a short note on the nucleus
Answers
★ NUCLEUS:-
→ Nucleus was first described by Robert Brown in 1831.
→ Later the material of the nucleus stained by the bar the basic dyes was given the name chromatin by Flemming,
→ Normally, there is only one nucleus per cell. Variations the number of nuclei are also observed. Some cells lack nucleus. E.g. mammalian erythrocytes and sieve tube cells of vascular plants.
→ The interphase nucleus contains:-
o Nuclear envelope:- Double layered membrane with a space between (10 - 50 nm) called the perinuclear space. It forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the ER and also bears ribosomes on it.
→ The nuclear envelope has minute pores, which are formed by the fusion of its two membranes. These are the passages for the movement of RNA and protein between the nucleus and the cytoplasm.
o Nuclear matrix (nucleoplasm).
o Chromatin:- A network of nucleoprotein fibres. It contains DNA and basic proteins (histones), some non- histone proteins and RNA. During cell division, chromatins condense to form chromosomes.
o Nucleolus:- One or more non-membranous spherical bodies. It is continuous with the nucleoplasm. It is a site for ribosomal RNA synthesis.