To complete this summary of genetic processes, there are small sections of DNA that exist independently of the nuclear DNA that we have been describing until now. They exist within bodies in the cell, though outside the nucleus, called mitochondria (The singular is mitochondrion)
There may be a hundred of them in each cell, though numbers vary between different kinds of cell. Their importance has generally been seen that they perform an important function in generating energy by breaking down food substances. More recently, however, a variety of other functions have been discovered to do with the cell's metabolism and other processes.
Their origin is thought to date back to almost the beginning of life on Earth.(1) The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria. Similar organisms exist today near for instance the vents of undersea volcanoes. These appeared some four billion years ago early in the Archean Eon when the atmosphere is thought to have contained almost no free oxygen.
The earth was still warm from the remaining heat of its formation and from the decay of radioactive elements. Although the atmosphere contained high concentrations of hydrogen, methane, ammonia and carbon dioxide, the temperature is thought to have been much as it is now. This was partly because the sun's radiation was much lower and because more was reflected away by the lack of clouds. Geological evidence points also to the presence of liquid water in shallow seas.
The conditions were such that the mixture of chemicals interacted with each combining to form various compounds, but how these somehow formed the earliest self-replicating organisms is still a matter of debate. In time cells formed with a protective membrane coat and, in time, the supply of preformed organic materials may have diminished.
Those that could make use of inorganic compounds, notably hydrogen sulphide, the so-called autotrophs, would have been favoured. At some point, some of them gained to ability to draw energy from the sunlight to use carbon dioxide to produce sugars and starches, becoming the cyanobacteria. As a result the concentration of oxygen in the atmosphere rose rapidly which other organisms became able to use. Once these appeared those organisms that could not produce nutrition but relied on preformed nutritional material, the heterotrophs, could flourish once again producing a self-regulating carbon cycle.
Such organisms were the forerunners of the chloroplasts that produce the chlorophyll in plants, and mitochondria, the energy producers in animals. The theory of symbiogenesis holds that, at some point, they became incorporated in other organisms, conferring the benefit of their energy processing, maybe gaining protection from the elements, but losing the ability to live independently.(2)
At the beginning of the 1980s genetic data began to be applied to human evolution and came to prominence in the so-called "mitochondrial Eve" hypothesis. This suggested that all living people could be traced back to a single female who lived in Africa approximately 200,000 years ago
This female would have been a member of about 10,000 people, the founding population of modern humans. It should not be thought that she was the only, or original female. Assuming equal numbers of males and females are born, at a given generation one quarter of mothers will bear only male offspring and the mitochondrial line will end.
The importance of the hypothesis was that it supported the idea of a single origin of humans migrating out of Africa, rather than the competing idea of multiple simultaneous evolution over a number of different regions.
The theory relies on the fact that mitochondrial DNA is inherited only through the maternal line. Textbooks used to suggest that this was because, while a female's ovum contains virtually all that would be needed for the developing baby, the male sperm was little more than genetic material in a protective membrane. In fact the sperm's mitochondria are concentrated in its tail which is lost at fertilisation. We now know that paternal mitochondria do, in fact, sometimes enter the ovum but are neutralised by tagging with a protein, ubiquitin, and possibly by other processes.(3, 4)
Since only maternal DNA is involved, there is no recombination between paternal and maternal genes, allowing more straightforward tracing of inheritance. Moreover the mutation rate of the 37 genes is higher than it is for nuclear DNA.(5)
Following this, attention turned to the Y chromosome which is passed solely through the paternal line, though results have not been so clear cut. (6)
Bibliography and good reading.
Bland.J., (2019) Mitochondria http://www.gender.org.uk/about/03gene/39_mitochond.htm.
Book graphics courtesy of Amazon.co.uk
Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
Last amended 11.07.19