Transposable Element Classes and LINEs (Recommend 1.5x Speed)

This educational video from a university genetics course provides an introduction to transposable elements, focusing on LINEs (Long Interspersed Nuclear Elements). The instructor begins by defining transposable elements as genetic elements that can move within a genome, contrasting them with viruses that have an independent form. He notes that transposable elements can cause mutations but also constitute a large fraction of genomes, citing that 43% of the human genome and 85% of maize genome are transposable elements. He then introduces the two main classes: Class I retrotransposons, which require reverse transcriptase, and Class II DNA transposons, which use transposase. The video focuses on non-LTR retrotransposons, specifically LINEs, which make up 32% of the human genome. The most common LINE in humans is L1, with 516,000 copies, each about 6 kb long, comprising 17% of the genome. The structure of a full-length L1 element is described, including its 5’ UTR, two open reading frames (ORF1 and ORF2), 3’ UTR, and poly-A tail. ORF1 encodes an RNA-binding protein, while ORF2 encodes a protein with endonuclease and reverse transcriptase activities. The mechanism of transposition is explained: L1 mRNA is transcribed, exported to the cytoplasm, translated, and then the proteins bind to the mRNA and bring it back into the nucleus. There, ORF2 nicks the target DNA at a consensus sequence (TTTT/A), and the poly-A tail of the mRNA anneals to the nicked strand, providing a primer for reverse transcription. This process is called target-primed reverse transcription (TPRT). The video acknowledges that the later steps of integration are poorly understood. Finally, it notes that new L1 insertions often lack the 5’ end due to premature termination of reverse transcription, making them non-functional. The video sets up a follow-up on SINEs and a genetic disorder caused by an L1 insertion.