Nucleus

Aug 09, 2022, 16:45 IST

What is Nucleus

Nucleus as a cell organelle was first described by Robert Brown as early as 1831. Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming..The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleoli (sing.: nucleolus).

Important parts of Nucleus

Nuclear membrane: It is double layered membrane separating the nuclear contents from the cytoplasm. It is perforated by nuclear pores which allow transport of molecules selectively, between cytoplasm and nucleoplasm.

Nucleoplasm: It is a granular, dense fluid present inside the nucleus. Chromatin and nucleolus are present inside the nucleoplasm.

Chromatin material: It is made up of deoxyribonucleic acid (DNA) and basic protein histones. Chromatin is in the form of highly coiled network of thread like structure.

Chromatin takes the shape of rod like structures, called as chromosome, at the time of cell division.

The DNA present in the chromatin is responsible for storing and transmitting hereditary characteristics from one generation to the next

Segments of DNA are called genes.

Nucleolus: It is more or less round structure not bounded by any membrane, consisting of ribosomal RNA (rRNA).

rRNA is transported to the cytoplasm, where it takes part in the synthesis of ribosomes after combining with proteins.

Functions of Nucleus

The nucleus controls all the metabolic activities of the cell.

It transmits hereditary traits from parents to the offsprings.

Cytoplasm

It is the fluid part of the cell which is present between the plasma membrane and nuclear membrane.

It contains various cell organelles, performing different functions of the cell.

Functions of Cytoplasm

It holds cell’s organelles in place.

It also gives the shape to cell structure.

Most of the chemical reaction (which keeps the cell alive) takes place in cytoplasm.

Important Facts about Nucleus

Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.

The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.

At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.

The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.

The interphase nucleus has a loose and indistinct network of nucleoprotein fibres called chromatin. But during different stages of cell division, cells show structured chromosomes in place of the nucleus.

Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA. A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes.

Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present.

Based on the position of the centromere, the chromosomes can be classified into four types. The metacentric chromosome has middle centromere forming two equal arms of the chromosome. The sub-metacentric chromosome has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm. In case of acrocentric chromosome, the centromere is situated close to its end forming one extremely short and one very long arm, whereas the telocentric chromosome has a terminal centromere.