Chemistry Reference
In-Depth Information
Heredity and Cell Structure
One of the most essential features of the cells, and hence of living organ-
isms, is their ability to reproduce by duplicating themselves: all living cells
can divide into two identical daughter cells. Prior to the division of a cell,
the chromatin in the nucleus of most living organisms clusters into
chro-
mosomes
(self replicating threadlike structures), and when the cell divides,
each chromosome is also divided into two new, identical chromosomes,
one for each new cell. Under normal conditions the chromosomes of the
body of a living organism always occur in pairs; the cells of the human
body, for example, contain 46 chromosomes, arranged into 23 pairs. Those
of other organisms, vegetable as well as animal, have different but char-
acteristic numbers of paired chromosomes. The sex cells of plants and
animals differ from their body cells in that they have only half as many
chromosomes as do body cells. In a human sex cell, for example, there are
23 unpaired chromosomes; each chromosome in a sex cell is one of a poten-
tial pair in the body cells. Table 62 lists the number of chromosomes of
body cells as well as sex cells in several plants and animals.
The chromosomes, only a few micrometers long, are made up of a
large number of
genes
, which are sequences of
deoxyribonucleic acid
(DNA), the substance that carries, encoded within its structure, the genetic
characteristics of living organisms (see Chapter 12). The genes in each chro-
mosome are arranged one next to the other, much like beads on a neck-
lace; they are also paired, in a way similar to the chromosomes in the nuclei
of the body cells. In other words, each chromosome pair has paired genes.
Since the genes carry the genetic characteristics of living organisms, there
are two genes, located opposite each other on a pair of chromosomes,
depicting each characteristic of the organism. During fertilization, a male
sex cell joins a female sex cell and, as a result, the body cells of the new
organism have the combined number of chromosomes and genes of the
sex cells of the mother and father.
8.2. BIOLOGICAL MATTER: ORGANIC AND BIOINORGANIC
SUBSTANCES
Living processes involve a vast number of
biological
or
biogenic
substances,
the substances that make up all living organisms and those that they create.
Chemical and biological studies have elucidated the composition and struc-
ture of a great number of these substances, the ways they are created in living
