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Only 1.2% of your genome codes for proteins.
What is the other 98.8% of your DNA?
Introns, promoters, and other things...
Over 44% of DNA consists of TRANSPOSONS.
Transposable elements are parasitic regions of DNA - "jumping genes".
They contain genes whose sole function is to make proteins that help the transposable element copy itself and jump to a new place in your genome.
Transposons increase their fitness, relative to the rest of your DNA, by making copies and inserting them into the DNA.
These transposons are then passed on to offspring .
What is the cost to the rest of your DNA?
Transposons can result in deleterious mutations if they insert themselves into a functional gene (e.g., hemophelia, tumors).
So if transposons are parasites, can natural selection limit their spread?
Yes. Transposons that insert themselves into functional genes usually result in host death, and are thus eliminated from the population.
Selection tends to favor transposons that insert themselves in non-coding regions of the DNA.
Most transposons are found near centromeres, where there is little coding DNA.

So can transposons ever be beneficial?
Antibiotic resistance! Some transposons code for resistance genes. These transposons spread from one bacterial cell to another via conjugation.
Transposons can help create NEW genes. When a transposon moves, it takes a chunk of protein coding DNA with it. Once inserted into the new region, these protein coding regions may get transcribed.
messeger RNA is reverse-transcribed (RNA→DNA)
forms a double-stranded DNA segment
this DNA segment is then inserted into DNA
Retrotransposons have a unique "footprint"
retrotransposons lack introns and regulatory sequences (these things not coded in mRNA)
found far away from original gene