Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
Nuclear DNA exists as a complex of proteins called ______ that condenses into ______ during cellular division.
(a) chromosomes, chromatin
(b) chromatids, chromosomes
(c) chromophores, chromatin
(d) chromatin, chromosomes
Ans:(d) chromatin, chromosomes
Chromatin
👉In eukaryotes, this organisation is much more
complex. There is a set of positively charged, basic
proteins called histones. A protein acquires charge
depending upon the abundance of amino acids
residues with charged side chains. Histones are rich
in the basic amino acid residues lysine and arginine.
Both the amino acid residues carry positive charges
in their side chains. Histones are organised to form
a unit of eight molecules called histone octamer.
The negatively charged DNA is wrapped around the positively charged
histone octamer to form a structure called nucleosome . A
typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute
the repeating unit of a structure in nucleus called chromatin, threadlike
stained (coloured) bodies seen in nucleus. The nucleosomes in
chromatin are seen as ‘beads-on-string’ structure when viewed under
electron microscope (EM)
👉Theoretically, how many such beads (nucleosomes) do you imagine
are present in a mammalian cell?
Ans: Normally, there are approx 30 million nucleosomes present in mammalian cells but it is not certain to the maximum point in case of humans. Diploid cell nucleus mainly contains this much number of nucleosomes with the number of cells that are present. 3/4 of whole human genome are occluded by 30 million nucleosomes.
👉The beads-on-string structure in chromatin is packaged to form
chromatin fibers that are further coiled and condensed at metaphase stage
of cell division to form chromosomes. The packaging of chromatin at higher
level requires additional set of proteins that collectively are referred to as
Non-histone Chromosomal (NHC) proteins. In a typical nucleus, some
region of chromatin are loosely packed (and stains light) and are referred to
as euchromatin. The chromatin that is more densely packed and stains
dark are called as Heterochromatin. Euchromatin is said to be
transcriptionally active chromatin, whereas heterochromatin is inactive.
👉Nucleus
Nucleus as a cell organelle was first described by Robert Brown as early
as 1831. Later the material of the nucleus stained by the basic dyes was
given the name chromatin by Flemming.
The interphase nucleus (nucleus of a
cell when it is not dividing) has highly
extended and elaborate nucleoprotein
fibres called chromatin, nuclear matrix
and one or more spherical bodies called
nucleoli (sing.: nucleolus)
👉The nuclear matrix or the nucleoplasm contains nucleolus and
chromatin. The nucleoli are spherical structures present in the
nucleoplasm. The content of nucleolus is continuous with the rest of the
nucleoplasm as it is not a membrane bound structure. It is a site for
active ribosomal RNA synthesis. Larger and more numerous nucleoli are
present in cells actively carrying out protein synthesis.
👉You may recall that the interphase nucleus has a loose
and indistinct network of nucleoprotein fibres called
chromatin. But during different stages of cell division, cells
show structured chromosomes in place of the nucleus.
Chromatin contains DNA and some basic proteins called
histones, some non-histone proteins and also RNA. A
single human cell has approximately two metre long
thread of DNA distributed among its forty six (twenty three
pairs) chromosomes.
👉Prophase
Prophase which is the first stage of karyokinesis of mitosis follows the
S and G2 phases of interphase. In the S and G2 phases the new DNA
molecules formed are not distinct but intertwined. Prophase is marked
by the initiation of condensation of chromosomal material. The
chromosomal material becomes untangled during the process of
chromatin condensation (Figure 10.2 a). The centrosome, which had
undergone duplication during S phase of interphase, now begins to move
towards opposite poles of the cell. The completion of prophase can thus
be marked by the following characteristic events:
l. Chromosomal material condenses to form compact mitotic
chromosomes. Chromosomes are seen to be composed of two
chromatids attached together at the centromere.
2. Centrosome which had undergone duplication during interphase,
begins to move towards opposite poles of the cell. Each centrosome
radiates out microtubules called asters. The two asters together
with spindle fibres forms mitotic apparatus.
Cells at the end of prophase, when viewed under the
microscope, do not show golgi complexes, endoplasmic
reticulum, nucleolus and the nuclear envelope.
👉Telophase
At the beginning of the final stage of karyokinesis, i.e.,
telophase, the chromosomes that have reached their
respective poles decondense and lose their individuality. The
individual chromosomes can no longer be seen and each set
of chromatin material tends to collect at each of the two poles
(Figure 10.2 d). 
This is the stage which shows the following
key events:
l. Chromosomes cluster at opposite spindle poles and their
identity is lost as discrete elements.
2. Nuclear envelope develops around the chromosome
clusters at each pole forming two daughter nuclei.
3. Nucleolus, golgi complex and ER reform.
Chromatid
👉Metaphase
The complete disintegration of the nuclear envelope marks
the start of the second phase of mitosis, hence the
chromosomes are spread through the cytoplasm of the cell.
By this stage, condensation of chromosomes is completed
and they can be observed clearly under the microscope. This
then, is the stage at which morphology of chromosomes is
most easily studied. At this stage, metaphase chromosome
is made up of two sister chromatids, which are held together
by the centromere (Figure 10.2 b). Small disc-shaped
structures at the surface of the centromeres are called
kinetochores. These structures serve as the sites of attachment
of spindle fibres (formed by the spindle fibres) to the
chromosomes that are moved into position at the centre of
the cell. Hence, the metaphase is characterised by all the
chromosomes coming to lie at the equator with one chromatid
of each chromosome connected by its kinetochore to spindle
fibres from one pole and its sister chromatid connected by
its kinetochore to spindle fibres from the opposite pole (Figure
10.2 b). The plane of alignment of the chromosomes at
metaphase is referred to as the metaphase plate. The key
features of metaphase are:
l. Spindle fibres attach to kinetochores of
chromosomes.
2. Chromosomes are moved to spindle equator and get
aligned along metaphase plate through spindle fibres
to both poles.
👉MUTATION
Mutation is a phenomenon which results in alteration of DNA sequences
and consequently results in changes in the genotype and the phenotype
of an organism. In addition to recombination, mutation is another
phenomenon that leads to variation in DNA.
One DNA helix runs continuously from
one end to the other in each chromatid, in a highly supercoiled form.
Therefore loss (deletions) or gain (insertion/duplication) of a segment of
DNA, result in alteration in chromosomes. Since genes are known to be
located on chromosomes, alteration in chromosomes results in
abnormalities or aberrations. Chromosomal
aberrations are commonly observed in cancer cells.
In addition to the above, mutation also arise due
to change in a single base pair of DNA. This is known
as point mutation. A classical example of such a
mutation is sickle cell anemia. Deletions and insertions
of base pairs of DNA, causes frame-shift mutations
