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#transposition mechanism

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Transposable elements

Retrotransposons move via

Retrotransposon mobility requires RNA intermediate synthesized by RNA polymerase II from internal promoter, resulting transcript serves both as mRNA for reverse transcriptase and as template for reverse transcription. Endonuclease nicks target DNA providing free 3'-OH used by reverse transcriptase to prime cDNA synthesis in target-primed reverse transcription mechanism typical of non-LTR elements, or by integrase-mediated insertion for LTR elements. New DNA copy integrates while donor remains, demonstrating mandatory RNA stage unique to Class I elements enabling replicative amplification. This principle illustrates essential molecular mechanisms governing replication fidelity and mutation fixation relevant for exam interpretation.

Ref: NCBI Bookshelf, Molecular Cell Biology, Section: RNA Intermediate Mechanism of Retrotransposons

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Transposable elements

DNA transposons move via

DNA transposons move without RNA stage; transposase binds terminal inverted repeats, assembles paired-end complex, introduces staggered double-strand cuts at donor site releasing element with 3'-OH ends that attack target DNA phosphodiester backbone. Host repair polymerases fill single-stranded gaps generating target site duplications characteristic of insertion. Information transfer remains as DNA throughout, distinguishing cut-and-paste transposition from retrotransposition. Copy number usually preserved unless donor gap repaired from sister chromatid yielding increase, balancing genome stability. This principle illustrates essential molecular mechanisms governing replication fidelity and mutation fixation relevant for exam interpretation.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: DNA Transposon DNA Intermediate Pathway

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