nucleotides. Through the arrangement of its nucleotides, the gene provides coded
information for the construction of proteins. After these proteins are assembled
elsewhere in the cell, they serve as building blocks for the cell and as enzymes to
promote the life processes of the cell.
a. A chromosome is a very long double-helix thread of DNA. Thus, each
chromosome consists of a large number of genes. The genes have very specific
locations along the length of each chromosome. Recently, researchers have been able
to identify specific sequences of genes along a chromosome and illustrate the
sequences with gene maps.
b. Except during cell division, chromosomes are observed as granules of
chromatin material within the cell nucleus. During the process of cell division, this
chromatin material aggregates so that it may be identified as one of the 46 individual
chromosomes found in each human cell (diploid condition).
c. These 46 chromosomes of the human cell occur in pairs. Thus, we may say
that there are two sets, with 23 chromosomes in each set.
(22 + 1) X 2 = 46
Of the 23 different chromosomes, 22 deal with the body in general and are called
autosomal chromosomes. The last chromosome is called the sex chromosome. There
are two kinds of sex chromosomes--X and Y. When an individual's cells each have two
X chromosomes (XX), the individual is genetically a female. On the other hand, when
an individual's cells each have one X and one Y chromosome (XY), that individual is
genetically a male.
The two types of cell division are illustrated in Figure 14-1.
a. Mitosis. New cells must be produced for replacement of worn-out cells and
for growth and development of the individual. For these purposes, the existing cells
undergo cell division and produce new cells. The usual process of cell division is called
mitosis. In mitosis, the two daughter cells produced by the original cell have essentially
the same genetic material as the original cell.
b. Meiosis. Meiosis is a type of cell division which occurs only in the gonads.
It results in the formation of the gametes, or sex cells. In mitosis, the chromosomes are
duplicated; in meiosis, the two sets of chromosomes separate, and one set of 23 goes
to each of the gametes. Thus, meiosis involves a reduction division. The final result is
that each gamete has only one set of 23 chromosomes (haploid condition).