Introduction

The genome comprises all the genetic material present in the haploid cell and, therefore, includes all the nuclear and mitochondrial genes. Two basic approaches that have been used to study genome structure and organisation can be described as cytogenetic and molecular genetic. The cytogenetic approach began with the differential staining of the giant polytene chromosomes from dipteran salivary glands in the 1930s. However, it was not until the late 1960s that differential staining methods were developed that could distinguish banding patterns in normal mitotic chromosomes (Caspersson et al., 1968). More selective staining methods have allowed more detailed resolution of banding patterns; and these have made considerable impact on understanding the organisation of genomes at a microscopic level.

The second approach has been to study the genome at a molecular level. This can be done directly by analysing the DNA, or indirectly by studying the products of transcription and translation, i.e. mRNAs and proteins. Nucleic acid hybridisation techniques, developed since the late 1960s, and DNA sequencing, developed in the mid-1970s, have been central to the molecular approach. Studying DNA sequences provides detailed information about the structure and organisation of genes, whereas studying the number of different mRNAs and proteins synthesised in a cell helps to estimate the number of genes being expressed in a particular cell or tissue.