6. Chloroplast genome

The chloroplast is the green plastid in land plants, algae and some protists. As the site in the cell where photosynthesis takes place, chloroplasts are responsible for much of the world's primary productivity, making chloroplasts essential to the lives of plants and animals alike. Agriculture, animal farming, and fossil fuels such as coal and oil are all "products" of photosynthesis that took place in chloroplasts. Other important activities that occur in chloroplasts (and several non-photosynthetic plastid types) include the production of starch, certain amino acids and lipids, some of the colorful pigments in flowers, and some key aspects of sulfur and nitrogen metabolism. The interactions between plastid and nuclear encoded transcription and translation process is elaborated in Figure 21.7.
All plastids considered to date contain their own DNA, which is actually a reduced "genome" derived from a cyanobacterial ancestor that was captured early in the evolution of the eukaryotic cell. The chloroplast genome encodes for all the rRNA & tRNA species required for protein synthesis. The ribosomes contain two small rRNAs in addition to the major species. The chloroplast genome codes for ~50 proteins, including RNA polymerase & some ribosomal proteins. Again the rule is that organelle genes are transcribed & translated the apparatus of the organelle. The chloroplast genome of the higher plants varies in length, but displays a characteristic landmark. It has a lengthening sequence, 10-24kb depending on the plants, that is present in two identical copies as an inverted repeat (Gene that are coded within the inverted repeats are present in two copies per genome & include the rRNA genes).

Figure 21.7: Model of the interactions between plastid and nuclear encoded transcription and translation products. TS: transit sequence: a N-terminal section of the polypeptide chain, essential for the penetration of the polypeptide across the membrane, subsequently being cleaved off proteolytically.