Nucleus - Brain of the Cell

Nucleus is also called the “brain of the cell”. Why so? Because it contains the genetic material and is responsible for a large number of functions. Nucleus is spheroid and is most prominent part occupying around 10% of the total cell volume.

Structure:
Inside the nucleus is present a structure called nucleolus which consists of rRNA and proteins but no DNA. It is the site of assembly of ribosomes which are important for the process of protein synthesis. Nucleolus disappears when the cell is dividing and reappears after the cell is formed. 

Then, there is chromatin consisting of long strands of DNA associated with proteins. When the cell is in resting stage, the chromatin is relaxed and when the cell is going to divide, chromatin condenses and forms what is known as  “chromosome” as can be seen in the figure.

Structure of Nucleus

Nucleus is surrounded by nuclear membrane/envelope which keeps the nucleolus and chromatin inside the nucleus. Nuclear membrane is double-layered. the outer layer is connected with another organelle as endoplasmic reticulum. The space between both the layers is  fluid-filled space called perinuclear space. 

Now, if the nucleus is membrane bound, then how do DNA, proteins or macromolecules pass through? For this, there are several opening in the nuclear membrane called the nuclear pores which are the sites for exchange of macromolecules.

Functions:
It stores the genetic information in the form of “chromatin”. The gene expression takes place in nucleus which includes transcription where DNA is translated to mRNA. This mRNA is then transported to cytoplasm as ribosomes (which are present outside the nucleus, described here) are required for translation.
So, basically, nucleus says,  ”Hy Dude! I contain your genetic information and will form proteins for you whenever necessary”