prepare a clear account on extranuclear genome in chloroplast and mitochondria
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In this article we will discuss about the genomes in chloroplast DNA and mitochondria DNA.
The phenomenon of extra-nuclear inheritance based on transmission of visible phenotypes through mitochondria and chloroplasts. Studies in the 70s revealed presence of DNA in these organelles. Both mitochondria and chloroplasts are present only in cells of lower and higher eukaryotic organisms. Detailed studies established that DNA in these organelles is similar to the DNA in prokaryotic bacteria.
The genomes of both mitochondria and chloroplasts code for all of their RNA species and some proteins that are involved in the function of the organelles. The DNA is in the form of a circular duplex molecule, except in some lower eukaryotes in which mitochondrial DNA is linear.
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Each organelle contains several copies of the genome, and because there are-multiple organelles per cell, organelle DNA constitutes a repetitive sequence. Mitochondrial DNA (mtDNA) varies enormously in size, whereas chloroplast DNA (ctDNA) ranges in size between 120 and 200 kb.
Chloroplast DNA:
Chloroplasts are present in green plants and photosynthetic protists. ctDNA sequence studied in a number of plants indicates uniformity in size and organisation. The differences in size are due mainly to the differences in lengths of introns and inter-genic regions as well as the number of genes. All cp DNAs contain a significant proportion of noncoding DNA sequences.
The ctDNA is double stranded circular, and devoid of histones and other proteins. In many cases, the GC content of cpDNA differs from that of nuclear DNA and mitochondrial DNA. Complete cpDNA sequences have been determined in tobacco (155, 844 bp) and rice (135, 42 bp).
Multiple copies of cpDNA are present in the nucleoid region of each chloroplast. In the green alga Chlamydomonas, one chloroplast contains 500 to 1500 cpDNA molecules. Chloroplasts divide by growing and then dividing into two daughter chloroplasts.
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The genomes of both mitochondria and chloroplasts code for all of their
RNA species and some proteins involved in the function of organelles.
Each organelle contains several copies of the genome, and because there are multiple organelles per cell, organelles DNA contains a repetitive sequence.
CHLOROPLASTS :
Chloroplast genome contains genes for some of the proteins required for transcription and translation of the encoded genes and also for photosynthesis.
The genes that are found in repetitive sequences are also duplicated in the chloroplast genome.
MITOCHONDRIA :
As mitochondria is dependent upon the nucleus genome it is considered as semi autonomous body.
The large size of mitochondrial genomes in plant are due to non coding inter-genic regions and their content of tandem repeats.
Most of the Other proteins found in mitochondria are encoded by the nucleus genome and transported into mitochondria.
The proteins in mitochondria ribosomes are encoded by the nucleus genome and transported into mitochondria from cytoplasm.