# INTRODUCTION:-
-> The nucleus is the control centre of eukaryotic cells.
-> It contains the cell's genetic material (DNA) and directs all cellular activities like growth, metabolism and reproduction.
-> It is the largest organelle in a eukaryotic cell and is absent in prokaryotes (Bacteria lack a true nucleus and have a nucleoid instead ).
-> The nucleus was discovered by ROBERT BROWN in 1831 in orchid cells.
-> JOACHIM HAMMERLING in 1934 proved that the nucleus controls cellular heredity and metabolism.
-> It is usually found in the centre of the cell ( sometimes pushed to one side due to the vacuole in a plant cell).
-> Generally, one nucleus is present in a cell, but some cells are multinucleated {Ex - skeletal muscle fibres and fungus}, whereas some are non-nucleated cells or enucleated {red blood cells}.
-> Nucleus is generally spherical or oval in shape. But the shape may vary.
Ex: lobed in WBCs and elongated in muscle cells.
-> Nucleus is usually 5-25 micrometres in diameter in size.
-> The size and number of nuclei are related to cell function and metabolic activities.
* STRUCTURE OF NUCLEUS:
-> The nucleus is formed by the nuclear envelope and contains nucleoplasm, chromatin and nucleolus.
A) NUCLEAR ENVELOPE (Nuclear membrane) :
-> It is a double membrane structure that separates the nucleus from the cytoplasm.
-> Each membrane of the nuclear envelope is about 7 to 10 nm thick.
1) OUTER MEMBRANE-
-> It is continuous with the endoplasmic reticulum and may have ribosomes attached.
-> It helps in the exchange of materials between the nucleus and cytoplasm.
2) INNER MEMBRANE-
-> It lies close to the nucleoplasm and provides structural support.
3) PERINUCLEAR SPACE-
-> Narrow space between the two membranes, about 10 to 15 nm.
-> It continues with the lumen of the endoplasmic reticulum.
4) NUCLEAR PORES-
-> They are large openings formed by nuclear pore complexes (NPCs).
-> Each nucleus has 3000 - 4000 nuclear pores.
-> It regulates the exchange of materials (mRNA, tRNA, ribosomal subunits, ions and enzymes ).
-> Between the nucleus and cytoplasm.
B) NUCLEOPLASM ( Nuclear sac or Karyolymph) :
-> They are gel like matrix that fills the interior of the nucleus, which is composed of nucleotides, enzymes, ions and nuclear proteins.
-> It provides the medium for chromatin and nucleolus.
-> It is the site for enzymatic activities such as replication, transcription and RNA processing.
C) CHROMATIN (genetic material) :
-> Network of DNA and protein fibres found in nucleoplasm.
-> It is made up of DNA + Histone proteins + non-histone proteins + RNA .
* TYPES OF CHROMATIN:-
1) EUCHROMATIN- It is lightly stained and loosely packed.
-> Transcriptionally active ( genes are expressed ).
2) HETEROCHROMATIN- It is darkly stained and tightly packed.
-> Transcriptionally inactive ( genes are silent ).
-> It carries hereditary information, condenses into chromosomes during cell division.
D) NUCLEOLUS:
-> It is a dense, spherical structure present inside the nucleus.
-> It is a non-membrane-bound structure .
-> It was discovered by FELICE FONTANA in 1781 and later described by BOWMAN in the 1840s.
* STRUCTURE OF NUCLEOLUS-
-> It contains r-RNA, DNA and proteins.
-> It is formed around Nucleolar organiser regions ( NOR) of certain chromosomes.
* FUNCTION OF NUCLEOLUS-
-> Synthesis of ribosomal RNA.
-> Assembly of ribosomal subunits (40s and 60s).
-> Involves information of signal recognition particle for protein targeting.
* CHEMICAL COMPOSITION OF NUCLEUS :
-> It contains 9 - 12 % of DNA, which carries genetic information.
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-> It contains 1 - 10 % of RNA for protein synthesis and gene expression.
-> 60 - 70 % of proteins are responsible for histone proteins and enzymes functioning in the nucleus.
-> Only 3 -5 % of lipids are required for the membrane structure of the nucleus.
-> Trace amounts of minerals such as Ca, Mg, P are essential and enough for structural and catalytic roles.
* FUNCTIONS OF NUCLEUS:
a) Genetic control centre - Contains DNA, which controls all cell activity and codes for proteins.
-> Direct synthesis of enzymes, structural proteins and regulatory elements.
b) Storage of genetic information - stores genetic information in the form of genes or chromosomes.
c) Control of cellular metabolism - nucleus regulates the metabolic activities by controlling enzyme production via gene expression.
d) Regulation of cell division - DNA replication occurs in the nucleus during the synthesis phase of the cell cycle.
-> Controls mitosis and meiosis by distributing genetic material equally.
e) Ribosome formation - nucleolus forms ribosomal subunits which later move to the cytoplasm for protein synthesis.
f) Transmission of heredity - passes genetic information from parent to offspring during reproduction.
g) RNA synthesis - nucleus synthesises three types of RNA :
- mRNA ( MESSENGER RNA): carries genetic code.
- rRNA (RIBOSOMAL RNA): which forms ribosomes.
- tRNA ( TRANSFER RNA): which brings amino acids during translation.
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