Biotechnology and Bioinformatics (1+1)

1.2.6. Mitochondrial DNA

DNA is also present inside a cell as extra-chromosomal gene s in mitochondria. Several mitochondria are present in each cell. In plants, the photosynthetic organelles, chloroplasts also contain DNA.

Unlike the chromosomal DNA, there is no meiosis and replication appears to be a simple copying process. Extra chromosomal genes present in mitochondria are normally circular molecules of around 16 kb in length. Because there are large numbers of mitochondria in an egg, but very few in a spermatozoan, it is hardly surprising to find that mt DNA present in a sexually reproduced offspring is usually inherited entirely from its mother. This maternal only inheritance of mt DNA is the normal situation in almost all animals (but Mytilus sp. is an exceptional one).

In contrast to the nuclear genome , the mitochondrial genes of animals are very efficient and have no intron s. In addition there is virtually no ‘ junk DNA ’ or repetitive sequence s in mitochondrial genome although the control region does often vary in length due to tandem repeats.

Mitochondria protein coding genes code for enzymes that are involved in electron transport system. They include seven subunits of NADH dehydrogenase, cytochrome b, cytochrome c oxidase and ATP synthetase.

The mitochondrial genome of fish contains 13 genes coding for proteins, two genes coding for ribosomal RNA, 22 genes coding for transfer RNA molecules and one non-coding section of DNA which acts as the initiation site for mt DNA replication and RNA transcription . This is called the control region.

The rate of mutation in animal mt DNA is higher than in the nuclear DNA (about 5 to 10 times higher). This means that the rate of evolution is greater in mtDNA than in nuclear DNA.

Analysis of mitochondrial DNA for determination of population relationships is particularly attractive for three reasons.

i) it is relatively small (For e.g., in rainbow trout -16.5X10³ bp and catfish – 17 kb) and less complex than nuclear DNA.

ii) mt DNA exhibits a more rapid evolution than nuclear DNA and thus, allows for detection of relatively recent sequence divergence.

iii) the inheritance of mt DNA is apparently strictly maternal, thereby avoiding the complication of sexual recombination of genetic material.

Restriction enzyme analysis of mt DNA from several populations of rainbow trout result in their differentiation. Analysis of mt DNA by restriction endonuclease s has been used to distinguish three species of catfishes from the Arabian Gulf (Arius bilineatus, A. thalassinus and A. teniispinis).