Is Nucleus a double membrane organelle?

Robert Brown discovered Nucleus in 1831, a double membrane organelle. During his experiments, he observed a structure in plant cells and termed it as “nucleus”.

Walther Flemming in 1879 discovered chromatin in the nucleus. He observed basic dyes stain some material of the nucleus which eventually turned out to be chromatin. Flemming was also a pioneer in discovering cytogenetics.

Image Source: www.sciencephoto.com

Image Source: www.uni-kiel.de

Structure of the Nucleus

The most elaborate and extended structure of the nucleus (a double membrane cell organelle) is observed in the interphase. Interphase is a stage of cell division when the cell and nucleus are not dividing but are preparing for division.

The Nucleus includes:-

• Nucleoplasm/Nuclear Matrix
• Nucleolus
• Double membraned nuclear membrane (nuclear envelope)
• Nucleoprotein fibres referred to as chromatin 

The double membrane of the nucleus has an outer membrane and an inner membrane. The outer membrane is continuous with the rough endoplasmic reticulum (a cell organelle part of the endomembrane system) hence bears 70s ribosomes on it. The area between the two membranes is particularly called perinuclear space and is about 10 nm-50 nm.

Image Source: www.genome.gov

The nuclear membrane is porous and has specialized minute pores that have the role to facilitate the movement of materials across the nucleoplasm and cytoplasm. These pores are located at places where outer and inner membranes fuse. RNA molecules, as well as proteins, pass from these nuclear pores in both directions. 

The nucleoplasm consists of a spherical nucleolus. Nucleolus does not have any membrane and is the site of active rRNA (ribosomal RNA) synthesis. rRNA is essential for ribosomes production hence essential for the cell. Basically, cells that perform protein synthesis require ribosomes hence rRNA, therefore they have larger and more numerous nucleoli.

Functions of the Nucleus (a double membrane organelle)

The nucleus is a double membrane cell organelle that performs a lot of multitasking:

•  It is the site where the majority of DNA is localised and contains the hereditary information of the cell. 
• It is the site where nucleolus is present and the majority of rRNA production takes place.
• It is responsible for regulating and controlling cellular activities.
• The nucleus particularly carries the genes. 

Chromatin and Chromosomes

Chromatin

 The nucleus (a double membraned cell organelle) during the interphase stage of the cell cycle when it is not dividing, has a network of chromatin which can be referred to as nucleoprotein fibres. They are loose and indistinct fibres. Chromatin essentially consists of DNA, non-histone proteins, histone proteins and RNA. Histone proteins are basic in nature and help in the packaging of DNA. Basically, the human DNA in a single cell is packaged with the help of histones into a 2-metre long thread of DNA.

Chromosomes

However, when the nucleus goes into the M-Phase of the cell cycle and starts dividing, the chromatin can be observed as distinctive structures called chromosomes. Another key point is that there are 46 chromosomes (twenty-three pairs) in a human somatic cell. 

Chromosomes have:-

• Two Chromatids that contain DNA (visible best during the Metaphase). 
• A primary constriction called the centromere (it holds the two chromatids of a chromosome).
• Disc-shaped kinetochores associated with the centromere (on the sides).

Image source: www.genome.gov

Chromosome Classification

Chromosomes in the nucleus (a double membrane cell organelle) can be classified into four types based on the position of the primary constriction (centromere).

1. Telocentric  – Centromere is present at the terminal position.
2. Acrocentric  – Centromere is present close to the end, this leads to the formation of one extremely short and one extremely long arm.
3. Sub-Metacentric – Centromere is present slightly away from the middle of the chromosome, this leads to the formation of one short arm and one long arm.
4. Metacentric  – Centromere is present in the middle of the chromosome, consequently, this leads to the formation of two equal arms.

During the Anaphase of cell division, Metacentric chromosomes appear to be V-shaped, Sub-metacentric chromosomes appear to be L shaped, Acrocentric chromosomes appear to be J shaped and Telocentric chromosomes appear to be I shaped. The shorter arm of the chromosome is known as the p arm while the longer arm is known as the q arm.

Image Source: NCERT

Satellite

The centromere is the primary constriction of the chromosomes. Although in some instances, there is a secondary constriction. This constriction is non-staining and hence is not easily observed. This constriction is always present at a constant location. This leads to the formation of a small fragment called a satellite.

Multi-nucleate / Uninucleate/ non-nucleate

In the majority of cells, there is a single nucleus (a double membraned organelle) present. These cells are called uninucleate cells. Some cells have more than one nucleus. They are called multinucleate cells. The mechanism by which multinucleate cells are formed are as following:-

Syncytium 

In this case, a fusion of cells leads to a multinucleate condition.

E.g- Slime mould-Physarum polycephalum (individual amoebae fuse)

Coenocytic

A condition when nuclear division is not followed by cytokinesis.

       E.g- Plasmodium

If cells do not have a nucleus they are termed non-nucleate. Red Blood cells also called erythrocytes and mature sieve tube cells of vascular plants do not have a nucleus.

Keep reading at MBD to find out more about the nucleus.

Thank you

Team MBD

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