SOLVED PROBLEM 1. Transposable elements have been referred to as “jumping genes” because they appear to jump from one position to another, leaving the old locus and appearing at a new locus. In light of what we now know concerning the mechanism of transposition, how appropriate is the term “jumping genes” for bacterial transposable elements?

Solution

In bacteria, transposition takes place by two different modes. The conservative mode results in true jumping genes because, in this case, the transposable element excises from its original position and inserts at a new position. The other mode is the replicative mode. In this pathway, a transposable element moves to a new location by replicating into the target DNA, leaving behind a copy of the transposable element at the original site. When operating by the replicative mode, transposable elements are not really jumping genes because a copy does remain at the original site.

SOLVED PROBLEM 2. Following from the question above, in light of what we now know concerning the mechanism of transposition, how appropriate is the term “jumping genes” for the vast majority of transposable elements in the human genome and in the genomes of most other mammals?

Solution

The vast majority of transposable elements in the characterized mammalian genomes are retrotransposons. In humans, two retrotransposons (the LINE called L1 and the SINE called Alu) account for a whopping one-third of our entire genome. Like bacterial elements, retrotransposons do not excise from the original site, so they are not really jumping genes. Instead, the element serves as a template for the transcription of RNAs that can be reverse-transcribed by the enzyme reverse transcriptase into double-stranded cDNA. Each cDNA can potentially insert into target sites throughout the genome. Note that while both bacterial elements and retrotransposons do not leave the original site, their respective mechanisms of transposition are dramatically different. Finally, while LTR-retrotransposons do not excise, they can become much shorter insertions due to the production of solo LTRs by recombination.

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