molecule. In a matter of min- 
utes, a total of 7.2 million free 
nucleotides are brought to 
each template and attached A 
to T and G to C. Finally, each 
new double strand retwists 
itself into a helix. (In 
prokaryotes, such as E. coli , 
the DNA exists as a large, 
single molecule rather than as 
multiple chromosomes.) 
All of this molecular maneuver- 
ing must be performed both 
rapidly and accurately. If 
nucleotides are lost, rearranged, 
or erroneously paired, the 
garbled instructions that 
result could lead to a non- 
functioning protein when the 
DNA’s code is translated. 
/ 2 3 4 
After replication in human 
cells, the DNA condenses into 
46 pairs of chromosomes. At 
this point, the membrane sur- 
rounding the nucleus breaks 
down, the chromosome pairs 
pull apart, and one member 
of each pair moves to the 
opposite pole of the cell. Then 
the cell divides, forming two 
identical daughter cells, each 
with 46 chromosomes. The 
production of sperm and egg 
cells is a more complex process 
in which a second division of 
the nucleus occurs, resulting 
in cells that have 23 chromo- 
somes each, instead of 46. 
When an egg is fertilized by a 
sperm, the full complement of 
46 chromosomes is restored. 
Each of us begins as a single, 
fertilized cell, a microscopic 
package that contains within 
its DNA the directions for 
everything that we can 
become. The single cell then 
divides again and again. Each 
new cell contains the same 
kinds of molecules and even 
the same amount of water as 
the “parent” cell. 
Some of our cells are very 
short-lived and are repeatedly 
replaced. Scavenger white 
blood cells, for example, cir- 
culate and consume invading 
particles for only a few days 
before they die. In contrast, 
our brain cells never repro- 
duce. Most live as long as we 
do, but when one dies, it is 
not replaced. 
At any instant, only certain 
genes in a cell are “on,” or 
“expressed,” and giving 
orders for the production of 
specific proteins. Some of the 
instructions that switch these 
genes on or off are generated 
as a result of interactions 
between the surface membrane 
and the environment. Thus, 
the commands from the 
nucleus are influenced by 
what goes on outside the cell 
as well as by the cell’s genetic 
program . 
An order to make a protein 
begins when the appropriate 
genes are transcribed from 
the DNA into strands of 
another kind of nucleic acid, 
called messenger ribonucleic 
acid (niRNA). Messenger 
RNA is manufactured by 
transcribing just one chain of 
II <) 10 11 
