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DIRECTOR OF PROTEIN SYNTHESIS 
20 
he nucleus is the 
biggest, densest, 
and most obvious 
structure in the 
eukaryotic cell — 
the first to be recognized by 
microscopists 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 
proteins) 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 every- 
thing the cell is and will 
he, and thus control the 
cell’s reproduction and 
heredity. DNA is a deceptively 
simple molecule, consisting of 
a sequence of subunits called 
bases. The bases are 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 joined 
together by chemical bonds, 
and the ends of the rung are 
attached to chains of chemically 
bonded sugar and phosphate 
molecules that are like the 
upright rails of the 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 joined to 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 deter- 
mines, 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 
enormous 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 replica- 
tion occurs is astonishing. For 
example, before a single 
Escherichia coli (a common 
intestinal bacterium, abbrevi- 
ated E. coli ) 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 mil- 
lion nucleotides on one 
side of the DNA molecule 
pulls away from its mate. 
As the molecule unzips, 
each half serves as a mold, 
or template, for a new 
