Describe ultra structure and morphology of chromosomes present in the nucleus
Nucleus is
filled with a Prein rich substance called Nucleoplasm or Karyolymph. In the
nucleoplasm are numerous fine stands in the form of network called chromatin
network or nuclear reticulum which is composed of nucleic acids, Deoxyribo
nucleic acid (DNA) and Protein. During cell division chromatin changes its
shape to form chromosomes. Chromosomes contain hereditary units called genes
that carry the hereditary information from generation to generation.
Chromosomes vary in number from species to species e.g. 20 in corn. Chromosomes
are elongated and each is composed of two parts the arm and centromere. Before
cell division each chromosome consists of two threads called chromoneres.
Chromosome has different shapes as under:
(i) Meta
centric
Chromosome with
equal arms resembling the letter V.
(ii) SUB META CENTRIC
Chromosome with
unequal arms and resembling the letter J.
(iii) ACROCENTRIC OR SUBTELOCENTRIC
Rod like
chromosomes with one arm very small and the other very long. The centromere is
subterminal.
(iv) TELOCENTRIC
Location of
centromere at the end of chromosome.
ULTRA STRUCTURE
OF CHROMOSOMES:
Eukaryotic
chromosomes are composed of chromatin, a complex of DNA and Protein. Most
eukaryotic chromosomes are about 60% Protein and 40% DNA. A significant amount
of RNA is also associated with chromosomes because they are the sites of RNA
synthesis. DNA of the chromosome exists as one very long double stranded fiber,
a duplex which extends unbroken through the entire length of the chromosome. If
the strand of DNA from a single chromosome were laid out in a straight line, it
would be more than 7 feet (2 meter) long. This is much too long to fit into a
cell. In the cell however DNA is coiled, thus fitting into much smaller space.
It we gently
disrupt a eukaryotic nucleus and examine DNA with an electron microscope, we
find that it resembles a string of beads. Every 200 ucleotides, DNA duplex is
coiled about a complex of histones which are small very basic polupeptides rich
in amino acids arginine and lysine. Eight of these histones form the core of an
assembly called a nucleosome. Because so many of their amino acids are basic,
histones are very positively charged. DNA duplex which is negatively charged,
is strongly attracted to the histones and wrap tightly around the histone core
of each nucleosome. The core thus acts as a ‘form’ that promotes and guides the
nucleosomes wraps up into higher order coils called supercoils.
Highly condensed
portion of the chromatin are called hetero chromatin. Some remain condensed
permanently, so that their DNA is never expressed. The remainder of the
chromosome called enchromatin is not condensed except during cell division,
when the movement of the chromosomes is facilitated by the compact packaging
that occurs at that stage. At all other times enchromatin is an open
configuration and its genes can be activated. The genes are carried in the
chromosomes. Chromosomes can be separately identified visually, but the genes
re very small units and have not molecules borne in the chromosomes of the cell
nuclei. It is observed that the chromosomes and genes behave so in inheritance
that the genes cannot be considered outside chromosome. At the time of meiosis
the separation of homologous pairs of chromosomes can be seen and it takes
place as also required for the segregation of gene pairs. Chromosomes are also
the kinds of organelles that segregate regularly when eukaryotic cells divide.
In early twentieth century it was that the chromosomes were the vehicle for the
information of heredity.
Comments
Post a Comment