An Introduction to the Invertebrates, Second Edition - tiera.ru

20 THE PATTERN OF EVOLUTION: METHODS OF INVESTIGATIONat the short end of the evolutionary time scale, to resolve changesthat occurred less than 15 million years ago such as the separationof genera and species. When the whole (small) genome can besequenced, mtDNA can also tell us about ancient changes. Moreoften, it is not the gene content of the mitochondria that is usedbut the order of genes round the chromosomal ring.2.7 How is molecular information obtained?2.7.1 Gene productsThe differences between proteins can be revealed and estimatedusing gel electrophoresis, a technique invented before the geneticmaterial itself became accessible for direct study. Techniquesapplied to DNA directly are now more commonly used.2.7.2 DNA hybridisationThe paired strands of DNA dissociate when heated because thebonds between corresponding nucleotides are broken. They recombineon being cooled. Single strands of DNA from related speciescan be put together and bonds will form, but only at the sites whichcorrespond. When such a ‘hybrid’ is carefully heated it dissociatesat a temperature lower than that required to dissociate perfectlymatched DNA, because fewer bonds will have been formed. Thedifference between the two temperatures (‘melting points’) can beused as a measure of the genetic similarity of the two species.2.7.3 Restriction site analysisRestriction enzymes cut DNA at predictable sites into fragmentsabout 4 to 6 nucleotides long. Fragments from different sources canthen be compared, to obtain information about a small part of thetotal molecule.2.7.4 Sequencing of nucleotidesThis process allows the identification of each nucleotide in thewhole sequence of a DNA molecule. This exhaustive process has beenmade easier by the polymerase chain reaction (PCR), which amplifiesa small quantity of material for rapid analysis, and by the automatedsequencing machine. This technique opened the way to the presentexplosion of molecular information.2.8 How is molecular information processed?Molecular evidence provides a large number of characters, all preciselydefined. A character is usually a given nucleotide at a givensite on the DNA molecule. Can the methods used for morphologicalcharacters be applied?