Final answer:
Both SINEs and non-LTR retrotransposons, such as LINES, lack long terminal repeat (LTR) sequences. Non-LTR retrotransposons use a target-site primed mechanism of retrotransposition, whereas LTR retrotransposons utilize an extrachromosomal mechanism.
Step-by-step explanation:
Retrotransposons are genetic elements that can move around the genome and are classified based on their replication mechanism. The two primary mechanisms are Extrachromosomally Primed Retrotransposition, which involves LTR retrotransposons, and Target-Site Primed Retrotransposition, associated with non-LTR retrotransposons. Non-LTR retrotransposons, such as LINES and SINEs, do not have the LTR sequences that define LTR retrotransposons.
LINES, although autonomous and capable of encoding the enzymes needed for transposition, are structurally distinct with their ORFs flanked by untranslated regions instead of LTRs. SINEs, being non-autonomous, lack the genes for the necessary enzymes and depend on the retrotranspositional machinery of other elements like LINES. Due to their lack of independence in movement and lack of LTRs, SINEs may also be referred to as retroposons. In their retrotransposition process, the LINEs provide the reverse transcriptase needed for replication. This type of retrotransposition differs from that of retroviruses and LTR retrotransposons, which rely on integrase-bound reverse transcriptase and contain the characteristic LTR sequences.