The nucleus is the largest and most prominent organelles in the animal cell.
Almost all eukaryote cells have a nucleus – red blood cells in mammals and phloem cells
in plants are a exception.
The Nucleus discovered by Robert Brown in 1831 With the help of his own made microscope or lenses
Every nucleus is surrounded by a nuclear envelope.
This consists of two membranes that
are separated by gap of 20 to 40 nm.
Nucleus may be irregular in shape and about 10 µm in diameter.
It contains the cell’s
DNA, which carries information that allows the cell to divide and carry out all its cellular
processes.
Nucleus controls the life and activities of the cell.
In animal cells nucleus is generally present in the central part. In plant cells nucleus is
pushed towards periphery due to a large central vacuole.
DNA/RNA and proteins (including enzymes) forms the chemical composition of coleus.
Nucleus consists of nuclear membrane, nucleoli, nucleoplasm and chromosomes.
The number of nuclear pores is highly variable. For example the undifferentiated cells
(e.g. eggs) have many pores (about 30000 per nucleus) while the differentiated cells (e.g.
erythrocytes)have only 3 or 4 pores/nucleus. Each pore has a definite structure.
Nucleolus is a darkly stained structure within the nucleus, has no membrane.
There are two regions of a nucleolus.
(i) The Peripheral Granular Area:
It contains the materials from which ribosomal subunits are formed.
(ii) The Central FibrillarArea:
It has large molecular weight RNA and rDNA.
The heredity material is in the form of chromosomes,which controls all the activities of
the cell. Chromosome is formed of DNA and proteins.
Nucleus is visible when the cell is in non-dividing stage. It contains chromatin network
and soluble sap called nucleoplasm. Dark staining chromatin, consisting of tightly packed
DNA, is known as Heterochromatin. The lighter, more loosely packed material is called
Euchromatin contains the DNA that is being actively read to produce proteins.
In
heterochromatin, the DNA is packed together and is not being read. Nucleus is stained
with basic dyes because of the chromatin material.
During cell division chromatin
material is converted into darkly stained thread like structures called chromosomes
Chromosome is made of arms and centromeres.
• Centromere is the place on the chromosome where spindle fibres are attached during
cell division.
Each chromosome consists of two identical chromads at the beginning of cell division
which are heldtogether at centromere.
There are four types of chromosomes on the basis of centromere position .
1)- Metacentric: Centromere is exat at center and forms two equal arms
2)- Submetacentric: centromere is near to center and forming two unequal arms
3)- Subtelocentric or Acrocentric : Centromere present near to tail/End of arm forming J like structure.
4)-Telocentric: Centromere is present on top of chromatid
• A chromatid is exact replica of the chromosome.
• The information to control cell activities is present on the chromosomes in the form of
genes.
• The chromosome number varies which may be 2n = 2 to 2n = 1200 (Pteridophytes).
The number of chromosomes in all individuals of the same species remains constant
generation after generation.
• Some examples of chromosome No. are: Frog = 26, Chimpanzee = 48. Fruit fly (Drosophila
melanogaster) = 8, Wheat = 42, Onion = 16, Potato. = 48. Garden pea = 14.
Penicillium(a
fungus) has two chromosomes (one pair), corn 20, wheat 42, sugarcane 80, some ferns
have more than 500 pairs, mosquito 6, fruit flmy 8, frog 26, honey bee 32, mouse 40 and
human cells have 46 chromosomes (23 pairs).
• The number of chromosomes in normal body cells is diploid (2n) while the gem cells
(sperms and eggs) have haploid chromosome number (11).
Examples: Human germ cells (eggs and sperms) = 23 chromosomes.
Drosophila germ cells = 4 chromosomes.
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