What Your Father Never Gave You But Mitochondrial Eve Did

No the answer is not the “Y-Chromosome.” True that a father does not pass a Y-chromosome to his daughters, but as it pertains to all his children (male and female) the answer is “Mitochondria.” Mitochondria are the sub-cellular organelles that process our energy. It is where energy from broken-down food molecules are converted to “ATP” the life-universal molecule for energy processes. The energy in the chemical bonds of the food substances which we ingest is converted into the energy of the bond of the third Phosphorus of Adenosine Tri-Phosphate (ATP) by the addition of a third Phosphate to Adenosine Di-Phosphate. It is a very complex process taught to all Biology Majors. An ATP molecule is like a “charged battery”. When ATP is converted to ADP (Adenosine Di-Phosphate) the energy from the third bond is used to do work (i.e., life processes). We need ATP for virtually everything we call life. We need it for muscle contraction, nerve impulse firing, thought, sight, everything we do. But this blog is not about physiology, it is about mitochondria. All higher plants and animals which have cells with a chromosome-bearing nucleus have mitochondria (the enlarged bean shaped organelle shown on left in diagram below) in the cell’s (the upper right in the diagram below) cytoplasm. The cytoplasm is the fluid filled area in cells between the nucleus and the outer cell membrane. The cytoplasm has a variety of “organelles” one of which is the mitochondria.

And the cells have lots of mitochondria, 100’s or even 1000! Cells which consume more energy have more mitochondria. Bacteria and related primitive life forms do not have organelles so they do not have discrete mitochondria. In fact they do not have a discrete nucleus with chromosomes. Bacteria, cyanobacteria (blue-green algae) and the bacteria-like algae that live in hot thermal springs also have circular DNA, and internal membranes that function like our mitochondria for energy metabolism. Now here is the first interesting fact. There are between 2 and 10 or so circular DNA’s inside our mitochondria which are remarkably like bacterial DNA. Our mitochondria duplicate their DNA and divide into two mitochondria pretty much like bacteria replicate their DNA. Studies of the specifics of the “genome” of bacteria and mitochondria have led most biologists to accept the “Endosymbiotic Theory” of the origin of the cells of higher plants and animals. It is believed that perhaps 1 to 2 billion years ago a primitive cell life form engulfed another and rather than consume or digest it, the engulfed primitive cell was retained in a symbiotic relationship. And over 1.5 billion years the relationship of the engulfed cell (today’s mitochondria and chloroplasts) to the host cell has evolved.

The endosymbiotic theory of mitochondrial origin is nowadays well confirmed and it is believed to be related to the increase of oxygen level in the atmosphere. The size of the symbiont’s genome was reduced due to endosymbiotic gene transfer (EGT) or gene loss during evolution. Genes involved in the synthesis of nucleosides (any one of the four nitrogenous bases bonded to a 5-carbon sugar like ribose or deoxyribose) and amino acids, anaerobic glycolysis, and cellular regulation were conveyed to the host and are currently encoded by nuclear DNA (nDNA) in humans. 

And thus the first mitochondria-like organelle appeared in a primitive nucleated organism. Similarly, plant’s chloroplasts (where photosynthesis occurs) have circular DNA. It is also believed that a primitive photosynthetic organism much like today’s blue-green algae (cyano-bacteria) was engulfed and became a symbiotic photosynthetic organelle for the evolutionary precursors of higher algae and plants.

Now back to what our fathers did not give us. A sperm does not contain any mitochondria which merge with the mitochondria of an ovum. A sperm contains 50–75 mitochondria in its midsection. The sperm’s mitochondria produce energy for the movement of the sperm. For comparison, in humans, the mature egg cell, or oocyte, contains from 100,000 to 600,000 mitochondria per cell, but each mitochondrion contains only one copy of mitochondrial DNA (mtDNA) initially.

A sperm has just enough mitochondria so it can swim so-to-speak, but it just passes an “N” or haploid set of chromosomes to the ovum. We get all our mitochondria from our mothers, who got theirs from their mothers, and so on and so on, infinitum. We have sequenced our mitochondrial DNA. It will be exactly like our mothers except for any mutations. And it will be like your grandmother on your mother’s side. And so on, and so on. The mitochondrial DNA measures only about 16,569 DNA base pairs (compared to 3.2 billion for your nuclear DNA in your chromosomes) and because it is circular and not a chromosome it does not re-combine (another future blog for Genetics Students).  So mitochondrial DNA is quite useful for mapping human populations. Tiny mutations and changes in mitochondrial DNA in human populations traveled with the human populations as humans spread out over the earth from east-central Africa about 150,000 years ago. The companies that provide you with information about your genetic heritage use the mitochondrial differences along with nuclear chromosomal DNA markers to give you a genetic makeup. 

Mitochondrial Eve – She is a theoretical woman who existed more than 150,000 years ago and all mitochondria in humans today can be traced back to her. Mutations in mitochondrial DNA (mtDNA) happen at a predictable rate in human populations. This is the so-called “clock.” Evolutionary geneticists look at the rate of mutations and changes in mtDNA across human populations and estimate that Mitochondria Eve existed 150,000 years ago. In order to be “Eve” she must have one or more female children who in turn had one or more female children and so on and so on until modern times. A woman who has no children or only male children could not pass on her mitochondria. If she or all of her female children and their female children died prior to having a female child, then she could not pass on her mitochondria. It is believed that human populations have experienced several bottlenecks where our numbers probably became quite low. In reality, a Mitochondrial Eve is not the first female of the human species, but merely the most recent female historically from which all humans can trace their mitochondrial ancestry. Mitochondrial Eve probably was a member of one of the small bottlenecked populations and likely located in east central Africa which is believed to be the cradle of humanity. Mitochondrial Eve dates back to around 100,000 to 250,000 years ago; the large range is indicative of the statistical variance of the data. Considering the history of human mitochondria, opposition to “three party babies (next blog) ” seems unfounded since the opposition is about mitochondrial DNA.

As human populations migrated out of Africa they took with them the mitochondria which would be shared by the entire human population on earth. Small subtle changes in the mitochondrial genome would be carried with the migrations of people and are present today in our mitochondria. The various ancestral DNA companies use these small differences was well as our nuclear DNA to trace our family’s origins.