Wednesday, July 09, 2008

Evolution: DNA part 2: DNA exchange at the periphery

As mentioned before, human DNA is the familiar double helix shape, wrapped up inside the nucleus of each cell. The physical molecule is referred to as DNA; packaged up (with proteins) it constitutes a chromosome; normal human cells have 23 pairs of chromosomes. Each DNA molecule contains a number of base pairs (the unit of genetic information, somewhat akin to a byte in digital information). The human genome of 46 chromosomes has about three billion base pairs.

But the narrative is not nearly as neat as that. We have to understand bacterial DNA, mitochondrial DNA, plasmids, and how they all interact and exchange information.

The basics of human genetic inheritance are relatively simple: sex-specific cells, the sperm and egg gametes, are haploid, only containing one copy of each chromosome pair, pending unification which results in the full complement.

Mitochondrial DNA is one of the complications. This is not a part the human genome located in the nucleus: it's genetic information located in mitochondria, cell elements outside the nucleus that generate chemical energy for the cell to operate. Each cell can contain from one to thousands of mitochondria, depending on the organism type and cell type. When gametes unite, the sperm's mitochondrial DNA is tagged for deletion, so allowing mtDNA to be a marker of matrilineal descent.

Mitochondrial DNA is small (in the order of 15,000 base pairs) and circular - which is pretty much the description of bacterial DNA. In fact, it is generally agreed that mitochondria are bacterial in origin, once endosymbiotic - that is, symbiants located inside the cell that came to be part of the cell's structure. How? A somewhat harder question to answer.

Bacteria are prokaryotic - that is, they lack a cell nucleus. Their DNA consists of a single continous loop. But bacteria, too, have strands of DNA that are separate from their chromosomes. These are called plasmids, rings of DNA that are capable of reproducing independently of bacterial reproduction. Plasmids could be seen as independent symbiotic life-forms in bacteria, similar to viruses except more useful, for example conveying antibiotic res

Bacteria reproduce through binary fusion: that is, they divide into two daughter cells. In the process, the bacterial chromosome is duplicated, one copy for each daughter.

This inheritance mechanism sounds simple, as if each bacterium could be uniquely traced to an ancestor. However, there is an additional mechanism for DNA change: horizontal gene transfer (HGT).

HGT can be seen as a counterpoint to inheritance mechanisms, which could be called vertical gene transfer. HGT, as the name suggests, involves the transfer of genetic information between organisms. This too can result in the spreading of drug resistance - in fact, this was how it was first noted, as far back as 1959. There are three mechanisms for this: transformation involves the absorption of foreign DNA; transduction occurs when a bacterial virus transfers genetic information from one bacterium to another; and in bacterial conjugation, bacteria that are touching can under certain circumstances exchange DNA.

HGT is common in prokaryotic bacteria, and even happens in some unicellular eukaryotes. It would be harder to say that this mechanism impacts on multicellular eukaryotes (having cells with nuclei), the evidence does suggest this has happened. It is suggested that this would have happened in the early stages of eukaryotic evolution.

However, this story is not complete. I noted in May a study of bdelloid rotifers (small marine animals), which found they had absorbed genetic material from a wide range of organisms: animals, plants, fungi and bacteria. The mechanism is not well understood, but it must have been relatively recently in evolutionary terms - in the order of tens of millions of years, maybe.

There are a number of gaps which need explaining, in particular how these mechanisms take place, and how they evolve. I hope to fill in some of the gaps in time, as far as scientific research and my learning permit.

So it would seem that bacteria play an important role in the spreading and exchanging of genetic information - even as far as humans.

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