BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 9. NUCLEIC ACIDS

9.4. Structure of nucleic acids

9.4.3. Mitochondrial DNA

A characteristic feature of eukaryotes is that a portion of their Genetic information is associated with molecules located outside the nuclear Chromosomes. Unlike other Organelles of Eukaryotic Cells, Cell/35.html">Mitochondria contain their own DNA, which differs from nuclear DNA in molecular weight and nucleotide composition. Mitochondrial DNA is predominantly represented by closed, circular, supercoiled, double-stranded molecules.

In humans, mitochondrial DNA consists of 16,569 Base Pairs and encodes 2 Ribosomal RNAs, 22 distinct Transfer RNAs, and 13 Proteins. The fact that a mere 22 tRNAs are sufficient to recognize all 64 codons indicates that a single tRNA molecule recognizes as many as 4 codons, whereas ordinary Ribosomes require at least 32. This alteration in the mitochondrial Genetic Code reduces Structure/19.html">The Importance of the third nucleotide in the codon, thereby requiring a smaller number of tRNAs.

The human Mitochondrial Genome encodes the synthesis of a small number of proteins. These include seven subunits of the NADH-oxidizing complex (60% of the genes), two subunits of ATP synthase, three subunits of cytochrome c oxidase, and one subunit of ubiquinol-cytochrome c reductase (cytochrome b). All other proteins—electron carriers, mitochondrial translocases, components of Protein transport into mitochondria, and factors required for the METABOLISM/31.html">Transcription, Translation, and Replication of mitochondrial DNA—are encoded by the nuclear genome.

The vast majority of DNA molecules are double-stranded. However, The Genome of certain bacterial, plant, and animal Viruses is represented by covalently closed single-stranded rings. Examples include the DNA of Bacteriophages M13 and ΦX174, as well as animal and plant Parvoviruses.