the human genome is the term used to describe the total genetic information (DNA content) in human cells. It comprises of two genomes - a complex nuclear genome and a very simple mitochondrial genome.
The human mitochondrial genome is defined by a single type of circular double stranded DNA whose complete nucleotide sequence has been established. It is 16569 bp in length and is 44% (G+C). The 2 DNA strands have significantly different base composition, the heavy(H) strand is rich in guanine, the light(L) strand is rich in cytosine. Human cells typically contain thousands of copies of the mitochondrial molecules, but the number can vary considerably in different cell types.
During zygote formation, a sperm cell contributes its nuclear genome, but not its mitochondrial genome to the egg cell. The cytoplasm and the mitochondria in the zygote is got from the egg cell. Therefore, the mitochondrial genome is said to be maternally inherited. Males and females both inherit their mitochondrial from mother but males do not transmit their mitochondrial to subsequent generations.
MITOCHONDRIAL GENES:- The human mitochondrial genome contains 37genes. For 28 of the genes the heavy strand is the sense strand, for the other nine the light strand is the sense strand of the 37 genes,
24 specify a mature RNA product, 22-mitochondrial t-RNA molecules and 2-mitochondrial rRNA molecules. The remaining 13 genes encode polypeptides which are synthesized on mitochondrial ribosomes.
Each of the 13 polypeptides encoded by the mitochondrial genome is a subunit of one of the mitochondrial respiratory complexes, the multichain enzymes of oxidative phosphorylation which are engaged in the production of ATP. However , there are about 100 different polypeptide subunits in the mitochondrial oxidative phosphorylation system and so the vast majority are encoded by nuclear genome and are translated on cytoplasmic ribosomes before being imported into the mitochondria. Unlike its nuclear counterpart, the human mitochondrial genome is extremely compact:approximately 93%of the DNA sequence represents coding sequence.
The human mitochondrial genome is defined by a single type of circular double stranded DNA whose complete nucleotide sequence has been established. It is 16569 bp in length and is 44% (G+C). The 2 DNA strands have significantly different base composition, the heavy(H) strand is rich in guanine, the light(L) strand is rich in cytosine. Human cells typically contain thousands of copies of the mitochondrial molecules, but the number can vary considerably in different cell types.
During zygote formation, a sperm cell contributes its nuclear genome, but not its mitochondrial genome to the egg cell. The cytoplasm and the mitochondria in the zygote is got from the egg cell. Therefore, the mitochondrial genome is said to be maternally inherited. Males and females both inherit their mitochondrial from mother but males do not transmit their mitochondrial to subsequent generations.
MITOCHONDRIAL GENES:- The human mitochondrial genome contains 37genes. For 28 of the genes the heavy strand is the sense strand, for the other nine the light strand is the sense strand of the 37 genes,
24 specify a mature RNA product, 22-mitochondrial t-RNA molecules and 2-mitochondrial rRNA molecules. The remaining 13 genes encode polypeptides which are synthesized on mitochondrial ribosomes.
Each of the 13 polypeptides encoded by the mitochondrial genome is a subunit of one of the mitochondrial respiratory complexes, the multichain enzymes of oxidative phosphorylation which are engaged in the production of ATP. However , there are about 100 different polypeptide subunits in the mitochondrial oxidative phosphorylation system and so the vast majority are encoded by nuclear genome and are translated on cytoplasmic ribosomes before being imported into the mitochondria. Unlike its nuclear counterpart, the human mitochondrial genome is extremely compact:approximately 93%of the DNA sequence represents coding sequence.
- All 37 mitochondrial genes lack introns and they are tightly packed ( on average one per 0.45kb).
- The coding sequences of some genes ( those encoding 8th and 6th subunits of mitochondrial ATPase) show some overlap
- In most other cases, the coding sequences of neighbouring genes are contiguous or separated by one or two non-coding bases.
- The mitochondrial genes encodes all the rRNA and t-RNA molecules it needs for synthesizing proteins but relies on nuclear encoded genes to provide all other components.(Such as protein components of mitochondrial ribosomes, amino acyl t-RNA synthetases etc.)
- The only significant region lacking any know coding DNA is the displacement(D) loop region.
- The replication of both H and L strands is unidirectional and starts in the D-loop.
- Unlike the transcription of nuclear genes, in which individual genes are almost always transcription of the mitochondrial DNA starts from the promoters in the D-loop region and continues in the opposing directions for the two different strands round the circle to generate large multigenic transcripts. The mature RNA, are subsequently generated by cleavage of the multigenic transcripts.
- These are 60 mitochondrial sense codons.
- Two of the four stop codons, UAA and UAG also act as stop codons. the other two are AGA and AGG which specifies arginine in nuclear genetic code.
- UGA which is a stop codon in the nucleus encodes for tryptophan in the mitocondria.
- AUA specifies methionine in the mitochondria as in nucleus it specifies isoleucine.
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