Comparison of DNA base sequences is used to elucidate relationships between organisms. These comparisons have led to new classification systems in plants. Similarities in DNA may be determined by DNA hybridisation.

If two species have recently shared a common ancestor, then they are closely related. If this is the case, then they will have much of the same DNA because when one species gives rise to another they initially have similar DNA.

Over time mutations will render the DNA very different through the accumulation of mutations. This means that species that have not shared a common ancestor recently will have very different DNA.

So by looking at DNA scientists can determine what other species a species is related to and therefore what type it should be classified as.

A recent example of where this has been useful is in the classification of plants. Before genetic comparison plants had been categorised due to physical characteristics, however when the DNA was examined, scientists found that some plants that looked very different had more similar genes than ones that looked similar. The whole plant kingdom had to be re-classified.

DNA hybridisation is one method of comparing the base sequence of DNA:

• Heat the DNA of two species, this will break the hydrogen bonds holding together the two sides of the chromosome
• Leave them to cool and reform hydrogen bonds between complimentary base pairs
• Some of the DNA will have formed with one strand from each species
• This 'hybrid' DNA is then heated in stages to see at what temperature it separates into single strands

A higher temperature means more energy was required because there are more hydrogen bonds, this means that there were many complimentary base pairs made, and that means that the DNA from both species was similar.

If the DNA was not similar there would not be very many complimentary base pairs, so the hybrid DNA would have few hydrogen bonds and not take very much energy to break them all. This means that hybrids strands that separate at a low temperature come from two species that are not genetically similar.