Non-Nuclear DNA: Yet Another Gift From Mom
The abbreviation for deoxyribonucleic acid, DNA, has become in effect a word. Few do not appreciate its centrality to life. It carries our inheritance from mother and father and will transmit certain of our characteristics to our children. It directs the synthesis and determines the amino acid sequence of hundreds of thousands of proteins, including all our enzymes and structural proteins. DNA is subject to mutations: some silent, some deleterious, some beneficial, underlying evolution when considered as an effect on speciation. The propensity of genes to mutate underlies the risk of cancer, a disease of evolution.
DNA is arrayed on 23 pairs of chromosomes in the human cell nucleus. The length of uncoiled DNA in a single cell nucleus is measured at over six feet, it is so tightly wound. This situation is analogous to packing 24 miles of extremely fine thread into a tennis ball. Moreover, the double helical molecule of DNA in a cell nucleus contains several billion pairs of nucleic acid bases. Our 19,000 protein-encoding genes occupy approximately two percent of the length of a cell’s DNA. Much of DNA is “non-coding” or assigned to make various types of RNA that among other things regulate gene function and account for a gene’s ability to direct the synthesis of many different proteins.
Little appreciated is that we have DNA outside our cell nuclei, mitochondrial (mt)DNA. The mitochondria are subcellular structures that are present in the cytoplasm of cells and are responsible for energy generation. This process provides the power for such events as muscle contraction and other activities that require biochemical energy. In the mitochondria, DNA is arrayed in a doughnut-like circular single chromosome. This single, circular chromosome contains 37 genes dedicated to, among other things, encoding proteins, notably enzymes that convert energy sources derived from food into chemical energy to drive processes such as the contraction of heart muscle.
A critical feature of mtDNA is that it is derived only from ones mother since it is only the egg’s contribution of mitochondria to the fertilized egg, not the sperm’s, that persists. mtDNA provides a timeline for comparing potential mutations and mutation rates in DNA. Thus, mtDNA has been used to characterize our matrilineal ancestry. Since it has been passed from mother to male and female children from time immemorial, paleontologists and geneticists have used the nature of mtDNA to trace our primordial mother, referred to as Mitochondrial Eve, using the Hebrew biblical first woman’s name as a metaphor.
Mitochondrial DNA has been traced back approximately 150,000 years to South Africa where it is posited that the Mitochondrial Eve represented the women from whom all living humans descended. This approximation is buttressed by being dated after the speciation of Homo sapiens and before the “Out of Africa Theory” that describes the migration that led to the populating of Europe, Southern Asia and Australia by Homo sapiens. The Mitochondrial Eve concept does not mean that this was the first woman, nor the only living female of the time, nor the first member of a new species. She is defined as the most recent woman from whom all living humans descend in an unbroken line through their mothers. Looking back all existing genetic lines converge on this one woman. It is the result of the Mitochondrial Eve having had at least two daughters who have unbroken female lineages through the present time. The size of the ancient human population in that period never decreased below thousands based on studies of nuclear DNA.
So when thinking about DNA, recall that a small, but critical, set of genes resides outside the nucleus in mitochondria and gives us our oomph! By the way, paleontologists and biostatistical geneticists can use a structure passed only from father to son, the Y chromosome, to look for the Y-Chromosomal Adam, but that is another story. In that story, Adam and Eve did not necessarily know each other.
Written December 2020