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Mechanism of Transposition

Why would transposable elements persist in cells even if they do nothing except copy themselves? Although some transposable elements contain additional coding DNA, many transposable elements code for only the factors necessary for copying and moving the sequence to another location in the genome. Transposable elements appear to be an example of ‘selfish’ DNA. DNA will persist if the effect of the DNA is to increase the probability that it will be copied and passed on to the next generation. If a particular sequence of DNA has the effect making the organism healthier or more likely to reproduce then the DNA sequence will become more common. Transposable elements spread by directly copying themselves. DNA that spreads by copying itself is sometimes called ‘selfish’ DNA because it does not contribute to the fitness of the organism.

View the animation below, then complete the quiz to test your knowledge of the concept.

1The ends of a simple transposable element contain
A)genes for the transposase enzyme.
B)inverted repeats.
C)direct repeats.
D)self-replicating genes.
E)antibiotic-resistance genes.

2What is the difference between composite transposons and IS elements?
A)Composite transposons are found only in bacteria, while IS elements are found only in animals.
B)Composite transposons are DNA molecules, while IS elements are RNA.
C)Composite transposons can carry out their own transposition, while IS elements require help from other transposons.
D)Composite transposons can carry antibiotic resistance genes, while IS elements do not.
E)None of the above.

3The target sequence which is recognized by transposons
A)is made up of single-stranded DNA.
B)contains the sequence for the trasnsposase gene.
C)is cut out and discarded during transposition.
D)is duplicated during transposition.
E)is inverted during transposition.

4The enzyme transposase is highly specific, and so IS elements can only move to a few locations in the genome.

5A composite transposable element contains an IS element at each end.

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