Page 28 - 11-Bio-4 The Cell

4. The Cell eLearn.Punjab

NUCLEUS

Presence of cell nucleus was reported in 1831 by Robert Brown. Its early discovery was undoubtedly
due to its prominence in many cells, where it stands out as slightly darker than the surrounding
cytoplasm. It controls the life and activities of the cell. In animal cells, it generally occupies the
central space, while in the case of plant cells it is pushed towards periphery due to the presence
of a large vacuole. Nucleus may be irregular or spherical in shape. Generally, the cells have one
nucleus and are called mononucleate. On the other hand, the cells with two nuclei are binucleate
and with more than two as multinucleate.

Nucleus is only visible when the cell is in non-dividing stage. It contains chromatin network and
soluble sap called nucleoplasm. In dividing cells, the nucleus disappears and the chromatin material
in it is replaced by chromosomes. The heredity material is in the form of chromosomes, which
controls all the activities of the cell. DNA, RNA and proteins including enzymes form the chemical
composition of the nucleus. Nucleus consists of nuclear membrane, nucleoli, nucleoplasm and
chromosomes.

Nuclear Membrane

Nucleus is surrounded by a nuclear membrane which separates the nuclear material from the
cytoplasm. The nuclear membrane is actually a nuclear envelope composed of two membranes.
The outer membrane is at places continuous with the endoplasmic reticulum, while the inner
membrane encloses the nuclear content. The outer and the inner membranes are continuous at
certain points resulting in the formation of pores, the nuclear pores. The nuclear pores allow the
exchange of materials between the nucleus and the cytoplasm. The number of nuclear pores is
highly variable. The undifferentiated cells (such as eggs) have numerous pores (about 30,000 per
nucleus), whereas differentiated cells such as erythrbcytes have only 3 or 4 pores/nucleus. Each
pore has a definite structure which controls the traffic of substances passing through them (figs.
4.7, 4.10 and 4.16).

Nucleolus

It is a darkly stained body within the nucleus, and is without any membranous boundary to separate
it from the rest of the nuclear material. There may be one or more nucleoli in the nucleus. The
ribosomal RNA (rRNA) is synthesized and stored in the nucleolus. It is composed of two regions,
the peripheral granular area composed of precursors of ribosomal subunits and the central fibrill

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