Contrast the structure of SINE and LINE DNA sequences. Why are LINEs referred to as retrotransposons?

SINEs (Short Interspersed Nuclear Elements) and LINEs (Long Interspersed Nuclear Elements) are types of repetitive DNA sequences found in the genome. They belong to the class of transposable elements, which can move to new positions within the genome. Let's first describe their structures briefly: SINEs: They are shorter DNA sequences (80-500 base pairs long) that are non-coding (do not code for proteins) and are usually derived from small RNA genes like tRNAs or 7SL RNA. LINEs: They are longer DNA sequences (over 1,000 base pairs long) that can code for proteins needed for their own transposition. They typically contain two open reading frames (ORFs), which code for the proteins required for their own mobilization. Now, let's contrast the structure of SINE and LINE DNA sequences.

1. Length: SINEs are shorter (80-500 base pairs) compared to LINEs (over 1,000 base pairs). 2. Protein-coding capacity: SINEs are non-coding, while LINEs have ORFs that code for proteins needed for their own transposition. 3. Origin: SINEs are derived from small RNA genes like tRNAs or 7SL RNA, while LINEs are not derived from these sources.

Retrotransposons are transposable elements that are transposed via an RNA intermediate. They use reverse transcription (conversion of RNA into DNA), followed by integration of the copied DNA into a new genomic location. This process requires the action of a reverse transcriptase enzyme, which is coded by the retrotransposon itself. LINEs are referred to as retrotransposons because they use a similar mechanism for their transposition: 1. Transcription: LINEs are transcribed into RNA by cellular machinery. 2. Translation: The RNA transcript is translated into proteins, including a reverse transcriptase enzyme, by the cell's ribosomes. 3. Reverse transcription: The RNA transcript serves as a template for reverse transcriptase to synthesize a complementary DNA (cDNA) copy of the LINE sequence. 4. Integration: The cDNA is integrated into a new genomic location by the action of the LINE-encoded endonuclease enzyme. In summary, LINEs are referred to as retrotransposons because they transpose via an RNA intermediate using reverse transcription, akin to the mechanism used by retroviruses and other retrotransposons.