To replicate before cell division, the 
DNA double helix separates and 
unwinds and each strand acts as a 
template for the formation of a mirror 
image according to the rules of base 
pairing: A with T, and G with C. 
This results in two daughter DNA 
molecules whose sequences are 
identical to those of the original DNA. 
After replication in human cells, 
DNA condenses into 46 pairs of 
chromosomes. At this point, the mem- 
brane surrounding the nucleus breaks 
down, and the chromosome pairs pull 
apart and move to the poles of the 
cell. Then the cell divides, forming 
two identical daughter cells, each with 
46 chromosomes. The production of 
germ cells, or sperm and eggs, 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 
complete complement of 46 chromo- 
somes is restored. 
Each of us begins as a single, ferti- 
lized cell, a microscopic package that 
contains within the DNA directions for 
everything that we can become. The 
single cell then divides again and 
again. As Mazia puts it, the story of 
the cell cycle is "double or nothing. 
With few exceptions, a living cell 
either reproduces or dies; the principle 
is so simple that no one has bothered 
to call it a principle. A cell is born in 
the division of a parent cell. It then 
doubles in every respect: in every 
part, in every kind of molecule, even 
in the amount of water it contains." 
Some of our cells are very short- 
lived. Scavenger white blood cells, 
for example, circulate and consume 
invading particles for only a few days 
before they die. In contrast, our brain 
cells never reproduce. Most live as 
long as we do, but when one dies, it 
is not replaced. 
At any instant, only certain genes in 
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