VI.2.1.1 Selfish DNA is the term used for those DNA segments that proliferate in the genetic pool precisely by virtue of molecular drive.
It has long been thought that there is a large amount of DNA in the eukaryotic genome that probably has no functional importance for its bearer or is even harmful for it – amongst plants, for example, it increases the risk of extinction of the given species. Sequential analysis of this DNA has revealed a large content of termination codons. Further, it has been found that these sequences are often surprisingly simple and uniform. In some cases, they can form long segments of a single nucleotide or dinucleotide. Consequently, it is obvious that a meaningful protein or biologically important ribonucleic acid could not be coded here.
Extensive discussions are held on the causes for the massive occurrence of these sequences. According to some authors, this DNA can be of great importance in regulating the expression of genetic information and in pairing of homologous chromosomes in cell division or could have some structural importance. However, at the present time, the opinion predominates that this is mostly selfish DNA (Dawkins 1976; Orgel & Crick 1980; Doolittle & Sapienza 1980). Selfish DNA is a term used to refer to DNA that does not provide any benefit to its bearer and, in fact, mostly brings it a smaller or greater disadvantage. At the very least, the cell must synthesize nucleotides for its replication and provide energy in the form of macroergic bonds.
DNA with these properties cannot be included in the genome of organisms through natural selection but only through molecular drive. This is thus apparently a kind of DNA whose sequence is suitably adapted for effective participation in processes of the gene conversion, transposition, uneven crossing-over ortemplate slippage types. For example, if, as a consequence of random mutations, a sequence is formed in the DNA consisting of monotonous multiple repetition of a certain nucleotide, it is highly probable that gradual lengthening of this repetitive segment will occur trough slippage of the nucleotide strand during evolution of the organism (see below).
Simultaneously, it makes no difference whether this segment brings its bearers an advantage or a disadvantage. Only if this segment were too long could it happen that the energy burden associated with its replication would be so large that it would basically put the organism at a disadvantage in competition with the other individuals in the population. In this case, a certain equilibrium state would be formed in the gene pool of the population. Molecular drive would act in one direction on the gene pool and would generate alleles bearing a longer section of repetitive DNA. Natural selection would act in the opposite direction and would remove individuals with very long repetitive segments from the population. Replication of the given segment would thus apparently be stopped. Population studies have shown that the content of repetitive DNA frequently differs dramatically amongst members of the population, where there is an even greater difference in the inter-population variability in the content of repetitive DNA (and thus total DNA). For example, it is known that the size of the haploid genome in the mosquito Aedes albopictus differs up to 2.5-fold in different countries.