Which statement about transposons is true?
First described by Barbara McClintock in maize controlling elements causing variegation, transposons are DNA segments capable of moving to new genomic locations altering genetic landscape. Two classes exist: Class I retrotransposons including LINE-1 autonomous encoding ORF1 RNA-binding and ORF2 with endonuclease and reverse transcriptase that reverse transcribes RNA intermediate into cDNA integrating at target-primed site, and SINEs like Alu non-autonomous relying on LINE machinery; copy number increases because donor remains while copy inserts. Class II DNA transposons such as Tc1/mariner encode transposase recognizing terminal inverted repeats TIRs, catalyzing cut-and-paste excision and integration via DDE motif. Transposition can disrupt coding exons, shuffle exons, create new regulatory elements, cause chromosomal deletions via unequal crossing over, providing raw material for evolution but also causing mutagenesis including hemophilia and cancer when inserting tumor suppressor. Presence in bacteria, plants, animals indicates universal distribution, controlled by piRNA pathway in germline, DNA methylation, and heterochromatin formation to limit activity, not solely beneficial.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Transposons and Mobile Elements.