Chloroplast Genome, Cytoplasmic Organellles, Cytoplasm, Multimeric Protein, Mendelian, Chloroplasts, Hypothesis

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	Chloroplast Genome - The chloroplasts of green plants are cytoplasmic organelles that house the various pigments and enzymes of the light harvesting photosynthetic apparatus. Even before the turn of the century it was clear that green pigmentation was one of the easiest traits to observe in plant breeding experiments. Although some pigmentation traits obeyed Mendel's laws, other colour traits were only transmitted through the female parent that provided the cytoplasm of the zygote.
These observations of cytoplasmic or maternal inheritance eventually led to the hypothesis that chloroplasts must carry genes. We know that chloroplasts contain a unique circular DNA genome that is completely different from the nuclear genome. The presence of a genetic system within chloroplasts had already been inferred from studies on non Mendelian inheritance in 1909, but the presence of organellar DNA and ribosomes was demonstrated only in 1962. Since then it has been shown that chloroplasts and other plastids contain all the machinery necessary for gene expression. The chloroplast genetic components form a large proportion of those in the leaf, comprising up to 15% of the total DNA and up to 60% of the total ribosomes. The chloroplast genome has been extensively characterized from a variety of species and cooperation between the chloroplast and nuclear genome in chloroplast biogenesis is currently under investigation.
Electron micrographs indicate that the chloroplast DNA is some 10 to 20 times smaller than the E. coli chromosomes. For example, the chloroplast genome of maize (corn) contains 140,000 base pairs of DNA. Such genomes are much too small to encode the approximately 1,000 different proteins found in chloroplasts. Instead, biosynthesis of the chloroplast involves an intimate collaboration between the nuclear and chloroplast genomes. In fact, every known multimeric protein component of chloroplasts is a mixture of the products of both nuclear and chloroplast genes. Most chloroplast proteins are encoded by nuclear DNA, translated in the cytoplasm, and imported into the chloroplast by a specific transport mechanism that enables polypeptides to cross the outer membrane of the organelle.
However, some 100 chloroplast specific proteins are synthesized within the chloroplast itself. These proteins are encoded by chloroplast DNA, transcribed by the chloroplast specific RNA polymerase, and translated by the chloroplast specific protein-synthesizing machinery. Since RNA cannot cross the outer membrane of the chloroplast, chloroplast ribosomal RNAs and tRNAs must be encoded in chloroplast DNA. Chloroplasts are not static organelles but can adapt to different physiological conditions, such as high or low levels of light. For example, when grown entirely in the dark, chloroplasts lack chlorophyll but retain carotenoid pigments. Thus many chloroplast genes are light regulated in certain cases by light sensitive promoters.