DNA hybridisation is another biochemical method for comparing how closely related two species are. It measures how similar the DNA of the two species is overall.The two strands of a DNA molecule are held together by hydrogen bonds between complementary base pairs. Heating breaks these bonds and separates the strands; the more hydrogen bonds there are between matching bases, the more heat is needed.The method:
1. DNA is extracted from both species and cut into fragments.
2. The DNA is heated so that it separates into single strands.
3. Single strands from the two species are mixed and cooled. Where the base sequences are complementary, strands from different species join to form hybrid DNA.
4. The hybrid DNA is heated again and the temperature at which the strands separate is measured.
5. This is compared with the separation temperature of DNA from one species alone.
The more complementary bases there are between the two species, the more hydrogen bonds form, so the higher the temperature needed to separate the hybrid strands. A small difference from the same-species value means the species are closely related.Worked example (illustrative data): human DNA separates at 86.0 °C. Human-chimpanzee hybrid DNA separates at 84.4 °C and human-lemur hybrid DNA at 78.0 °C. The difference for the chimpanzee is 86.0 − 84.4 = 1.6 °C, and for the lemur 8.0 °C. The chimpanzee is more closely related to humans.
1. DNA is extracted from both species and cut into fragments.
2. The DNA is heated so that it separates into single strands.
3. Single strands from the two species are mixed and cooled. Where the base sequences are complementary, strands from different species join to form hybrid DNA.
4. The hybrid DNA is heated again and the temperature at which the strands separate is measured.
5. This is compared with the separation temperature of DNA from one species alone.
The more complementary bases there are between the two species, the more hydrogen bonds form, so the higher the temperature needed to separate the hybrid strands. A small difference from the same-species value means the species are closely related.Worked example (illustrative data): human DNA separates at 86.0 °C. Human-chimpanzee hybrid DNA separates at 84.4 °C and human-lemur hybrid DNA at 78.0 °C. The difference for the chimpanzee is 86.0 − 84.4 = 1.6 °C, and for the lemur 8.0 °C. The chimpanzee is more closely related to humans.