# 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 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 nucleus controls cellular heredity and metabolism.
-> It is usually found in the centre of the cell ( sometimes pushed to one side due to vacuole in plant cell).
-> Generally one nucleus is present in cell but some cells are multinucleated {Ex - skeleton muscle fibres and fungus} and 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 micrometer 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 nuclear envelope and contains nucleoplasm , chromatin and nucleolus.
A) NUCLEAR ENVELOPE (Nuclear membrane) :
-> It is a double membrane structure that separates nucleus from the cytoplasm.
-> Each membrane of nuclear envelope is about 7 to 10 nm thick .
1) OUTER MEMBRANE-
-> It is continuous with endoplasmic reticulum and may have ribosomes attached .
-> It helps in exchange of materials between nucleus and cytoplasm.
2) INNER MEMBRANE-
-> It lies close to the nucleoplasm and provides the structural support.
3) PERINUCLEAR SPACE-
-> Narrow space between the two membranes about 10 to 15 nm .
-> It continues with lumen of endoplasmic reticulum.
4) NUCLEAR PORES-
-> They are large openings formed by nuclear pore complexes (NPCs) .
-> Each nucleus has 3000 - 4000 nuclear pores .
-> It regulates exchange of materials ( mRNA , T-RNA , ribosomal subunits , ions and enzymes ).
-> Between nucleus in cytoplasm .
B) NUCLEOPLASM ( Nuclear sac or Karyolymph) :
-> They are gel like matrix that fills 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 non - membrane bound structure .
-> It is discovered by FELICE FONTANA in 1781 and later described by BOWMAN in 1840s.
* STRUCTURE OF NUCLEOLUS-
-> It cxontains 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 targetting .
* CHEMICAL COMPOSITION OF NUCLEUS :
-> It contains 9 - 12 % of DNA which carries genetic information.
c
-> It contain 1 - 10 % of RNA for protein synthesis and gene expression.
-> 60 - 70 % of proteins are responsible for histone protein and enzymes functioning in the nucleus.
-> Only 3 -5 % of lipids are required for membrane structure of the nucleus.
-> Trace amount of minerals such as Ca , Mg , P are essential and enough for structural and catalytic role .
* FUNCTIONS OF NUCLEUS:
a) Genetic control centre - Contains DNA which control all cell activity and coding for proteins .
-> Direct synthesis of enzymes , structural proteins and regulatory elements .
b) Storage of genetic information - store 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 synthesis phase of 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 or RNA :
- mRNA ( MESSENGER RNA) -: carries genetic code.
- rRNA (RIBOSOMAL RNA) -: which forms ribosomes .
- tRNA ( TRANSFER RNA) -: which brings amino acids during translation.
No comments:
Post a Comment