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Chloroplast Genome Organization
Chloroplast is the chlorophyll containing organelle that carries out photosynthesis and starch grain formation in plants. Like mitochondria, chloroplasts contain DNA and ribosomes, both with prokaryotic affinities. The DNA of chloroplast (cpDNA) is a circle that ranges in size from 120-190 kb.

The chloroplast DNA, like mitochondrial DNA, controls the production of tRNAs, ribosomal RNAs and some of the proteins found within the organelle.

In more than 25 chloroplast DNAs from different plant, species that have been sequenced, there seems to be about 87-183 genes in the chloroplast genome. The chloroplast genome codes for all the rRNA and tRNA species needed for protein synthesis.

The ribosome includes two small rRNAs in addition to the major species. The tRNA set may include all of the necessary genes. The organelle genes are transcribed and translated by the apparatus of the organelle.

About half of the chloroplast genes codes for proteins involved in protein synthesis. The endosymbiotic origin of the chloroplast is emphasized by the relationship between these genes and their counterparts in bacteria.

The organization of the rRNA genes in particular is closely related to that of a cyanobacterium, which pins down more precisely the last common ancestor between chloroplast and bacteria.

Introns in chloroplast fall into two general classes. Those in tRNA genes are usually located in the anticodon loops, like the introns found in yeast nuclear tRNA genes. Those in protein coding genes resemble the introns of mitochondrial genes.

This places the endosymbiotic event at a time in evolution before the separation of prokaryotes with uninterrupted genes. The role of the chloroplast is to undertake photosynthesis. Many of the chloroplast genes code for proteins that are located or function in thylakoid membranes.

But quite strangely, even some protein complex found on thylakoid membrane are coded by nuclear genes. Thus on the thylakoid membranes you find proteins coded both by chloroplast genome and nuclear genome. Other chloroplast complexes are coded entirely by one genome.
 

 
  • Chloroplast Genome Organization
  • Origin of Cytoplasmic Organelles
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