NUCLEUS



NUCLEUS:

The nucleus ¹( karyon¹) is the most conspicuous and the largest organelle of a eukaryotic cell. It is visible  with light microscope. However, its fine structure has been revealed by electron microscope.

Discovery: The nucleus was observed by a Dutch microscopist Anton Von Leeuwenhoek in the red corpuscles of fishes. However, a Scotch botanist Robert Brown was the first to describe the nucleus seen in an orchid cell  in 1813. He stared that the nucleus was a regular cell organelle and called it by this name.

Importance of the nucleus:

The nucleus is present in all eukaryotic cells.Certain mature cells, however,do not have a nucleus. The mammalian red blood corpuscles lose their nuclei  at maturity, and survive for a few months only. The food _ conducting phloem cells, called sieve tubes, of flowering plants also lose their nuclei at maturity but they remain functional for several years. Generally, however, if they nucleus is removed from a cell, say Amoeba or Acetabularia, the cell does not survive for long unless the nucleus is restored. Hence, the nucleus is essential for the survival of the cell. The cells deprived of nuclei cannot divide and differentiate. 

      Prokaryotic cells do not have an organized nucleus with a nuclear envelope. They have a nucleoid, a highly folded circular  DNA molecule without a membrane. 





Number: Most cells are uninucleate, i.e., have a single nucleus. Some cells are binucleate,i.e.,have two nuclei, e.g., Paramecium caudatum, a protozoan. A few cells are multinucleate, having many  nuclei. Certain  animal and plant cells, such as striated muscle fibres and latex vessels,  become multinucleate by cell fusion. The multinucleate cells formed in this manner are called syncytia ( singular is syncytium). Opalina, a protozoan, becomes multinucleate by repeated nuclear divisions without corresponding  cytoplasmic divisions. The multinucleate cells formed in this way are termed plasmodio ( singular is plasmodium), or coenocytes.Fungi, such as Rhizopus, are coenocytic multinucleate because the cross wall are not formed or are irregularly formed in the mycelium. The alga Vaucheria is also coenocytic.


Position: In some cells,the nucleus has a relatively fixed position, usually near the centre. In others, it may move around freely and may be found anywhere in the cell. Gland cells have a nucleus at the base. In mature plant cells, nucleus is pushed to one side from the centre by the enlarging sap vacuole. Adipose cells have a peripheral nucleus. 

    Form: Shape of the nucleus may be related to the shape of the cell. The nucleus is spherical in rounded, cuboidal and many__ sided cells, oval in cylindrical cells, and disc__ like in flat ( sequamous) cells.The nucleus is lobed in certain leucocytes. Plant cells have lens__ shaped nucleus, being pressed against the wall by a large central vacuole. The macronucleus of ciliates has interesting shapes, bean__ like, horseshoe__ shaped, beaded and even branched .






          Size: The size of the nucleus varies with the cell type and species. However, a relationship seems to exist between the nuclear mass and the cytoplasmic mass in each   cell type .Size of the nucleus is also related to the number of chromosomes sets in has. The haploid cells have smaller nuclei than the polyploid cells. Generally,  the nucleus varies from 5 to 25  ųm in diameter. 

Note : L. Nucleus = kernel, G. Karyon = kernel

Green Structure:

The nucleus typically consists of 5 types of organelles. It is bounded by a thin nuclear envelope, or karyotheca.Within the envelop, is a clear fluid substance,  the karyolymph, or nuclear sap, in with the nuclear solutes are dissolved.  Suspended in the nuclear sal are a network  of protein __ containing fibrils called nuclear matrix ; fine intermingled nucleoprotein filaments collectively referred to as chromatin; and one or more rounded bodies called nucleoli ( singular, nucleolus).





