<P> In humans, the heavy strand of mtDNA carries 28 genes and the light strand of mtDNA carries only 9 genes . Eight of the 9 genes on the light strand code for mitochondrial tRNA molecules . Human mtDNA consists of 16,569 nucleotide pairs . The entire molecule is regulated by only one regulatory region which contains the origins of replication of both heavy and light strands . The entire human mitochondrial DNA molecule has been mapped . </P> <P> The genetic code is, for the most part, universal, with few exceptions: mitochondrial genetics includes some of these . For most organisms the "stop codons" are "UAA", "UAG", and "UGA". In vertebrate mitochondria "AGA" and "AGG" are also stop codons, but not "UGA", which codes for tryptophan instead . "AUA" codes for isoleucine in most organisms but for methionine in vertebrate mitochondrial mRNA . </P> <P> There are many other variations among the codes used by other mitochondrial m / tRNA, which happened not to be harmful to their organisms, and which can be used as a tool (along with other mutations among the mtDNA / RNA of different species) to determine relative proximity of common ancestry of related species . (The more related two species are, the more mtDNA / RNA mutations will be the same in their mitochondrial genome). </P> <P> Using these techniques, it is estimated that the first mitochondria arose around 1.5 billion years ago . A generally accepted hypothesis is that mitochondria originated as an aerobic prokaryote in a symbiotic relationship within an anaerobic eukaryote . </P>

Where is the code for traits found and how are they produced