Functional rearrangements in DNA

Mutations are passed on to daughter cells. In multicellular organisms, they might be transmitted to the organism's offspring. Over a series of generations they accumulate. The effect is to produce variation within the species. Variation is the raw material of evolution and the wellspring of life's diversity. But simple "point mutations" (changes in a single base, altering a single gene at a single point) are not the only means of altering the DNA. There are more dramatic ways, rarer than point mutations but probably much more influential on the course of evolution.

Sometimes a piece of a DNA molecule can be excised and the ends spliced back together. Occasionally the excised piece can be turned round and re-inserted back to front. This often produces nonsense, but not always. Suppose a deranged editor altered the sentence:-

Napoleon was heard to declare that he was able ere he saw Elba before his final battle by deleting either the underlined segment, or both the underlined and italicised segments, and splicing the remaining pieces together. The resulting sentences would have quite different meanings:-Napoleon was heard to declare before his final battle Napoleon was heard to declare battle.

There are editors who do this sort of thing. But few would be deranged enough to re-insert a letter by letter inversion of the excised underlined phrase, producing:-

Napoleon was heard to declare able was eh ere Elba saw eh taht before his final battle, though the utterance might throw light on the outcome of Waterloo. The inverted segment is nonsense, but it almost makes sense. A few more point mutations in the right places could make it intelligible again.

Deleting a segment of a DNA molecule can produce new genes: intelligible sentences, so to speak, but with meanings different from the original. Inverting and re-inserting the excised fragment can produce something from which sense can be rescued. These are examples functional rearrangements of DNA, and they are significant factors in evolution. For instance, suppose the deleted or inverted region contained a gene promoter or the binding site for a repressor. In this case, deletion or inversion would radically alter the transcriptional activity of at least one gene. Such cases are known.

A genome might contain DNA sequences that can be joined in alternative ways by cutting and splicing. To continue the linguistic analogy, consider the sentence:-

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