Chemical Composition:

The nucleus is composed of about 80% proteins, 12% DNA, 5% RNA and 3% lipids.The proteins include enzymes,  such as  polymerases, which help in the synthesis of DNA and RNA. Traces of minerals, such as calcium, sodium,  potassium, magnesium, are also present. Phosphorus is a component of DNA and RNA  

 Nucleus are the Functions:

Nucleus is considered the control centre of a cell. However, the control is a two__ way affair.  Substances from the cytoplasm enter the nucleus and influence the  DNA  to charge the particular mix of  RNA ( and hence proteins) formed.  Thus, the types of   proteins synthesised in  a cell change according to its needs. The nucleus serves many functions __ 

(i) Cell Meintenance and Growth: Nucleus maintains the cell and being about its growth by directing the synthesis of structural proteins. 

(ii) Cell Metabolism: Nucleus regulates cell metabolism by directing the synthesis of functional proteins ( enzymes).

(iii) Genetic Information: Nucleus also contains genetic information for reproduction, development and behaviour of the organism besides for structure and metabolism. 

(vi) Cell Replication: Nucleus takes part in the formation of ribosomes. 

(v) Ribosome Formation: Nucleus takes part in the formation of ribosomes.

 (vii) Variation: Nucleus develops genetic variations that contribute to evolution. 

(vii) Cell Differentiation:Nucleus brings about cell differentiation by keeping only certain genes operational in particular cells.

   Although the nucleus controls the cytoplasmic activities, it cannot exist by itself.

Nuclear Envelop ( Karyotheca):

Structure: Nuclear envelop consists of 2 unit membranes: outer and inner, with a narrow ( 200__ 400 A° wide), fluid __ filled intermembrane, or pronuclear, space in between. Each membrane resembles the  cell membrane in structure. The outer membrane bears ribosomes on its   cytoplasmic surface and is rough. It is the continuous with endoplasmic reticulum at certain places. Thus, the perinuclear space is continuous with  the  channels of the endoplasmic reticulum. These interconnections allow the nuclear envelope to shrink or expand rapidly by losing material to, or gaining it from, the endoplasmic reticulum. The nuclear envelope may expand rapidly when a dormant cell resumes activity and starts preparing  DNA or RNA . The inner membrane is free of ribosomes and is smooth, but has a dense layer, the nuclear lamina, closely associated with its inner surface. The nuclear lamina is a network of protein fibres, and gives support and form to the inner membrane. It connects chromatin to the inner membrane, and keeps most of the  chromosomes in the periphery of the nucleus.  The nuclear envelope is semipermeable, and is perforated by minute, circular nuclear pores.There may be 1,000 to 10,000 pores per nucleus.  At the edges of the pores, the inner and outer membranes of the nuclear envelope are continuous. Each pore is fitted with a complex protein structure called pore complex. The pore complex projects into both cytoplasm and nucleoplasm. It consists of two rings, the annoli, each having 8 subunits, and sending spokes into the pore. The spokes enclose a channel. The pore complex controls the passage of macromolecules, such as tRNAs, mRNAs, ribosomal proteins, nucleotides, enzymes, and of ribosomal subunits. The RNA molecules are very large and hydrophilic, and can not pass through the lipid bilayers
of the nuclear envelope. Therefore, the pores are necessary for their passage. The nuclear envelope disappears for a short time during cell division. It persists even during cell division in protozoans. 






           Origin: The nuclear envelope is formed during telophase by coming together and fusion of small vesicles into which the nuclear envelope breaks up during prophase.

         Functions: 

Nuclear envelope serves five functions ___ 


(i) It maintains the shape of the nucleus.

( ii) It keep the nuclear contents in place and distinct from cytoplasm. 

( iii) It regulates the flow of materials into and out of the nucleus. 

( vi) Its pores allow the exist of ribosomal subunits and tRNAs and mRNAs.

(v) Nuclear envelope preserves the stability of the genetic material,  protecting it from respiratory breakdown that occurs in the cytoplasm. 

