Nucleus

 Nucleus Structure and Function

What is a Nucleus?

The word nucleus means “central part” or “middle portion”.

The nucleus is a membrane bounded organelle present in all eukaryotes. It is surrounded bt a double membrane of phospholipid bilayer.

Robert Brown in 1831, discovered the nucleus. The other for nucleus is Karyon and the study of nucleus is called karyology.

The nucleus is present in the center of the cell. In plant cells, it is slightly on the side due to the presence of a large vacuole but in animal cells, it is mostly found in the center of the cells. 

The nucleus is prominent in eukaryotes while in prokaryotes, the nucleus is absent rather their DNA is freely found in the cell, the region where nucleus is present is called nucleoid region.

The nucleus is so important organelle that it s also known as “the brain of the cell”.

Nucleus Structure

The nucleus is most of the time round-shaped structure. It is very large organelle as compared to other organelles of the cell. It has following main components:

Nuclear envelope or Nuclear Membrane

The nuclear membrane is the outer envelope of the nucleus. It is a double-layered structure that comprises of inner nuclear membrane and an outer nuclear membrane. The nuclear membrane is composed of two phospholipid bilayers that join together at the ends. The space between the phospholipid bilayers is called perinuclear space.

Nuclear pore

The region at the ends of phospholipid bilayers of the nuclear membrane contains spaces which are called nuclear pores. These pores play an important role in the transport of materials inside and outside of the nucleus.

Lamin Proteins or Lamina Layer

On the inner side of the nuclear membrane, there is a layer of thread-like filamentous proteins called lamin proteins. These proteins provide mechanical support to the nucleus so that the nucleus may bear any kind of stress or compressive forces and they help the nucleus to maintain its shape. These proteins also have functions in gene expression and DNA replication because they can hold the chromatin material inside. 

Nucleoplasm

The nucleoplasm is a jelly-like fluid found in the nucleus. Different structures are present in the nucleoplasm which enter the nucleus from the cytoplasm through nuclear pores e.g. ATP, hormones, ions or some parts of proteins, etc. or they have to go from the nucleus to the cytoplasm e.g. Mrna, Trna, etc.

Nucleolus

The nucleolus is a gloomy and precipitated region in the nucleus because of the presence of ribosomal RNA and proteins. Ribosomes are assembled and proteins are synthesized in the nucleolus.

Chromatin material

Apart from the nucleolus, the nucleus contains chromatin material i.e. DNA lying in twisted form and histone proteins. DNA is wrapped on histone proteins. This structure is called a nucleosome. 

The loosely condensed chromatin material is called euchromatin which is important for gene expression while the highly condensed chromatin material is called heterochromatin. The DNA is tight and compact in heterochromatin material due to which no gene expression occurs here.

Functions of the Nucleus

• It controls all the cellular activities of the cell. 
• It brings about the growth of cells.
• It contains instructions for protein synthesis. 
• It contains DNA which is the hereditary material and very important for sustaining life.
• It controls hereditary characteristics of an organism.
• It is a site for transcription in which messenger RNA is synthesized for protein synthesis. 
• It is a storage house for proteins produced from ribosomal RNA.