The average human adult consists of about 50 trillion _cells_—50
trillion microscopic, more or less self-contained, blobs of life. He
begins life, however, as a single cell, the _fertilized ovum_.
After the fertilized ovum is formed, it divides and becomes two cells.
Each daughter cell divides to produce a total of four cells, and each of
those divides and so on.
There is a high degree of order and direction to those divisions. When a
human fertilized ovum completes its divisions an adult human being is
the inevitable result. The fertilized ovum of a giraffe will produce a
giraffe, that of a fruit fly will produce a fruit fly, and so on. There
are no mistakes, so it is quite clear that the fertilized ovum must
carry “instructions” that guide its development in the appropriate
direction.
These “instructions” are contained in the cell’s _chromosomes_, tiny
structures that appear most clearly (like stubby bits of tangled
spaghetti) when the cell is in the actual process of division. Each
species has some characteristic number of chromosomes in its cells, and
these chromosomes can be considered in pairs. Human cells, for instance,
contain 23 pairs of chromosomes—46 in all.
When a cell is undergoing division (_mitosis_), the number of
chromosomes is temporarily doubled, as each chromosome brings about the
formation of a replica of itself. (This process is called
_replication_.) As the cell divides, the chromosomes are evenly shared
by the new cells in such a way that if a particular chromosome goes into
one daughter cell, its replica goes into the other. In the end, each
cell has a complete set of pairs of chromosomes; and the set in each
cell is identical with the set in the original cell before division.
[Illustration: Mitosis]
Interphase
Prophase
Metaphase
Anaphase
Telophase
Interphase
[Illustration: _To study chromosomes, scientists begin with a cell that
is in the process of dividing, when chromosomes are in their most
visible form. Then they treat the cell with a chemical, a derivative of
colchicine, to arrest the cell division at the metaphase stage (see
mitosis diagram on preceding page). This brings a result like the
photomicrograph above; the chromosomes are visible but still too tangled
to be counted or measured. Then the cell is treated with a
low-concentration salt solution, which swells the chromosomes and
disperses them so they become distinct structures, as below._]
[Illustration: Cell after treatment with salt solution]
[Illustration: _The separate chromosomes in a dividing cell are
photographed and then can be identified by their overall length, the
position of the centromere, or point where the two strands join, and
other characteristics. The photomicrograph can then be cut apart and the
chromosomes grouped in a karyotype, which is an arrangement according to
a standard classification to show chromosome complement and
abnormalities. The karotype below is of a normal male, since it shows X
and Y sex chromosomes and 22 pairs of other, autosomal, chromosomes. By
contrast, the cells in the upper pictures are abnormal, with only 45
chromosomes each._]
In this way, the fundamental “instructions” that determine the
characteristics of a cell are passed on to each new cell. Ideally, all
the trillions of cells in a particular human being have identical sets
of “instructions”.[1]