CoachChemically, a chromosome is one immensely long, coiled molecule of DNA (with packing proteins): a double strand — the famous twisted ladder — whose rungs are pairs of four chemical letters. The order of the letters along the strand is the information: a gene is a stretch of that sequence, as a sentence is a stretch of a page. Four letters, three billion of them in one human deck — enough text for a build’s whole instruction set, coiled into every nucleus.
示例
Example 65.5 (Seeing it yourself)
DNA is kitchen-extractable: mash a banana or an onion, add salty water and a little washing-up liquid (membranes undone — Definition 45.1’s envelopes are its target), strain, and layer cold spirit on top: at the boundary rises a tangle of whitish threads, spoolable on a toothpick. Those threads are DNA — the actual molecule of this chapter, in actual centimetres.