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chloroplast genome

Chloroplast Genome<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Most organelles genome so far characterized take the form of a single circular molecule of <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />DNA of unique sequence (denoted mtDNA in the Mitochondria &cpDNA in the chloroplast).There are few exceptions where mtDNA is a linear molecule, generally in lower eukaryotes.

Chloroplast genomes are relatively large, usually ~140kb in higher plants&<200kb in lower eukaryotes. This is comparable to size of a large bacteriophage,e.g. T4 at ~65kb.There are multiple copies of the genome per organelle, typically 20-40 in a higher plant multiple copies of the organelles per cell, again 20-40.

Evolution: Suppose that these organelles originated in endosymbiotic events, in which primitive cells captured bacteria that provided the function that evolved into mitochondria& chloroplast. At this point, the proto-organelles must have contained all the genes need to specify its functions. At some later time, some or most of the organelle gene must have been transferred to nucleus. Homologies suggest that both Mitochondria & chloroplast have evolved separately from lineages that are common with Eubacteria, & chloroplast sharing an origin with Cyanobacteria.

What genes are carried by chloroplast?

The chloroplast genome codes for all the rRNA& tRNA species needed for protein synthesis. The ribosomes include two small rRNAs in addition to the major species.The tRNAset resembles that of Mitochondria in including fewer spp. than would suffice in the cytoplasm. 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 lengthing sequence,10-24kb depending on the plants, that is present in two identical copies as an inverted repeat(Gene that are coded with in the inverted repeats are present in two copies per genome &include the rRNA genes). The organization of genome is summarized TABLE.

Table (1) The chloroplast genes codes for 4rRNAs,30tRNAs&~40 proteins.

Genes

Type

Genes

Type

RNA coding

Thylakoid memb.

16s rRNA

1

PS-І

2

23s rRNA

1

PS-П

7

4.5s rRNA

1

Cyt b/f

3

5s rRNA

1

H+-ATPase

6

tRNA

30

Others

Gene Expression

NADH dehydrogena

6

R Proteins

19

Ferridoxin

3

RNA polymerase

3

Ribulose BP cblase

1

Others

2

Unidentified

29

Total

110

The Chloroplast Genome

The genome of the chloroplasts found in Marchantia polymorpha (a liverwort, one of the Bryophyta) contains 121,024 base pairs in a closed circle. These make up some 128 genes which include:

  • duplicate genes encoding each of the four subunits (23S, 16S, 4.5S, and 5S) of the ribosomal RNA (rRNA) used by the chloroplast
  • 37 genes encoding all the transfer RNA (tRNA) molecules used for translation within the chloroplast. Some of these are represented in the figure by black bars (a few of which are labeled).
  • 4 genes encoding some of the subunits of the RNA polymerase used for transcription within the chloroplast (3 of them shown in blue)
  • a gene encoding the large subunit of the enzyme RUBISCO (ribulose bisphosphate carboxylase oxygenase)
  • 9 genes for components of photosystems I and II
  • <?xml:namespace prefix = v ns = "urn:schemas-microsoft-com:vml" /><?xml:namespace prefix = w ns = "urn:schemas-microsoft-com:office:word" />6 genes encoding parts of the chloroplast ATP synthase
  • genes for 19 of the ~60 proteins used to construct the chloroplast ribosome

All these gene products are used within the chloroplast, but all the chloroplast structures also depend on proteins

  • encoded by nuclear genes
  • translated in the cytosol, and
  • imported into the chloroplast.

Recombination b/w the repeats can further an inversion of the short single copy sequences(SSC) b/w than with respect to the long single copy sequence(LSC) that constitutes the rest of the genome. The complete sequence of chloroplast DNA has been determined for a liverwort(a moss)& for tobacco.In spite of a considerable difference in overall length b/w 121&155kb,the gene organization is similar &overall number of genes almost identical. Most of their products are thylakoid components of the thylakoid membranes or converted with redox reactions.

TABLE(2):The chloroplast genome is a circle, divided by two inverted repeats into the SSC&LSC-

Liverwort

Tobacco

Inverted repeats(IRs)

10,058

25,339

SSC

19,813

18,482

LSC

81,095

86,684

Total genome

121,024

155,844

The protein synthesis apparatus displays resembles to the bacterial apparatus. Many of the ribosomal proteins are homologus to those of E.coli & some of the ribosomal protein genes are present in clusters that resemble corresponding clusters in E.coli. genes for RNA polymerase subunits α,β are homologus to the genes core enzymes in E.coli,yet the gene for β subunits has an intron.

The presence of interrupted genes suggest that chloroplast originated before prokaryotes lost their introns.The inform fall intron general classes, there in tRNA genes are usually (although not inevitably) located in the anticodons loop,like the introns found in yeast nuclear tRNA genes.

The role of the chloroplast is to undertake photosynthesis &may of its genes code for proteins of the complexes of the thylakoid membranes. The constitution of these complexes shows a different balance from that prevailing for mitochondrial complexes.

TABLE(3):Photosynthetic complex of the thylakoid membrane may be coded by the chloroplast or nuclear genome or both:

Complex

Subunits synthesized in chloroplast

Subunits synthesized in cytoplasm

Light harvesting

None

3 subunits 20-30 kb

PS-

P700/A1,P700/A2

-

PS-

32k P680,44k,D2,Cyt559

-

Cyt b/f

Cyt f,b6,sub 4

-

H+ATPase

αβ γ І П

γδП

RICE PLANT GENOME

Structure of plant chloroplast genome

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