One of the fundamental laws of modern biology—which states that the DNA
content of somatic cells is constant for any given species—was first set
forth in a research report of 1948. This finding means that in any given
species, such as a mouse or a man, all cells except the germinal cells
contain the same amount of DNA. Germinal cells, that is, the sperm cells
of the male semen and the female egg, contain exactly half the amount of
DNA of the somatic cells. This must be the case, since DNA is the
hereditary material, and each individual’s heredity is shaped half by
his father and half by his mother. One ten-trillionth of an ounce of DNA
from a father and one ten-trillionth of an ounce of DNA from the mother
together contain all the specifications to produce a new human being.
A large amount of DNA must be manufactured by an individual organism as
it develops from a fertilized egg (one single cell) to an adult
containing several million cells. For instance, a mouse cell contains
about 7 picograms of DNA (one picogram is one millionth of a microgram,
or one millionth of one millionth of a gram). A whole mouse contains in
its body approximately 25 milligrams (25 thousandths of a gram) of DNA,
and all this DNA was synthesized by the cells as the mouse grew to
adulthood. Since the amount of DNA per cell remains constant and since
each cell divides into two cells, it is apparent that each new cell
receives the amount of DNA characteristic of that species.
Once we realize that a cell that is making new DNA (as most cells do)
must divide to keep the amount of DNA per cell constant, it follows that
a cell that is making DNA is one that is soon destined to divide. If we
can now mark newly made DNA with a radioactive isotope, we can actually
mark and thus identify cells that are preparing to divide. The task can
be divided into two parts: (1) to label the newly made DNA and (2) to
detect the newly made, labeled DNA.