Development of Chloroplasts

Germination of a seed results in growth of a shoot, in which the initial plastids exist with the cells as simple, double-membrane-enclosed vesicles that contain deoxyribonucleic acid (DNA), ribosomes, and a set of enzymes needed for expression of the DNA. These structures, only about 20 percent of the size of a mature chloroplast, are called proplastids. When the shoot reaches the light, the plastid begins the synthesis of chlorophyll, which is required for nearly all remaining aspects of development. Synthesis of lipids, which form the framework of thylakoid membranes, is stimulated within the inner membrane of the envelope.

Proteins are also imported into the chloroplasts after synthesis on cy-tosolic ribosomes as precursor molecules. Such proteins contain an extension at their amino-terminal end, designated the transit sequence, that serves as a targeting signal for import into the chloroplast. As soon as the protein reaches the stroma, the transit sequence is removed by a specific protease. The chloroplast envelope contains an elaborate apparatus made of numerous protein molecules that function to guide proteins through the membranes into the interior. While some proteins remain embedded in the membrane, others pass through the envelope into the stroma. Of these, a relative few are also transported across thylakoid membranes into the thylakoid lumen. The two major proteins that are imported are the precursor of the small subunit of rubisco, which is released into the stroma, and the chlorophyll-binding proteins, which are integrated into large light-harvesting antenna complexes within the envelope inner membrane. These complexes absorb and funnel light energy to reaction centers to drive the light reactions of photosynthesis. The addition of lipids, pigments, and proteins causes expansion of this membrane, which pinches off into vesicles that subsequently fuse to construct the large thylakoid structure in the interior of the organelle.

Chloroplasts grow and divide along with the cell they reside in as the plant grows. Nearly one hundred copies of the chloroplast genome, a circular, rather small molecule of DNA, are present in each chloroplast. The genes are expressed by transcription to make messenger ribonucleic acid

genome the genetic material of an organism

False color transmission electron micrograph of a developing chloroplast in a tobacco leaf.

genome the genetic material of an organism

False color transmission electron micrograph of a developing chloroplast in a tobacco leaf.

cyanobacteria photosyn-thetic prokaryotic bacteria formerly known as blue-green algae eukaryotic a cell with a nucleus (eu means "true" and karyo means "nucleus"); includes pro-tists, plants, animals, and fungi prokaryotes single-celled organisms without nuclei, including Eubacteria and Archaea diatoms hard-shelled single-celled marine organisms; a type of algae

(mRNA), which is translated on chloroplast ribosomes. These ribosomes, about one million in total number per chloroplast, are synthesized inside the chloroplast and are slightly smaller than the cytosolic ribosomes that are encoded by nuclear DNA. Therefore, chloroplasts are able to synthesize their own proteins, but in fact make only about 10 percent of the proteins they contain. Although a chloroplast may contain 500 to 1,000 different proteins, the chloroplast genome contains only 70 to 80 genes for proteins among its total of about 150 genes. The remainder of the proteins are encoded in nuclear DNA and imported.

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