Nuclear Sap ( Nucleoplasm, Karyolymph): 

Structure: The nucleoplasm is a clear, nonstraining, fluid material in the nucleus. It contains raw materials ( nucleotides), enzymes ( DNA and RNA polymerases) and metal ions ( Mn++ ,Mg++ ) for the synthesis of DNA and RNAs. It also contains histone and  nonhistone proteins for combination with DNA, and other proteins for the formation of ribosomal subunits. The contents occur in the nuclear sap in true or colloidal solution. The RNAs ( rRNA, tRNAs, mRNAs) and ribosomal subunits synthesized in the nucleoplasm finally leave the nucleus.

Function: The nucleoplasm has the following functions:__

(i) It is the seat of synthesis of DNA, RNAs and ribosomal subunits. 

( ii) It supports the chromatin material and nucleoli.

( iii) It provide turgidit to the nucleus.

Nuclear Matrix: 

Structure: The nuclear matrix is a network of fine, crisscrossing, protein__ containing fibrils which are joined to the nuclear envelope by their ends. It forms a sort of nuclear skeleton. It remains intact after the chromatin and DNA have been removed.

Functions: The nuclear matrix has the following functions ___ 

( i)  It maintains the shape of the nucleus.

 (ii) Chromatin fibres are anchored to nuclear matrix.

 (iii) The machinery for various nuclear activities, such as  replication and transcription, is associated with the matrix. 

Chromatin: 

Structure: The term chromatin¹ mean ' coloured material ' and indicates the ability of this material to easy take up of stain for microscopic examination. The chromatin occurs in a nondividing  ( interphase) nucleus as fine filaments termed the chromatin fibres. The latter lie crisscross and give the appearance of a diffuse network, often called the nuclear,or chromatin, reticulum. The chromatin fibres are simply greatly extended chromosomes. 

1: G. Chroma = colour.

           During cell division, short, thick, rod __ like organelles, called the chromosomes, are formed by condensing and tight coiling of the chromatin fibres. A fixed number of chromosomes appear in the cells of each species. A chromosomes consists of two identical halves, the chromatids, held together at one point, the centromere. The centromere appears as a narrow region, called Primary constriction,  of the chromosome. The chromosomes may have additional constrictions termed secondary constrictions near their ends. Part of the chromosome beyond the secondary constriction is termed statellite.The secondary constrictions are always constant in their positions and can be used as markers. The chromosomes having satellites are known as sat chromosomes, or marker chromosomes. Certain secondary constrictions are the sites for the formation of nucleoli and are known as nucleolar organizers.The chromosomes having nucleolar organizers are called nucleolar chromosomes. A chromatid contains a single fine chromatin fibre, the chromonema.It is  very long and greatly coiled to be accommodated in a short chromatid. The chromosomes bear hereditary units, the genes.








              Types of chromosomes: A chromosome may have either equal or unequal arms. This depends upon the position of centromere. Therefore, the chromosome is of four types.





        (i) Metacentric chromosome: The chromosome in which the centromere is near the middle and the two arms are almost equal in length, is called a metacentric chromosome. 
   
       (ii) Submetacentric chromosome: The chromosome in which the centromere is slightly away from the middle point and consequently its  one arm is slightly short and the other arm slightly long, is called submetacentric chromosome. 

        (iii) Acrocentric chromosome: The chromosome in which the centromere is near the end and consequently its  one arm very long, is called acrocentric  chromosome. 

   (vi) Telocentric chromosome: The chromosome in which the centromere is at the tip of chromosome and the  arms are on one side only, is called telocentric chromosome.

    Chemical Composition: A chromatin fibre is a continuous linear  DNA  double strand associated with proteins of two types: basic histones and acid or neutral nonhistones. It also contains some  RNA , certain metallic ions and some enzymes,  such as DNA and  RNA polymerases . Histones are structural proteins.  Nonhistones are functional  proteins . 

      The chromatin of prokaryotic cells is a circular DNA with very little protein.

          Types: After cell division, the chromosomes change back into chromatin fibres. Most of the fibres become uncoiled, extended and scattered. They form the Euchromatin ( true chromatin) of the interphase nucleus. It stains lightly. Some chromatin fibres remain coiled and compacted in the interphase also. They constitute the heterochromatin ( the other chromatin). It stains deeply. It lies close to the nuclear lamina.

