ABSTRACT
This chapter describes the main methods which have allowed biologists to elucidate the physical organization of plant genomes. Three general approaches are available to analyze genomes. These are determination of their physical organization, determination of their genetic organization and determination of expression patterns. The initial level of analysis of physical structure of a genome was microscopic observation. It was carried out at stages of the cell cycle where chromosomes are individualized or decondensed. Differences in genome size became obvious when it was possible to measure the deoxyribonucleic acid content per nucleus, using either extraction methods and dosage or cytophotometric methods. The proportion of repeated sequences in plant genomes appears to be quite variable, up to 90% in the most complex genomes, such as wheat. Cytology describes two types of chromatin: heterochromatin and euchromatin. Transcriptionally active chromatin, which in vivo is the most decondensed state, is at least packaged as 30 nm fibers.
