Page 128 - The Cell in 40 Topics

s you know, cells replicate themselves by dividing. But what
happens to DNA as a result of this division process? There is
one single DNA strand in each cell. Yet the newly emerging
“twin” cell will also need complete DNA of its own. In order
to achieve this, a series of processes takes place, every stage of which is an
independent miracle. As a result, an exact copy of the “original” DNA is
formed shortly before cell division takes place, and this copy is transferred
to the new-formed cell.
In order to replicate itself, DNA first separates itself into two opposite
parts. This takes place in a most interesting manner. The DNA molecule,
which resembles a spiral staircase, split down the middle, much like a zip-
per being unzipped, by an enzyme called DNA helicase. As the arms of the
DNA separate from one another, helix-stabilizing enzymes keep both arms
fixed, in order that they should not wrap round one another again (Figure
101).
The DNA has now been divided into two halves. The missing halves
of each are completed with materials available around them. The task of
making good these gaps is performed by DNA polymerase. Thus, two new
DNA molecules are produced (Figure 102).

126