18 
ASTONISHING UNIFORMITY OF LIFE 
11 cells — whether 
from a bacterium, 
plant, mouse, or 
human — are 
made of the 
same basic materials: nucleic 
acids, proteins, carbohydrates, 
water, fats, and salts. For this 
reason, scientists seeking to 
understand both normal and 
disease processes in humans 
can learn a great deal by 
studying similar systems in 
“model organisms” like 
bacteria, slime molds, yeast, 
and fruit flies. 
physician and author Lewis 
Thomas in The Lives of a Cell. 
“It is from the progeny of this 
parent cell that we take our 
looks; we still share genes 
around, and the resemblance 
of the enzymes of grasses to 
those of whales is a family 
resemblance.” 
“The uniformity of the earth’s 
life, more astonishing than its 
diversity, is accountable by 
the high probability that we 
derived, originally, from a 
The genetic material in all liv- 
ing cells is deoxyribonucleic 
acid (DNA), a large molecule 
that directs the making of 
duplicate cells. DNA also 
directs the building of proteins 
according to a code. Even the 
simplest living cells — the 
mycoplasma — contain a rela- 
tively large amount of DNA, 
enough to code for perhaps 
1,000 different proteins. 
Every human cell has about 6 
feet of very tightly wound 
DNA strands contained 
within its nucleus, and every 
adult carries billions of miles 
of ultrathin DNA strands in 
his or her body. 
! 
Each cell is separated from 
the rest of the world by a 
membrane so thin that it 
single cell,” noted the late 
