3. Cell organelles - Nucleus

      Nucleus   is  a    membrane   enclosed    organelle   found    in    Eukaryotic

cell.   It  controlls  all   the   cell   activities.   so,    it       is     called   as       the  

brain      of   a    cell.

STRUCTURE  AND FUNCTIONS OF  NUCLEUS:

      The  nucleus  is the largest cellular organelle  in animal. The nucleus  has two membranes namely inner and outer membrane. The space between the two cellular membrane is called perinuclear space. It covers the nucleus and seperates the genetic material from surrounding cytoplasm, also  seperates  nucleoplasm and cytoplasm. The outer membrane  is  contact with rough endoplamic reticulam  and ribosomes. There are small holes found in the nuclear membrane is called as nucleopore. It transports nucleic acids and  proteins. Pores are about 100 nm in total  diameter. There are 3000 to 4000 pores are present  in a typical Mammalian cell throughout its envelope. Inside the nucleus there is a sap called nucleoplasm. It is highly viscous liquid. Inside the sap there is net like structure called chromatin reticulam. It contains DNA the genetic material  and responsible for genetic function of a cell and RNA serves as the copy  used in the actual manufacture of cell proteins. Both DNA and RNA are called as nucleic acids. DNA is B-form double helix. It  is double strandard molecule  consisting of a long chain of nucleoidides.  DNA contains deoxribo nucleic acid, its name derived from the that its molecules contains  one oxygen atom less than that of ribose. The nitrogen bases present in then nucleic acids are made up of Pyrimidines and purines. There are two purine bases: Adenine  and Guanine  and three types of pyrimidine bases Cystonsine, Thymine, and Uracil. Adenine,  Guanine, Cytosine and Thymine occur  in DNA.
 The pentose sugar present in RNA is ribose. Adenine, Guanine, Cytosine and Uracil occur in RNA. RNA is A- form single strandard  helix with shortage chains of nucleodides. 

structure of DNA

      

            chemical structure of DNA and
                    mRNA

FUNCTIONS OF DNA and RNA:

DNA carries information from one part of the cell to another. RNA transforms  the genetic code from the nucleus  to the ribosomes  to make proteins. There are many types of like mRNA, tRNA. Nucleus  controls the vital activites of cells like cell division and  reproduction.