Class notes
Biol 430 Nucleus Notes
- Course
- BIOL 430
- Institution
- Central Washington University
This is a comprehensive and detailed note on nucleus for Biol 430.
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NUCLEUSCHAPTER NOTES
(A general and basic understanding of the molecules involved in the nucleus per lecture from the
professor)
Learning Objectives
1. Breakdown the nucleus and what’s involved in it.
2. Define each of the terms / study for future quizzes.
3. Mention function and importance of the nucleus.
INTRODUCTION –
Nucleus is usually the most conspicuous organelle of eukaryotic cell. However, well defined
nucleus is absent in prokaryotic cells. Nucleus is the repository of genome and the source of
informational macromolecules that govern the synthetic activities of the cytoplasm. It is
surrounded by a bilaminary nuclear envelop having pore complexes that permit the nuclear-
cytoplasm transport of materials. In the animal cells, it generally lies in the centre, surrounded on
all sides by the cytoplasm.
However, in plant cells it is often pushed to one side of the cell due to the presence of large
central sap vacuole. The shape of nucleus is variable according to cell type. It is generally
spheroid but ellipsoid or flattened nuclei may also occur in certain cells. In certain WBC (white
blood cells) the nucleus is dumbbell shaped. In human neutrophil it is trilobed. Most cells
contain a single nucleus, known as mono or uninucleate cells. Cells with two nuclei are known as
binucleate cells e.g. Paramecium. Sometimes more than two nuclei are present in a single cell.
Such cells are called polynucleate or multinucleated cells.
Such cells in animals are called syncytial cells (e.g. osteoblast) and such plants are termed
coenocytes (e.g. siphonal algae). Cells having distinct nucleus are called eukaryotic cells,
whereas cells without definite nucleus are called prokaryotic cells (e.g. bacteria). The latter
possess scattered chromatin material (DNA) in the cytoplasm called nucleoid. The mature
mammalian erythrocytes also do not possess any nucleus. Size of nucleus is not constant and is
generally correlated with DNA content. The nuclear size is variable depending upon the number
of chromosomes (DNA content).
NUCLEUS
History of the Nucleus
Nucleus was observed by a Dutch Microscopist, Antonie van Leeuwenhoek in 1710, as a
centrally placed clear area in the blood cells of amphibians and birds. Fontana (1781) recorded
an ovoid structure in each of the isolated epidermal cells of eel's skin. However, Robert Brown
, (1831) was the first to use the term nucleus for a prominent body present in the orchid cell. He
stated that nucleus was the regular feature of the cells and initiated the concept of nucleated cells.
Structure of the Nucleus
The nucleus consists of various parts. It is bounded by a thin but clearly defined covering, the
nuclear envelop or karyotheca. Within the envelope is a clear fluid substance called nucleoplasm
or nuclear sap or karyolymph is present in which the solutes of the nucleus are dissolved.
Suspended in the nucleoplasm are network of protein-containing fibrils called nuclear matrix;
fine intermingled nucleoprotein filaments collectively referred to as the chromatin; and one or
more spherical bodies known as nucleoli (singular, nucleolus). There are no membranes or
microtubules inside the nucleus. Protozoans that form a mitotic spindle within the nuclear
envelop, however, have microtubules in their nuclei (Fig. 1).
Chemical Composition: The nucleus is composed of about 9-12% DNA, 5% RNA, 3%
lipids, 15% simple basic proteins such as histone or protamines, about 65% complex acid
or neutral proteins, including enzymes such as polymerases for the synthesis of DNA and
RNA, organic phosphates and inorganic salts or ions such as Mg++, Ca++ and Fe++ .
Functions: The nucleus acts as a control center of the cell. It serves the following main
functions:
o It maintains the cell by directing the synthesis of structural proteins.
o It regulates cell metabolism by directing the synthesis of enzymatic proteins.
o It contains genetic information for reproduction, development and behavior of the
organism besides for structure and metabolism.
o It brings about cell replication when needed.
o It is the site for the formation of ribosome subunits.
o It brings about cell differentiation by keeping only certain genes operational.
o It develops genetic variations that result in evolution.
NUCLEAR ENVELOPE
The nuclear envelop separates the nucleoplasm from the cytoplasm. It consists of two unit
membranes: outer and inner. Each unit membrane is about 75Å thick, and is a trilaminar
lipoprotein like the plasma membrane. The two unit membranes are separated by a space called
the inter membrane or perinuclear space. It is about 250Å wide. The outer or cytoplasmic surface
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