (UV) light.To make a probe, scientists must first know the base sequence of the target, for example the sequence of a disease-causing allele. A single strand with the complementary sequence is made, many copies are produced using PCR, and the label is attached.Worked example: part of an allele has the sequence 5' ATGGCTTAC 3'. The probe must be complementary and antiparallel:3' TACCGAATG 5' 
A probe must be long enough that its sequence is unlikely to occur elsewhere in the genome by chance, or it would bind to many sites. With four bases, a particular sequence of n bases is expected by chance once in every 4n bases. A 15-base sequence would be expected about once in 415 ≈ 1.1 × 109 bases.Probes are used to screen people for disease-causing alleles, to check whether genetic engineering has worked, and in genetic fingerprinting. The way a probe binds to its target is called DNA hybridisation (next lesson).
• a radioactive label (often phosphorus-32), detected by exposing the sample to X-ray film, a technique called autoradiography
• a fluorescent label, which glows under ultraviolet (UV) light.To make a probe, scientists must first know the base sequence of the target, for example the sequence of a disease-causing allele. A single strand with the complementary sequence is made, many copies are produced using PCR, and the label is attached.Worked example: part of an allele has the sequence 5' ATGGCTTAC 3'. The probe must be complementary and antiparallel:
A probe must be long enough that its sequence is unlikely to occur elsewhere in the genome by chance, or it would bind to many sites. With four bases, a particular sequence of n bases is expected by chance once in every 4n bases. A 15-base sequence would be expected about once in 415 ≈ 1.1 × 109 bases.Probes are used to screen people for disease-causing alleles, to check whether genetic engineering has worked, and in genetic fingerprinting. The way a probe binds to its target is called DNA hybridisation (next lesson).