              Nucleosomes: Electron microscope shown as chromatin fibre as a chain of similar subunits called nucleosomes 






A nucleosome consists of a core particle wrapped by DNA strand. The core particle is an octomer  of 8 histone molecules. Each nucleosome is connected to the next by a short DNA linker. A nucleosome and a linker are together referred to as a chromatosome

 



  Function: The chromatin forms chromosomes during cell division.

 Nucleolus ( little Nucleus): 

Discovery: Nucleolus was discovered in 1781 by Fontana, it was described by Wagner in 1840, and was so named by 
Bowman in the same year ( 1840).

    Structure: Nucleolus is a dense, rounded, dark__ staining, granular organelle without a limiting membrane. It consists largely of  RNAs  and proteins . It also contains  DNA which produces precursor RNAs for the formation of ribosomes. Each cell has a fixed number of nucleoi, usually 1 to 4, may be numerous¹.The nucleoli are formed at specific sites, called the nucleolar organizers, present on certain chromosomes .  Calcium is necessary for maintaining configuration of the nucleolus.The nucleoi are larger and more numerous in the cells which are actively engaged in protein synthesis. The nucleus shows four regions with electrons microscope.







1: Upto 1600 in the oocyte of the toad, Xenopus.

(i) Matrix: It is amorphous, homogeneous,  proteinaceous ground substance of the nucleolus.

(ii) Fibrillar Region:  It contains fine fibrils that represent RNA precursor molecules in early stages of processing. 



(iii) Granular Region: It contains spherical granules which represent ribosomes subunits ( rRNA + ribosomal proteins) 

 

(iv) Nucleolar Chromatin:  It consists of chromatin fibre that is a part of the nucleolar chromosome. It synthesises rRNA.



Functions: Nucleolus serves the following functions:___ 

( i) Nucleolus synthesizes and stores RNA. 

(ii) It also receives ribosomal proteins from the cytoplasm for storage. 

(iii) It forms ribosomal subunits by wrapping the rRNAs with ribosomal proteins. The ribosomal subunits later leave the nucleus through the nuclear pores.

( iv) Nucleolus also plays a role in cell division. 

Functions of nucleus:

Nucleus  is an essential  and integral part of the eucryotic cell which stores genetic information in its DNA molecules, and also controls cellular activities. 

1): Chromatin part of nucleus  possesses all the genetic information that is required  for growth and development  of the organism, its reproduction, metabolism and behavior. 


2): It controls cell metabolism  and other activities  through the information  of RNAs ( mRNA, rRNA, tRNA) which control synthesis of particular type of enzymes. 


3): Ribosomes  are formed in nucleolus  part of the nucleus.


4): Changes in genetic material present in the nucleus are associated with variations. 


5): Nucleus  with the help of RNAs, directs the synthesis of  specific structural proteins and other chemical required  for cell growth and cell maintenance. 


6); Nucleus  directs cell differentiation by allowing  certain particles  sets of genes to operate .

7): Replication of nucleus is essential for cell replication. 

 Transmission to Daughter Cells: of all the components of the nucleus, only the chromosomes are transmitted from one cell generation to the next.


Differences between Euchromatin and Heterochromatin 

Euchromatin:

1): It is narrow ( diameter  30__ 80 A°), lightly stained somewhat diffused.

2): It does not have granules. 

3): It forms the bulk of chromatin.

4): It possesses active genes.

5): It takes part in transcription. 


6): It is affected by a number of factors like pH, temperature and hormones. 


7): Crossing over is quite common.

8): Euchromatin replicates early.


Heterochromatin

1): Heterochromatin is thicker ( diameter 250 A or more), darkly stained and is condensed. 


2): It has granules of various  sizes.

3): It is present at certain places in the chromatin. 


4): It does not possess  active genes. 


5): Transcription is absent in heterochromatin .

6): It is not influenced  by these factors. 


7): It inhibits crossing  over.


8):Heterochromatin replicates late in the Sphase .



































































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