An example of an extended metaphor used in science writing:
"DNA is an alphabet, and a very simple one at that. It is formed of just four letters – A, C, G and T. These are also known as bases. But because our cells contain so much DNA, this simple alphabet carries an incredible amount of information."
Nessa Carey (2015) Junk DNA. A journey through the dark matter of the genome. London: Icon Books Ltd.
This is a metaphor (not a simile or analogy) as the text suggests not that DNA is in some way like an alphabet, but rather that it is an alphabet. DNA stands for dioxyribonucleic acid, a class of chemical substances not a system of symbols, so the identity is not, literally, accurate. A reader has to appreciate a comparison is being made.
(On the following page, the comparison is referred to as an 'alphabet analogy', and it is suggested that "a gene can be thought of as a sentence of three-letter words'.)
A, C, G, and T are, indeed, four letters but there is then a shift in saying they are also known as bases: they are letters used as symbols for bases, but not actually bases. That is there is ambiguity as A, C, G, and T can stand for four letters in an alphabet or for the four bases that are included in DNA structures, but these are different meanings. So, this is like saying 'a bank is a financial institution; it is also found at the side of a river' or 'a bar is a place where you can buy a drink; it is also an injunction stopping you going there'.
That DNA cannot be identified with an alphabet becomes clear when the text suggests
- "because our cells contain so much DNA, this simple alphabet carries an incredible amount of information",
this cannot be sensibly be read as "
- "because our cells contain so much [of this alphabet], this simple alphabet carries an incredible amount of information".
Rather, DNA would have to mean something like 'text', i.e.,
- "because our cells contain so much [text], this simple alphabet carries an incredible amount of information".
So, we have a kind of category error, as an alphabet and a text are different kinds of things and not synonymous.
