Final answer:
Retrotransposons make up varying proportions of plant genomes, sometimes exceeding 70%, and were once thought to be 'selfish' elements without function. However, it's now understood that they contribute to genetic diversity and can impact gene function. Retrotransposons and ERVs represent a substantial part of mammalian genomes as well.
Step-by-step explanation:
In some plants, retrotransposons can account for a significant proportion of the DNA in the genome, varying widely from as low as 4% to more than 70%. These elements move by a process where RNA is transcribed at one location and then reverse-transcribed back into genomic DNA at a different location. Initially, they were labeled as 'selfish DNA' because they seemed to serve no purpose other than their own replication.
However, views on transposons have shifted, since they contribute to genetic diversity and can have functional roles within the genome. While not all transposons are active, the active ones can affect gene expression and, in some cases, be linked to evolutionary adaptations. Retrotransposons, including endogenous retroviruses (ERVs), make up a substantial portion of the mammalian genome, with ERVs constituting 8% of the human genome.