ABSTRACT
A very important first step in the analysis of the newly isolated gene is the confirma tion of the origin of the gene. Especially in experiments that use either non-sterile plant material as a primary source or analyse gene expression induced upon patho gen infection, it is necessary to obtain certainty of the plant origin of the gene in question. This is best done by Southern analysis (Southern, 1 975), where the genomic DNA (normally 5-1 0 �g) of the plant is digested with a series of restriction endo nucleases (single or double restrictions) and the fragments are separated according to their size by agarose gel electrophoresis. Following transfer (Southern blotting) to a nylon or nitrocellulose membrane, the genomic plant DNA is hybridized to a labelled probe of the cDNA sequence in question (Southern hybridization) . The labelling can be radioactive e2p) or non-radioactive, with a number of very sensitive labelling systems available from a range of suppliers. Southern analysis may also provide information about the copy number of genes and the existence of gene families. Standard protocols can be found in many molecular biology manuals (e.g . , Sambrook et ai. , 1 989; Glover and Hames, 1 995).
