GENOME ORGANIZATION-Mitochondria

GENOME ORGANIZATION

The genetic evidences indicate that mitochondria contain genes distinct from those located in the nucleus. Mitochondrial DNA’s was identified on the basis of its unique density. The amount of mitochondrial DNA found in eukaryotic cells varies. In animal mitochondrial DNA-typically accounts for 0.1 to 1.0 percent of a cells total DNA contents in Yeast its value is high as 15%. The DNA located in mitochondria exhibits several features that distinguished it from nuclear DNA. One major difference is that mt DNA lacks histone and hence not packaged into nucleosomes.

Mt DNA is typically Circular. The mitochondrial DNA circles of animals cells are the smallest about 15,000-20,000 bp in size. Mt DNA of fungi and protists are larger i.e. 20,000-1,00,000bp and plant mt DNA are largest ranging from 2,00,000bp to million bp in length. One to the tendency of large circular DNA’s to break during isolation many reports suggest that plant mt DNA is linear rather than circular. Few unicellular prokaryotes such as Paramecium have linear mol of mt DNA’s.

In animals, fungi and protists mitochondrial contains few dozens to several 100’s mol of DNA, all of which are identical.

The first direct evidence for an association between cytoplasmic inherited tracts and mt DNA was produced by Piotr Solnimski et.al. they demonstrated that mt DNA obtained from yeast bearing petite mutation has a different density than the mt DNA of normal yeast Petites is induced by treatment with ethidium bromide as a model system. These studies revealed that ethidium bromide stimulates the breakdown of mt DNA and inhibits mt DNA replication. Long term exposure to ethidium bromide causes the complete destruction of mt DNA, but if treatment is stopped before all the DNA has been degraded, the remaining DNA resumes replication until the total context of mt DNA reaches normal levels. But the new mt DNA is grossly abnormal containing few of its sequence present in the original mt DNA. The presence of abnormal mt DNA in petite yeast strongly argues that this DNA is responsible for the abnormalities in mitochondrial structure and respiratory activity observed in petites mutants.

Mitochondrial DNA-replication

The replication of mt DNA is carried out by DNA p-ax that operates independently of nuclear DNA p-ase, as mt DNA synthesis can take place anytime during the cell cycle i.e. they are not restricted to S-phase when nuclear DNA replicates. In 1971 Jerome Vinogard and Piet Borst independently discovered that mt DNA mols exhibit a small 100p.displaced from the main circle and mode of replication is semi conservative. Denaturation of DNA mols causes a small fragment of S.S.DNA to be released, suggesting that loop is caused by displacement of one of the two strand of double helix, therefore it is termed as ‘D’ loop (Displacement loop)

mt DNA codes for ribosomal RNA’s, t-RNA’s and small mitochondrial polypeptides

Several evidence suppose the conclusion that mt DNA serves as a template for transcription of mt RNA’s. 2^nd-mitocondria contains their own RNA-p-ase that can be distinguished from nuclear. RNA’s by differing susceptibility to inhibitors. 3^rd ethidium bromide inhibits mt DNA synthesis and not nuclear DNA synthesis supports the conclusion that independent RNA synthetic pathways exist in nuclear and mitochondria.

mitochondria synthesis proteins using a unique sets of ribosomes, tRNA’s and protein factors.

The mRNA’s transcribed by mt DNA are translated into polypeptides within mitochondria using. Protein synthesizing machinery that differs from the machinery present in the cytorol.

Ribosomes:     Mitochondrial ribosomes differs from cytoplasmic ribosomes in size & chemical contraction  (Table – I)

I – sizes of various types of ribosomes

Source	Intact Ribosomes	Ribosomal Sub units	Ribosomal RNA’s
Mitochondria (mammals)	55-605	Large – 455 Small - 355	165 125
Mitochondria (yeast)	755	Large – 535 Small - 355	215 145
Mitochondria (Plants)	785	Large – 605 Small – 405	265 185, 55
Cytoplasm Eukaryotes	805	Large – 605 Small - 405	285 185, 5.85, 55

Transfer RNA’s

Mitochondria contains t-RNA’s and amino-acyl t-RNA synthetase. The amino-acyl t-RNA synthetase are coded by nuclear DNA by t-RNA’s are transcribed from mitochondria differs in sequence from the corresponding t-RNA’s found in cytosol.  Mammalian mt DNA codes for only 22t RNA’s following wobble rules Another unusual features of mitochondrial t-RNA is presence of the t-RNA for formylmethionine. This type of t-RNA is once thought to be found in bacterial only. Its presence in mitochondria suggest that protein synthesis in the two organelle inhibits some similarities to the protein synthesis of bacterial.

Protein synthesis factors

Protein synthesis factors of mitochondria are structurally distinct from its counterpart in the cytosol. Several protein synthesis factors present in mitochondria are functionally inter changeable with those obtained from bacterial cells.

Altered Genetic code

 Examining base sequence of mitochondrial DNA’s has revealed that mRNA is translated into polypeptide chair using a genetic code that is slightly different from universal code system difference in codon usage can occur even among the mitochondria of different organism   (Table – II)

II – Examples of Altered Genetic Code Usage in Mitochondria

S.N.	Codon	Universal code system	Mammals	Yeast
1	AUA	Ile	Met	Met
2	UGA	Stop	Trp	Trp
3	AGA, AGG	Arg	Stop	Arg
4	CUU CUC CUA CUG	Leu	Leu	Thr

Mitochondria synthesize polypeptides whose genes resides in mt-DNA.

Nuclear gene co-operates with mitochondrial gene in making mitochondrial protein.