The nucleus is the control centre as it regulates metabolic activities and cell division. The nucleus is surrounded by a nuclear membrane, which consists of outer and inner membranes that run parallel to each other and are separated by the perinuclear space. The outer membrane of the nuclear envelope is connected with the endoplasmic reticulum.
At a number of places, the two membranes fuse and give rise to minute pores called nuclear pores. These pores allow two-way movement of the RNA and protein molecules between the nucleus and the cytoplasm. The inner membrane is connected with the nucleoplasm that contains a spherical structure called the nucleolus, and a network of nucleoprotein fibres called chromatin.
When a cell begins to divide, the chromatin condenses and coils to form chromosomes. The chromosome consist of a primary constriction or centromere that has disc-shaped structures called kinetochores attached to its side. The parts present on either side of the centromere are called the arms of the chromosomes.
Based on the position of the centromere, chromosomes can be classified as metacentric, sub-metacentric, acrocentric and telocentric.
At times, chromosomes have secondary constrictions at a fixed location, lending the appearance of a small fragment called a satellite. These areas of secondary constriction help in the formation of a nucleolus after cell division. Chromosomes in the nucleus carry the genetic code of an organism.