• Chromosomes are present in nucleus.
• They are long threads made up of chromatin
• The chromatin is deoxyribonucleic acid (DNA)
associated with protein.
• The chromatin material is in two forms.
• It is dispersed and poorly stained.
• It lies in the central region of nucleus.
• Nuclei with euchromatic appearance are
known as open-face nuclei and are larger in
• This state or form is associated with high
degrees of synthetic activity.
• Euchromatin is also called active chromatin.
• It is clumped and densely stained.
• It is peripheral and lies close to the nuclear
envelope, leaving gaps at the nuclear pores.
• Nuclei with heterochromatic appearance are known
as closed-face nuclei and are relatively smaller in
• This state is associated with low degrees of
• Heterochromatin is also called inactive chromatin.
• This is the diagram of a nucleus showing
heterochromatin and euchromatin. Look at the
nuclear envelope and nuclear pores. The
heterochromatin is peripheral and lies close to
nuclear envelope. Heterochromatin is absent in the
region of nuclear pores.
• The number of chromosomes in each cell is fixed
for a given species. There are 46 chromosomes in
each somatic cell of insan (human being).
• These chromosomes are in pairs. So there are 23
pairs of chromosomes in each somatic cell of
insan. In other words, there are two sets of
chromosomes. In each set there are 23
• 46 chromosomes are the diploid number or
double number of chromosomes and 23 is
haploid number or half number of
• There are two haploid sets of chromosomes.
The number of sets is called ploidy.
• If more than two sets are present the cell is
said to be polyploid.
• Each chromosome in a haploid set is unique in
size and shape.
• There are 23 different chromosomes. There
size and shapes are different from one
• In the diploid set there are two identical
chromosomes of each type of chromosome.
They are called homologous pairs.
• However the sex chromosomes are not
identical in man. In woman the sex
chromosomes are also identical.
• So in man there are 22 pairs of chromosomes
that are identical.
• are homologous pairs of chromosomes. These
are called autosomes.
• And there is one set of sex chromosomes.
They are not identical.
• They are heterologous chromosomes.
• So man is heterogametic sex.
• There is one X and an unequal Y chromosome giving
a diploid complement of 44 autosomes + X and Y
• The woman (female) is the homogametic sex. Here
the sex chromosomes are identical or homologous.
Both sex chromosomes are X chromosomes.
• The diploid complement of a woman is 44
autosomes + 2 X sex chromosomes.
• During interphase the chromosomes are
usually highly extended filaments and form a
diffuse network collectively termed
• During cell division the chromosomes shorten
• They become characteristically visible during
metaphase, of course under microscope.
• Each pair of homologous chromosomes has a
characteristic common basic structure.
• Each chromosome consists of two parallel and
identical filaments called chromatids.
• They are joined together at a narrowed region
called centromere or kinetochore. Here is the
primary constriction of the chromosome.
• There is a pale-staining region in centromere.
• The centromere is attached to the spindle
fibers during cell division.
• The free ends of the chromatids are known as
• The diagram of a chromosome during metaphase
showing two chromatids.
• Major parts of the chromosome are highlighted.
• A pale staining area is visible in the centromere.
• Each chromatid is divided by the centromere into
• One is p arm and the other is q arm.
• The centromere may be near the middle of the
• Here the two arms of chromatids are nearly equal.
• This type of chromosome is called metacentric
• The centromere may not be near the middle of the
• Here the two arms of chromatids are not equal.
• One arm is short and the other is long.
• This type of chromosome is called submetacentric
• The centromere may be near one end of the
• Here the short arm is very short and the long arm is
• This type of chromosome is called acrocentric
• In some mammals the centromere may be placed at
the extreme end of the chromosome.
• Here the short arm is negligible. This type of
chromosome is called telocentric chromosome.
• In certain chromosomes, in addition to
primary constriction, there is another
narrowing (the secondary constriction) near
one end of each chromatid, separating a
portion of chromatid from main arm.
• This portion is given the name satellite body.
• These constrictions and terminal satellite bodies are
believed to be often associated with the
organization of nucleoli, which may be attached to
the chromosome at this point in interphase.
• Each complete diploid set of chromosomes
contains the cell’s hereditary instructions, or
genome, and each somatic cell has an
identical genetic complement.
• The human genome contains an estimated
50,000 to 100,000 structural genes per
haploid set or three billion base pairs.
• Each chromosome has a linear sequence of
• Every gene is situated at a characteristic
position called gene locus.
• This sequence is highly stable.
• Mutations are permanent and inheritable
changes in genome.
• They are brought about by external forces
such as ionizing radiation, or exposure to
• When the affected region is restricted these
are called gene mutations.
• When whole arms or relatively large segments
are involved, they are termed chromosomal
• Such mutations are some of the principal
factors responsible for the inheritable
variations which all populations show.
• Within members of such a population,
homologous chromosomes may determine a
number of alternative characters since they
may carry some mutant genes; such groups of
alternative genes are termed alleles.
• In any genome homologous chromosomes may
possess, at a particular locus, identical alleles
(the homozygous condition) or non-identical
ones (the heterozygous condition).
• Since, in the formation of gametes, the
homologous chromosomes are segregated into
haploid sets, the alleles which they bear are also
• The various types of recombination occur at
• Normal resolution chromosome banding
reveals 350 bands per haploid set,
• whereas high-resolution chromosome
banding reveals up to 1300 bands per haploid