Haploid and Diploid Chromosomes
We have seen
that during cell division the chromatin network in the nucleus becomes
condensed into a number of thread-like or rod-like structures called
chromosomes. The number of chromosomes in each cell is fixed for a given
species, and in man it is 46. This is referred to as the diploid number (diploid = double). However, in spermatozoa and in ova the
number is only half the diploid number i.e., 23: this is called the haploid number (haploid = half).
The 46 chromosomes in
each cell can again be divided into 44 autosomes and
the two sex chromosomes.
The sex chromosomes may be of two kinds, X or Y. In a man there are 44
autosomes and one X chromosome, and one Y
chromosome; while in a woman there are 44 autosomes and two X chromosomes in
each cell. When we study the 44 autosomes we find that they really consist of
22 pairs, the two chromosomes forming a pair being exactly alike (homologous chromosomes). In a woman the two X chromosomes form another such pair;
but in a man this pair is represented by one X and one Y chromosome. We shall
see later that one chromosome of each pair is obtained (by each individual)
from the mother, and one from the father.
As the two sex
chromosomes of a female are similar the female sex is described as homogametic; in contrast the male sex is heterogametic.
Each cell of the body
contains within itself a store of information that has been inherited from
precursor cells. This information (which is necessary for the proper
functioning of the cell) is stored in chromatin. Each chromosome bears on
itself a very large number of functional segments that are called genes. Genes represent ‘units’ of stored information which guide
the performance of particular cellular functions, which may in turn lead to the
development of particular features of an individual or of a species. Recent
researches have told us a great deal about the way in which chromosomes and
genes store and use information.
The nature and functions of a cell depend on the proteins
synthesized by it. Proteins are the most important constituents of our body.
They make up the greater part of each cell and of intercellular substances.
Enzymes, hormones, and antibodies are also proteins.
It is, therefore, not surprising that one cell differs from
another because of the differences in the proteins that constitute it.
Individuals and species also owe their distinctive characters to their
proteins. We now know that chromosomes control the development and functioning
of cells by determining what type of proteins will be synthesized within them.
