Page 25 - Biology_F5

Biology for Advanced Level Secondary Schools

fusion of the outer and inner membranes Functions of the nucleus
of the envelope. The nucleus contains (a) It controls all metabolic activities of
chromatin, consisting of DNA, which is the cell.
bounded by basic proteins called histones. (b) It contains hereditary information of
During nuclear division, chromatin the cell, hence, it transmits genetic
stains more intensely and becomes information to daughter cell.
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more conspicuous because it condenses (c) It takes part in the formation of
into more tightly coiled threads called ribosomes and RNA, thus it controls
chromosomes. However, some remains protein synthesis.
tightly coiled and continue to stain intensely (d) It controls cell division when required
and this is called heterochromatin. It is as DNA replication is essential for cell
seen as a dark patch, usually occurring division.
near the nuclear envelope. The remaining
loosely coiled chromatin is euchromatin. Exercise 1.5
It contains DNA that is genetically active
during interphase. 1. Describe the structure of the
nucleus of a cell.
The nucleolus 2. Explain the role of nucleolus.
The nucleolus is a cellular structure
present in the nucleus of a eukaryotic 3. Why is the nucleus said to be the
cell. The nucleolus appears as a rounded controller of all cellular activities?
and darkly stained structure inside the
nucleus. It contains nucleolar organisers Endoplasmic reticulum
that synthesise ribosomes by transcribing The endoplasmic reticulum is a network
and assembling ribosomal RNA (Figure of flattened membranes bound sacs called
1.9). The nucleoli vary in number from cisternae. Some of the endoplasmic

one to many nucleoli within a single cell reticula are encrusted with ribosomes
nucleus of a plant or an animal cell. and termed Rough Endoplasmic Reticula
(RER), while those without ribosomes are
Ribosomes known as Smooth Endoplasmic Reticula
Rough endoplasmic (SER). The quantity of both rough and
reticulum smooth endoplasmic reticula in a cell
Nucleolus can slowly interchange from one type
to the other, depending on the changing
Chromatin
metabolic activities of the cell.
Nucleoplasm

Nuclear pore Rough endoplasmic reticulum (RER)
The surface of the rough endoplasmic
Nuclear
envelope reticulum contains protein-manufacturing
ribosomes, giving it a “rough” structure
Figure 1.9: Structure of a nucleus or surface (Figure 1.10a). However, the

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