LINEs and SINEs | Poly A retrotransposon | Genetic organization of LINE and SINE | Mol bio
Understanding LINEs and SINEs: Transposable Elements
Introduction to LINEs and SINEs
- LINE stands for Long Interspersed Nuclear Elements, while SINE stands for Short Interspersed Nuclear Elements. These are types of transposons that play significant roles in the genome.
- Both LINEs and SINEs are categorized as interspersed retrotransposable elements, which transpose using RNA intermediates. They differ from retroviral retrotransposons by lacking inverted terminal repeats.
Characteristics of LINEs and SINEs
- LINEs are autonomous retroelements capable of coding for their own reverse transcriptase enzyme, whereas SINEs are non-autonomous and rely on enzymes derived from LINEs.
- The size difference is notable: mouse LINE sequences exceed 6 kilobase pairs, while human SINE sequences range from 200 to 400 base pairs.
Features of LINE Elements
- At first glance, LINE elements resemble protein-coding genes with five prime (5') UTR and three prime (3') UTR regions but lack inverted terminal repeats.
- The open reading frame (ORF2) within a LINE encodes two crucial proteins: an RNA binding protein and a reverse transcriptase/endonuclease activity-containing protein essential for transposition reactions.
Mechanism of Integration into the Genome
- The process begins with transcription by RNA polymerase II, producing mRNA that resembles typical mRNA found in the cytoplasm, complete with UTR regions and a polyadenylation site.
- ORF2-derived proteins bind to the line mRNA forming a complex that guides integration into target DNA through hybridization followed by strand synthesis facilitated by reverse transcriptase activity. This results in successful genomic integration of these transposons.
Selectivity in Transposition
- A critical question arises regarding how LINE selectively integrates its mRNA rather than random transcripts like those from other genes; it has been observed that ORF2 proteins exhibit higher binding affinity towards line mRNA due to specific sequences present within it. This selectivity minimizes random integrations into the genome despite some rare occurrences still happening.
Functions of LINE and SINE Elements
- Like other transposons, both can have mutagenic effects by disrupting gene function upon insertion or may lead to epigenetic silencing of certain genes.
- Recent research indicates they can serve as sites for long non-coding RNAs or act as enhancers regulating gene expression; particularly, SINE elements have shown potential in this regulatory capacity.