Nucleus:
These are most important and most prominent part of the cell, also called as the brain of cell.

• these are only visible in non dividing cells.
• in animal cell they are central in position with exception of skeletal muscle fubre.
• in plant cell, they are pushed to peri phery due to presence of large vacuole.
• they may be irregular or spherical in shape.
• a cell containing single nucleus is called "mononucleate" two as "binucleate" and with more then two as "multinucleate".

Structure:
Nucleus is composed of nuclear membrane, nucleoli, nucleoplasm and chromosomes.
 Nuclear membrane:

• Nuclear membrane also called as nuclear envelop seperates the nuclear material from the          cytoplasm.
• It is a doble layered structure. outer layer continuous with the endoplasmic reticulum and the inner ine encloses the nuclear contents.
• These membrane have same structure as per fluid mosaic model.

Nuclear pores:

• nuclear pores results from the fusion of outer and inner membranes. they are composed of speciallized transport proteins called nucleoprotiens.
• they act as a gateway for the exchange of materials with the cytoplasm.
• their number is variable depending upon the differentiation of the cell i.e. undifferentiated cell like eggs have 30,000 pores per nucleus while erythrocytes well differentiated cells have 3 to 4 cells per nucleus.

Nucleoplasm:

• it is transparent semifluid ground substance.
• it contains DNA, RNA and enzymes.

Nuclelolus:

• nucleolus is non membranous, darkly stained body within the nucleus.
• nucleoli may be one or more.
• they appear during interphase and disappear during cell division. RNA is synthesized and stored in it.
• nucleolus is composed of two regions. peripheral granular area containing precursors for ribosomal subunits and central fibril area containing RNA and DNA.
• it the factory for ribosome synthesis.

Chromosome and Chromatin:

• each chromosome is a thread like structure resulting from the organization of chromatin material during cell division.
• chemically chromosomes are composed of DNA and PROTEINS.
• under microscope  they appear to be made of arms and a centromere, the place where spindle fibres are attached during cell division.
• centromere is the place on the chromosome and kinetochore is a place on centromere where spindle fibres are attached during cell division.

Function:

• nucleus is the brain of cell.
• it controls all the metabolic activities of cell.
• it has all the genetic information in a cell.
• nucleus plays an important role in inheritance.

Number of chromosomes in different species.

• man has 46 chromosomes.
• chimpanzee has 48.
• onion has 16.
• garden pea has 14.
• frog has 26.
• drosophila has 8.
• potato has 48.
• pigeon has 80.