"In mammalian cells, only about 1% of the total cellular DNA is composed of mitochondrial DNA whereas in other organisms (for example in the egg cells of amphibians) there is a much higher percentage of mitochondrial DNA. " - UDNA is located in nucleus. DNA is also present in mitochondria (but less than 0.1% of the total DNA)" - Harpers 30th/eMitochondrial DNA (mtDNA or or mDNA)oMitochondrial DNA is the only non-chromosomal DNA in human cells. Mitochondrial DNA is only a small portion of the DNA in a eukaryotic cell (less than 0.1% of the total DNA).oIn humans, the 16,569 base pairs of mitochondrial DNA encode for only 37 genes. Human mitochondrial DNA was the first significant part of the human genome to be sequenced.oMitochondrial DNA is almost always maternally inherited. Thus diseases caused by mutation in mitochondrial DNA are always inherited from mother to next generation. All children from affected mother will inherit the disease. But no children will inherit the disease from affected father. Both male and females are affected. Affected son does not transmit the disease to next generation.oMitochondrial vs. Nuclear DNAoSimilar to the nuclear genome, the mitochondrial genome is built of double-stranded DNA, and it encodes genes. However many interesting features distinguish human mitochondrial DNA from its nuclear counterpart, including the following:oThe mitochondrial genome is circular, whereas the nuclear genome is linear.oThe mitochondrial genome is built of 16,569 DNA base pairs, whereas the nuclear genome is made of 3.3 billion DNA base pairs.oThe mitochondrial genome contains 37 genes that encode 13 proteins, 22 tRNAs, and 2 rRNAs.oThe 13 mitochondrial gene-encoded proteins all instruct cells to produce protein subunits of the enzyme complexes of the oxidative phosphorylation system, which enables mitochondria to act as the powerhouses of our cells.oThe small mitochondrial genome is not able to independently produce all of the proteins needed for functionality; thus, mitochondria rely heavily on imported nuclear gene products.oOne mitochondrion contains dozens of copies of its mitochondrial genome. In addition, each cell contains numerous mitochondria. Therefore, a given cell can contain several thousand copies of its mitochondrial genome, but only one copy of its nuclear genome.oThe mitochondrial genome is not enveloped, and is it not packaged into chromatin.oThe mitochondrial genome contains few, if any, noncoding DNA sequences. (Three percent of the mitochondrial genome is noncoding DNA, whereas 93% of the nuclear genome is noncoding DNA).oSome mitochondrial coding sequences (triplet codons) do not follow the universal codon usage rules when they are translated into proteins.oSome mitochondrial nucleotide bases exhibit functional overlap between two genes; in other words, the same nucleotide can sometimes function as both the last base of one gene and the first base of the next gene.oThe mitochondrial mode of inheritance is strictly maternal, whereas nuclear genomes are inherited equally from both parents. Therefore, mitochondria-associated disease mutations are also always inherited maternally.oMitochondrial genes on both DNA strands are transcribed in a polycistronic manner: Large mitochondrial mRNAs contain the instructions to build many different proteins, which are encoded one after the next along the mRNA. In contrast, nuclear genes are usually transcribed one at a time from their own mRNA.">