137 Homologous DNA Sequences

Key Idea: DNA hybridisation compares DNA similarity between species and can be used to measure relatedness. DNA hybridisation is a technique used to quantify the DNA similarity between species. More closely related species have fewer genetic differences than more distantly related species. The method provides information only about how much of the DNA is the same and cannot provide specific information about what the similarities or differences are. Although it has largely been replaced by DNA sequence analysis, DNA hybridisation is still used in microbial studies and has been used to determine the date of human divergence from apes, which has been estimated at 10 and 5 million years ago.

DNA hybridisation technique

1. DNA from the two species to be compared is extracted, purified and cut into short fragments. 2. The mixture is heated so the DNA separates. The DNA from the two species is mixed together. 3. As it cools, bonds form between compatible nucleotides.

Hybrid double-stranded DNA forms. 4. If species share low similarity, the hybrid DNA will have few bonds (and the strands will be weakly held together). The number of bonds (and therefore the strength of the hybrid

DNA) increases with increasing similarity. 5. The similarity is measured by heating the hybrid DNA to force it to form single strands. The greater the similarity, the more heat that is required to break the hybrid DNA apart.

Human DNA Chimpanzee DNA

Fragments of double stranded DNA held together by hydrogen bonds.

Heat the DNA samples. Heat disrupts the hydrogen bonding so the strands separate.

Single-stranded DNA from the two species is mixed 1. How can DNA hybridisation give a measure of genetic relatedness between species?

2. Why do the double strands of DNA break when they are heated?

3. What is responsible for the hybridisation between the

DNA strands?

4. The graph below shows the results of a DNA hybridisation between humans and other primates.

(a) Which primate is most closely related to humans?

(b) Which primate is most distantly related to humans?

5. Hybrid DNA from species A and B comes apart at a lower temperature that of species A and C. Which species is A most closely related to?

Primate species Similarity of human DNA to that of other primates DNA similarity (%)

0 20 40 60 80 100

Human

Chimpanzee Gibbon

Rhesus monkey

Vervet monkey Capuchin monkey Galago 58.0% 100%

97.6% 94.7%

91.1%

90.5%

84.2%

Cool the samples

Hybrid DNA

These bases match These bases do not match