Nucleus Structure And Functions | NMDCAT 2021

nucleus mdcat | nucleus structure and function mdcat | mdcat lecture | nucleus in urdu
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 undiferentiated cells (such as eggs) have numerous pores (about 30,000 per
nucleus), whereas diferentiated cells such as erythrbcytes have only 3 or 4 pores/nucleus. Each
pore has a deinite structure which controls the traic of substances passing through them (igs.
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 area consisting of large molecular weight RNA and rDNA. It is the nucleolus where ribosomes are
assembled and are then exported to the cytoplasm via nuclear pores.
Chromosomes
Nucleus is often deeply stained with basic dyes because of the chromatin material. During cell
division chromatin material is converted into darkly stained thread like structures known are
chromosomes. Under a compound microscope, chromosomes appear to be made of arms and
centromeres. Centromere is the place on the chromosome where spindle ibres are attached during
cell division. Each chromosome consists of two identical chromatids at the beginning of cell division
(chromatid is exact replica of the chromosome) which are held together at centromere (Fig.4.15).
A chromosome is composed of DNA and proteins. All the information necessary to control the
activities of the cell is located on the chromosomes in the form of genes, which are transferred
from one generation to the other. The number of chromosomes in all individuals of the same
species remains constant generation after generation. In man, each cell contains 46 chromosomes,
frog cell has 26 and chimpanzee has 48 chromosomes. There are 8 chromosoms in the fruit ly,
Drosophila melanogaster, 16 chromosomes in onion, 48 in potato, and 14 in garden pea. The number
of chromosomes in normal body cells is diploid (2n), whereas haploid chromosome number (n)
is present in germ cells, e.g human sperms and eggs have 23, while those of Drosophila have 4
chromosomes.

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[@laibabibi3787]

Sir pr RBCs k pass toh nucleus hee nai hota toh nuclear pores km yah ziada🤔

[@ALIACADEMYAR7]

Immature stage me hota ha

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Right

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Lekin sir Apne kaha RBC ka nucleus nahi hota tho nucleo pore kese ho sakte he ??

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Sir RBC me nucleus nh fir pore???

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Sir cardiac muscle are mononucleated🙂

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Rat cardiac cells are multi nucleated

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Differentiated cell mai R.B.C ki example di par os Mai to nucleus hi ni to nuclear pores Kahan se agai???

[@ALIACADEMYAR7]

Jb immature stage me Hota ha