Cell: The Unit of Life

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The centrioles form the basal body of cilia or flagella, and spindle fibres

that give rise to spindle apparatus during cell division in animal cells.

10. Nucleus

Nucleus was first discovered by Robert Brown (1831)

The material of the nucleus stained by the basic dyes was given the

name chromatin by Flemming.

Normally, a cell has only one nucleus. Some cells have more than one.

Some mature cells lack nucleus. E.g. mammalian RBC and sieve tube cells

of vascular plants (enucleated or anucleated cells).

Function of a cell is controlled by nucleus.

1

Double layered membrane with a space between (10 - 50 nm)

called perinuclear space. It is a barrier between the materials

present in nucleus & cytoplasm. Outer membrane usually

remains continuous with ER and also bears ribosomes on it.

Nuclear envelope has minute pores formed by the fusion of its

two membranes. These are the passages for the movement of

RNA and protein between nucleus and cytoplasm.

Nuclear envelope:

2

Nuclear matrix (nucleoplasm): It contains nucleolus and

chromatin.

3

Chromatin: A network of nucleoprotein fibres. It contains DNA and

basic proteins (histones), non-histone proteins and RNA. During

cell division, chromatins condense to form chromosomes.

4

Nucleolus: One or more non-membranous spherical bodies.

Its content is continuous with the nucleoplasm as it lacks a

membrane. It is a site for active ribosomal RNA synthesis. (NEET

2018)