10.1: Genes and DNA

Genes are a part of DNA, the genetic material packed into our chromosomes (Figure 10.1). Thus, DNA is the master file of recipes for all our proteins, including the enzymes that direct the construction of substances other than protein, e.g., cholesterol. Our DNA provides a complete set of directions for producing the entire human body.

Each of us (except identical twins) has unique DNA, making each of us different from everyone else. But the DNA in every cell of a single person is identical—brain and skin cells alike. Cells use the genes in DNA selectively, depending on a cell’s specialty. Think of different factories making ice cream, bread, and sausages, using only one big recipe book. Different cells make what they need, using only the pertinent recipes in identical DNA.

DNA is made of two long strands bridged together by molecules called bases (Figure 10.1, 10-2). The bases on one strand fit with (match) the bases on the other. There are only four kinds of bases (A,T,G,C) that form two matching pairs: A always pairs with T. G always pairs with C. Discovering DNA’s structure was momentous because it revealed how genetic material (DNA) could be duplicated from cell to cell (Figure 10.2).

Figure 10.1: DNA is the Genetic Material Packed in Chromosomes

The language of DNA uses the bases as letters. It spells out its message by lining up the bases in a certain sequence, just as we line up letters of our alphabet to write books. How can a language with just 4 letters (bases) encode our entire genetic make-up? The sequence of bases is read in sets of 3 (3-letter words). With 4 letters, we can make 64 (43 = 4X4X4 = 64) 3-letter words—more than enough to code for the 20 kinds of amino acids needed to make protein (Table 10-1).*

Sixty-one of the words code for amino acids; 3 words mean stop, to signal the end of the directions to make the protein. This genetic language is the same in an oak tree, a grasshopper, or a human. The genetic language is the same in all living things, plant or animal.

Genes make up less than 2% of our DNA and have precise locations, e.g., the gene for TPA/ Activase (the clot-dissolving enzyme discussed in Chap. 7) has an exact position in the DNA in chromosome #8. Human DNA has about 20,000 protein-coding genes and about 3 billion pairs of bases.**

^{*These very words appear on a computer screen via a binary code: off (zero) and on (one). With a “word” size of 7, there are 128 ways to arrange the 0s and 1s (27 = 2X2X2X2X2X2X2 = 128), more than enough to code for the upper and lower case of the 26-letter alphabet, punctuation, numbers, carriage return, etc. (e.g., the code for E is 1000101).
**We’re not tops here. The tiny water flea Daphnia has about 31,000 genes, and the flower Paris japonica has about 149,000 billion pairs of bases.}