Chloroplast DNA Replication and Protein Synthesis

Chloroplast DNA Replication and Protein Synthesis  

Chloroplast DNA replicate by a semi conservative mechanism involving the formation of D Loops.(Fig A) Chloroplast contains 70S ribosomes. Chloroplast ribosomes resembles with prokaryotes in following features-in use of N-formylmethionine as initiating codon and in basic structures of their RNAs. Chloroplast synthesize polypeptides whose genes reside in chloroplast DNA.

Chloroplast protein synthesis has been studied the first product of chloroplast protein synthesis to be identified was the large subunit of rubisco, the CO₂-fixing enzyme of the calvin cycle that consists of a large catalytic subunit and a smaller regulatory subunit. synthesis of the small rubisco subunits is selectively inhibited in the presence of cycloheximide, whereas formation of the large subunit is blocked by chloromphenicol. Thus the small subunit in the chloroplast. Verification of this conclusion has come from experiments showing that isolated chloroplasts synthesize the large but not the small rubisco subunit. Moreover the gene coding for the large rubisco subunit has been identified in  chloroplast DNA, thereby providing further support for the idea that this polypeptide is synthesized in chloroplasts. although rubisco is a soluble protein located in the chloroplast stroma, many polypeptides synthesized by chloroplasts have turned out to be thylakoid membrane proteins. Among them are subunits of protein complexes involved in photosynthetic electron transfer (photosystem II, the cytochrome b₀f complex and photosystem I) and subunits of the ATP-synthesizing CF₀-CF₁ complex.

Chloroplasts also synthesize some of the polypeptides found in chloroplast ribosomes and chloroplast RNA polymerase. As in the case of mitochondria, all the polypeptides synthesized by chloroplasts are encoded by chloroplast DNA.