THE ASTONISHING 
UNIFORMITY OF LIFE 
All cells — whether from a bacterium, 
plant, mouse, or human — are made 
of the same basic materials: nucleic 
acids, proteins, carbohydrates, water, 
fats, and salts. 
"The uniformity of the earth's life, 
more astonishing than its diversity, is 
accountable by the high probability 
that we derived, originally, from a 
single cell," notes physician 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 genet® material in all these 
cells is deoxyribonucleic acid (DNA), 
a large molecule that directs the 
making of duplicate cells. DNA also 
directs the building of proteins accord- 
ing to a complex code. Even the 
simplest living cells — the myco- 
plasma — contain a relatively large 
amount of DNA, enough to code for 
up to a thousand different proteins. 
Every human cell has about 6 feet of 
very tightly wound DNA strands con- 
tained within its nucleus, and every 
adult carries about 100 billion miles 
of ultrathin DNA strands in his or her 
body — a distance greater than the 
diameter of the solar system. 
Each cell is separated from the rest 
of the world by a membrane so thin 
that it cannot be seen under a light 
microscope. Despite its ethereal 
nature, the surface membrane is ex- 
ceedingly powerful, controlling every- 
thing that goes into and out of the cell 
and relaying vital messages. Similar 
membranes enclose or make up a 
large number of the cell's organelles. 
There is a fundamental distinction 
between the two major categories of 
cells. Prokaryotic cells, which include 
bacteria, mycoplasma, and blue-green 
algae, do not have.a..membrane 
around their nuclear region. Eukaryotic 
("proper nucleus") cells, in contrast, 
have a double membrane separating 
the nucleus from the cytoplasm, as well 
as many other internal membranes to 
segregate their organelles. The cells 
of all animals and plants (except blue- 
green algae) and one-celled protozoa 
are eukaryotes. 
Only eukaryotic cells are able to 
combine with one another to form 
multicellular systems — an important step 
up the evolutionary ladder. And while, 
in general, prokaryotic cells produce 
only exact duplicates of themselves, 
eukaryotic cells are capable of differ- 
entiation into many kinds of cells, at 
least in higher organisms. This gives 
eukaryotic cells certain obvious advan- 
tages. However, prokaryotes have 
advantages of their own: simpler 
n-utritiona I requirements and much more 
rapid growth and division. As Daniel 
Mazia of Stanford University points 
out, the differences between the two 
types of cells are simply "different ways 
of making a living." A cell's job — 
which, for many cells, centers on the 
manufacture of proteins — is divided 
into many steps. In eukaryotes, the 
production of protein begins in the 
most prominent organelle — the nucleus. 
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