Medical Biology
Lec -3-
Shorooq Wessam
Cell nucleus
In cell biology,the nucleus is a membrane-enclosed organelle found in most
eukaryotic cells. It contains most of the cells genetic material,organized as
multiple long linear DNA molecules in complex with a large variety of
proteins,such as histones to form chromosomes . the genes within these
chromosomes make up the cells nuclear genome . the function of the nucleus is
to maintain the integrity of these genes and to control the activities of the cell by
regulating gene expression , the nucleus is the largest cellular organelle in
animals ,in mammalian cells the average diameter typically varies from 11 to 22
micrometers (µm) and occupies about 10% of the total volume . the viscous
liquid within it is called nucleoplasm and is similar to the cytoplasm found
outside the nucleus .
The main structural elements of the nucleus are the nuclear envelope,a double
membrane that encloses the entire organelle and keeps its contents separated
From the cellular cytoplasm,and the nuclear lamina,a meshwork within the
nucleus that adds mechanical support much like the cytoskeleton supports
The cell as a whole.Although the interior of the nucleus does not contain any
membrane-delineated bodies ,its contents are not uniform and a number of
subnuclear bodies exist made up of unique proteins , RNA molecules and DNA
conglomerates . the best known of these is the nucleolus ,which is mainly
invoved in assembly of ribosomes .after being produced in the nucleolus ,
ribosomes are exported to the cytoplasm where they translate mRNA .
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1- Nuclear envelope and pores
The nuclear envelope consists of two cellular membranes , an inner and an
outer membrane arraged parallel to one another and separated by 10 to 50
nanometers (nm) . the nuclear envelope completely encloses the nucleus and
separates the cells genetic material from the surrounding cytoplasm , serving as
a barrier to prevent macromolecules from diffusing freely between the
nucleoplasm and cytosplasm . the outer nuclear membrane is continuous with
the membrane of rough endoplasmic reticulum (RER) and is similarly studded
with ribosomes . the space between the membranes is called the perinuclear
space and is continuous with the RER lumen .
Nuclear pores which provide aqueous chnnels through the envelope are
composed of multiple proteins collectively referred to as nucleoporins . the size
of pores allows the free passage of small water – soluble molecules while
preventing larger molecules such as nucleic acids and proteins . ribosomal
subunits and some RNAs are transported through the pore complexes in process
mediated by transport factors known as karyopherins.
2- Chromosomes
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The cell nucleus contains the majority of the cells genetic material in the form
of multiple linear DNA molecules organized into structures called
chromosomes . during most of the cell cycle these are organized in a DNA –
protein complex known as chromatin and during cell division . asmall fraction
of the cells genes are located instead in the mitochondria .
3- Nucleolus
The nucleolus is a sub-organelle of the cell nucleus , it is roughly spherical
and is surrounded by a layer of condensed chromatin . no membrane separated
the nucleolus from the nucleoplasm. The main role of the nucleolus is considred
to be ribosome biogenises , since its fundamental component rDNA codes for
preribosomal RNA .additionally recent research pointed out that the nucleolus is
also responsible for the trafficking of varios prominent small RNA species .
4- Centrosome
The centrosome is the main microtubule organizating center (MTOC) of the
cell as well as a regulator of cell – cycle progression . although the centrosome
has a key role in efficient mitosis , centrosomes are composed of two
orthogonally arranged centrioles surrounded by an amorphous mass of
pericentiolar material . centrosomes are often associated with the nuclear
membrane during interphase of the cell cycle . in mitosis the nuclear membrane
breaks down and the centrosome nucleated microtubules can interact with the
chromosomes to build the mitotic spindle , also it has a central role in making
cilia and flagella .
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