THE NUCLEUS, THE CELL'S 
"COMMAND CENTER" 
The nucleus is the biggest, densest, 
most obvious structure in the eukaryotic 
cell — the first to be recognized by mi- 
croscopists and the first to be isolated 
in the biochemists' centrifuge. 
For many years, nobody knew what 
the nucleus did. In the 19th century, 
several researchers noted that before 
a cell divided, the nucleus divided. 
But it was not until the beginning of 
the 20th century that scientists grasped 
the connection between the rodlike 
chromosomes (tightly packed bundles 
of DNA and protein) that had been 
observed in the nucleus and the 
transmission of hereditary traits. At 
that point, the importance of the 
nucleus became clear. 
The nucleus is the cell's command 
center. The chromosomes contain the 
genes (made of DNA) that give 
directions for everything the cell is and 
will be, and thus control the cell's re- 
production and heredity. DNA is a 
deceptively simple molecule, consist- 
ing of a sequence of subunits, called 
bases, linked together to form a 
double helix that can be visualized as 
an immensely long, corkscrew-shaped 
ladder. Each rung in the ladder is 
made up of two bases fitted together, 
and the ends of the rung are attached 
to chains of sugar-phosphates that are 
like the upright rails of a ladder. A 
unit of DNA containing one sugar 
molecule, one phosphate molecule, 
and one base is called a nucleotide. 
There are only four different bases: 
adenine (A), thymine (T), guanine (G), 
and cytosine (C). They pair with each 
other so that A is always opposite T 
and G is always joined to C. Thus, 
the sequence of bases on one side of 
the ladder (for example, AGCGT) is 
complementary to, and determines, the 
sequence on the other side (TCGCA). 
This is the "genetic alphabet"— a small 
set of "letters" with which, as with the 
ABC's, an infinite number of messages 
can be written. 
As might be expected, the nucleus is 
constantly active. Before cell division, 
all of the information contained within 
the DNA must be duplicated, in a 
process called replication. The speed 
with which replication occurs is aston- 
ishing. For example, before a single 
Escherichia coli (a common intestinal 
bacterium) splits in two, which it does 
every 20 minutes, the 360,000 turns 
of its DNA helix must first be unwound. 
Next, each of the 3.6 million nucleo- 
tides on one side of the DNA molecule 
pulls away from its mate. As the mole- 
cule "unzips," each half serves as a 
mold, or template, for a new molecule. 
In a matter of minutes, 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 helical 
DNA exists as a single ring floating in 
the cytoplasm and does not condense 
into separate chromosomes.) 
All of this molecular maneuvering 
must be performed both rapidly and 
accurately. If nucleotides are lost, 
rearranged, or erroneously paired, the 
garbled instructions that result could 
lead to a nonfunctioning protein when 
the DNA's code is translated. 
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