Molecular Basis

Abstract

Green plants have independent genetic and protein-synthesizing systems in the nucleus and the cytoplasm. In the latter, cytoplasmic organelle mitochondria and chloroplast, the major energy conversion sites with a vital role in the vigour and productivity determinations of a plant are considered as the carriers of cytoplasmic genes including c-genes, which are inherited and transmitted maternally. This is based on the premise that both these organelles are independently functioning and able to synthesise some proteins, independent of nucleus, due to the presence of specifically unique DNA, RNA, ribosomes and enzymes. The availability of site- specific restriction endonucleases provides a sensitive assay of sequence variability in chloroplast (cp) and mitochondria (mt) DNA, which is of relatively lower complexity compared to nuclear DNA. Differences between fragmentation patterns provide an index of genetic divergence, since the number and size of fragments is a function of the occurrence of a specific nucleotide sequence in DNA. Genes and DNA sequences have been characterised for nucleus, chloroplast and viruses of eukaryotes and for chromosomes, plasmids and viruses. Detailed restriction maps of cp-DNA of several higher plant species are now available (Leaver et al. 1982) and even location and sequence of chloroplast-specific r-RNA’s, t-RNA’s and several of the major protein-coding genes are known (Bedbrook and Koldnner 1979). In contrast, too little is known about the precise organisational pattern, informational content and functional properties of higher plant mt-DNA. Intramolecular heterogeneity, intraspecific variation, the unknown coding capacity of mt-DNA’s, difficulties of transmission genetic studies and the lack of characterised mt-mutants have limited the available information about mt-genome of higher plants compared to that of yeast and mammals.