Page 50 - The Miracle in the Cell

THE MIRACLE IN THE CELL
Self-Replication of DNA

Cells multiply by dividing. Thus a human body starts out as a
single cell, which then divides to form two and then 4, 6, 8, 16, 32 cells
and so on.
But after this process of division, what happens to DNA? Any
one cell contains only one helix of DNA. But clearly, the newly divid-
ed cells will each need their own DNA. To overcome this problem a
new series of processes takes place, each step of which is a miracle in
itself. Just before the cell divides, its DNA is replicated, so that each
new cell receives an exact copy of the original.
Microscopic observations reveal that before the cell divides, it
must reach a certain size. Once it does, the process of division is initi-
ated. As the cell begins to widen into the shape required for division,
the DNA starts to replicate itself.
What this means is that the cell as a whole "decides" to divide,
and its different parts begin to act accordingly. Clearly, the cell does-
n't possess the knowledge to carry out such a cooperative process suc-
cessfully. The process of division starts with a silent command, and
with the DNA leading the way, the rest of the cell follows suit.
In order to replicate itself, DNA first separates into two opposite
strands-an event that takes place in a very interesting manner. The
DNA molecule resembles a spiral ladder, whose rungs open up like a
zipper. The DNA now separates into two strands, and each of the two
opposite, incomplete strands then completes itself with materials
found in the cellular environment. In this way are produced, two new
DNA molecules (Figure 2.7). At each stage of the process, specialized
proteins called enzymes, operating like highly advanced robots, carry
out their respective duties. To explain the process completely would
take many pages, since what seems a very simple procedure at first
glance is actually made up of innumerable steps that are highly com-
plex.
The new DNA molecule resulting from this pairing is checked

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