Cell: Structure and Function - Nucleus and Chromosomes

The Nucleus was discovered by Robert Brown in $1831$ and serves as the control center of the cell, containing the genetic information in the form of $DNA$.

The nuclear envelope is a double-membraned structure with a perinuclear space measuring $10$ to $50\ nm$. It contains nuclear pores that facilitate the nucleo-cytoplasmic traffic of $RNA$ and proteins.

The nucleoplasm contains the nucleolus (plural: nucleoli) and chromatin. The nucleolus is the site for active $rRNA$ (ribosomal $RNA$) synthesis and is not membrane-bound.

Chromatin is a network of nucleoprotein fibers containing $DNA$, basic proteins called histones, non-histone proteins, and $RNA$.

During cell division, chromatin condenses to form chromosomes. Each chromosome has a primary constriction called the centromere, where disc-shaped structures called kinetochores are located.

Chromosomes are classified by centromere position: Metacentric (middle), Sub-metacentric (near middle, forming one short and one long arm), Acrocentric (near the end), and Telocentric (terminal).

The nucleosome is the repeating unit of chromatin, consisting of approximately $200\ bp$ (base pairs) of $DNA$ wrapped around a histone octamer. The octamer consists of two molecules each of $H2A$, $H2B$, $H3$, and $H4$.