Imagine that the genome (基因组) is a book. The book consists of 23 chapters, with thousands of stories made up of paragraphs, words and letters on different levels. There are one billion words in the book, which makes it longer than 500 dictionaries. If I read the genome out to you at the rate of one word per second for eight hours a day, it would take me a century. If I wrote out the human genome, one letter per millimeter, my text would be as long as the River Danube. This enormous document, however, all fits inside the extremely small nucleus (核) of a tiny cell that fits easily upon the head of a pin (针尖).
The idea of the genome as a book is not, strictly speaking, even a metaphor (比喻). It is true to a great extent. A book is a piece of digital information, written in one-directional form and controlled by a system that translates a small alphabet (字母表) of letters into a large dictionary of meanings through the order of their groupings. So is a genome. The only difference is that all English books read from left to right, while some parts of the genome read from left to right, and some from right to left, though never both at the same time.
While English books are written in words of different lengths using twenty-six letters, genomes are written entirely in three-letter words, using only four letters. And instead of being written on flat pages, they are written on long chains of DNA molecules (分子). The genome is a very clever book, because in the right conditions it can both photocopy itself and read itself.
A. They are nearly of the same length. B. They are made up of DN
A. C. They can reproduce each other. D. They are written on flat pages.