Functions of Chromosomes

1. Prepared by: Sarmistha Hada
B.Sc.(Ag)
21C/15

2. WHAT IS A CHROMOSOME
 The word ‘Chromosome’ is derived from two Greek words;
Chroma meaning colour and Soma meaning body.
 Chromosomes are the rod shaped dark stained bodies as
seen during metaphase stage of mitosis when the cells are
stained with a suitable basic dye and viewed under a light
microscope.
 Chromosomes were first described by Strausberger in 1875.
 The term ‘Chromosome’ was first used by Waldeyer in 1888.
 Chromosomes are clearly visible as distinct bodies during
the stages of cell division only.

3. Although chromosomes were first described in 1875, their
role in heredity was suggested independently by Sutton and
Boveri in 1902 through their chromosomal theory of
inheritance. It is now universally accepted that genes are
located in chromosomes and responsible for transfer of
genetic information. The various functions of chromosomes
are explained below.

4. Chromosome Provides The Genetic Information
 It is universally accepted that DNA is the genetic material,
and that in eukaryotes almost all the DNA is present in
chromosomes. Thus the most important function of
chromosomes is to provide the genetic information for
various cellular functions essential for survival, growth,
development, reproduction etc. of organisms.
 Chromosome contains the genes which are the basic unit
of inheritance and help in transfer of genetic information
from parents to offsprings.

5. Packaging of Genetic Material
 Each somatic cell of human contains DNA of about 174cm
long if stretched end to end, but the nucleus of the cell
which contains the DNA is only 6microns in diameter. The
nucleus would not be able to accommodate such large
amount of DNA without DNA packaging.
 All the DNA in human somatic cell is packaged compactly
into 23 pairs of chromosomes with the help of histone and
non histone proteins

7. Chromosome Protects The Genetic
Material
 A very important function of chromosomes is to protect the
genetic material (DNA) from being damaged during cell
division.
 If all the DNA present in human somatic cells is fully extended,
it would measure about 174 cm. A DNA molecule of such a
length would be prone to breakage even under the most gentle
situations prevailing in a cell.
 But in chromosomes, this DNA is not only packaged in 46
relatively very small and sturdy chromosomes, it is also
complexed with histones and other proteins. which protect the
DNA both from chemical (e.g., nuclease enzymes) and physical
forces.

8. Distribution of DNA to Daughter Cells
The properties of chromosomes ensure a precise distribution
of DNA to the daughter nuclei during cell division. As a result,
each of the two daughter nuclei produced through one mitotic
division receives the same number of chromosomes as the
parent nucleus. On the other hand, each of the four nuclei
produced after one complete meiotic division gets only half
the number of the chromosomes as the parent nucleus.

9. Regular and Directional Movement of the
Genetic Material
Chromosome movement is believed to occur mainly due to
the shortening of spindle fibers. These fibers attach to the
centromeric regions of chromosomes. Thus ensuring a
regular and directional movement of the genetic material
during cell division.

10. Regulation of Gene Action
 Gene action in eukaryotes is believed to be regulated through
histone and non-histone proteins associated with chromosomes.
 Histone acetylation and deacetylation causes changes in
transcription activity and thus regulates gene expression.
 Histone acetylation effectively removes the positive charge from
the histone tail and reduces interaction between histone and
nucleosome. This opens up the nucleosome and allows
transcription mechinery to come in contact with the DNA
template leading to gene transcription.
 Repression of gene transcription is achieved by the reverse of this
mechanism.

11. Correct Replication of DNA
 Semiconservative replication of a linear DNA molecule inevitably
leaves a short sequence at the two ends of this molecule in
unreplicated state. This will lead to a progressive shortening of
the molecule after every round of replication. Chromosome
organization avoids this shortening as follows.
 The telomeres are made up of several copies of a short
repeating sequence. The enzyme telomerase adds copies of this
sequence at the chromosome ends. Therefore, the shortening
that occurs due to replication is adequately compensated for by
telomerase action, and the chromosome length is maintained.

12. Helps in Sex Determination
 Humans have 22 pairs of autosomes which determine the
somatic characters and 1 pair of allosomes which are
responsible for sex determination
 Human females contain 2 X chromosomes and produces only
one type of gamete, i.e., X. the male on the other hand has 1 X
and 1 Y chromosome, is heterogametic and produces two types
of gametes X and Y. fusion of X sperm with ovum results in
development of female sex and fusion of ovum with Y sperm
results in male sex.
 This system of sex determination is found in Drosophila, human
and some other mammals.