NUCLEUS

$\displaystyle \small \bullet$ Nucleus is a cell organelle present in eukaryotic cells. It is usually single but sometimes absent or more in number.
$\displaystyle \small \bullet$ It contains all the genetic information for controlling all essential processes related to metabolism and transmission.
$\displaystyle \small \bullet$ Nucleus was first described by Robert Brown and was named chromatin by Flemming.
$\displaystyle \small \bullet$ Nucleus has four parts:
Nuclear membrane or Nuclear envelop
 $\displaystyle \small \circ$ It has a double membranous covering, an outer and an inner membrane. These two are separated by perinuclear space.
 $\displaystyle \small \circ$ The outer membrane is connected with endoplasmic reticulum and bears ribosomes.
 $\displaystyle \small \circ$ The nuclear pores are the passages through which movement of RNA and protein molecules takes place
Nucleoplasm
 $\displaystyle \small \circ$ Part of protoplasm that lies inside the nuclear membrane called nucleoplasm or matrix.
 $\displaystyle \small \circ$ It is a clear, non-staining, fluid material which contains raw materials, enzymes and metal ions for the synthesis of RNAs and DNA.
 $\displaystyle \small \circ$ The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.
Nucleolus
 $\displaystyle \small \circ$ It is a naked, round and slightly irregular structure which is attached to the chromatin at a specific region.
 $\displaystyle \small \circ$ Nucleolus is the site of active ribosomal RNA synthesis.
Chromatin
 $\displaystyle \small \circ$ A loose and indistinct network of nucleoprotein fibres is called chromatin.
 $\displaystyle \small \circ$ Chromatin is distinguished into:
Euchromatin is loosely coiled, lightly stained, metabolically active and diffused.
Heterochromatin is highly coiled, more stained and metabolically inactive region.

Chromosomes
$\displaystyle \small \bullet$ Chromatin fibres condense and  change into chromosomes during cell division. It consists of DNA and histone proteins.
$\displaystyle \small \bullet$ Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochore are present.
$\displaystyle \small \bullet$ Classification of chromosomes based on the position of the centromere:
Metacentric: the centromere is in the centre and two arms of equal size.
Sub-metacentric: the centromere slightly away from the centre with one short and one long arm.
Acrocentric: the centromere is towards end with one long arm and another tiny arm.
Telocentric: the centromere is at one end with only one arm
Acentric: chromosome without centromere. It do not attach to spindle in cell division.
$\displaystyle \small \bullet$ Sometimes a few chromosomes have non-staining secondary constrictions, which gives the appearance of a small fragment called the satellite.

MICROBODIES
$\displaystyle \small \bullet$ Many membrane bound minute vesicles are called microbodies.
$\displaystyle \small \bullet$ Microbodies contain various enzymes.
$\displaystyle \small \bullet$ They are present in both plant and animal cells.