NUCLEUS

Eukaryotic cell have double membrane bounded except for mature mammalian erytrhocytes and phloem sieve tubes element. Prokaryotic cell do not have
membrane bounded nucleus. There is usually one round or oval nucleus in each cell but the shape, position, and genetic materials can vary from cell to cell. The diameter of nucleus is about 4 – 10 µm and it surrounded by a double membrane. The outer membrane is continuous with the endoplasmic reticulum and covered with ribosom. The space between the outer membrane and the inner membrane is known as  perinucleus. The nuclear envelope is perforated by numeorus pores. These are aqueous protein channel which regulate exchange of substance between the nucleoplasm and cytoplasm. The nucleoplasm contain chromatin, nucleolus, nucleotides, enzym and mineral ions. Chromatin consists of long DNA strand coiled around histones protein. These condense to form chromosomes during cell division. During interphase, some of the chromatin stran are tightly coiled and are called heterochromatin. The remaining loosely coiled chromatin are called euchromatin. Each cell have one or more rounded darkly stained nucleolus. Nucleolus contain DNA, RNA, and proteins. The DNA contain the genetic information to synthesise ribosomal RNA (rRNA) needed in the synthesise of ribosomes.

Functions of a nucleus :

a.       Chromosomes in nucleus contain the genetic materials of he cell.

b.      The nucleus acts as the centre to controll cell activities and cell division.

c.       Production of RNAs and  rRNA needed for enzym and protein synthesis.