The term selfish DNA refers to a section of DNA on the genome that is not expressed phenotypically. Selfish DNA is non-transcribed and non-coding; it does not contribute anything to the benefit of the organism and is selectively neutral in most cases. Hence, it is known to have two distinct properties: (i) the DNA sequence spreads by forming additional copies of itself within the genome; and (ii) it makes no specific contribution to the reproductive success of its host organism.
The selfish DNA, once occurred, is passively replicated and passed on from parent to the offspring. The changes in its occurrence in the population are due to drift, and such a selection acts to reduce it only in case it is accumulated to such an extent that the cell cycle becomes sluggish by the requirement to replicate it all or if a non- coding sequence interferes with the construction of the organism.
Provided the quantity of a particular sequence is not excessive, it is not transcribed, and it accumulates in parts of the genome where is does not interfere with genetic regulation and transcription. There is every chance that selfish DNA would evolve and develop.
The two types of selfish DNA are:
• Passive DNA: If the sequence itself does not influence the chance of its spreading in the DNA and is retained, it is passive kind of selfish DNA. It could accumulate as ” junk DNA ” in the genome.
• Parasitic DNA: A particular sequence might have a chance of spreading through the DNA. These sequences are more active and are called parasitic kind of selfish DNA and would proliferate until checked by natural selection.