Nucleus

The nucleus is a dense protoplasmic double-membrane structure that is the largest cell organelle controlling cellular metabolism and containing DNA that is the genetic information transmitted to the next generation.

The word “Nucleus” is derived from the Latin word ”kernel”. In other words, the nucleus is considered the controlling centre of eukaryotic cells. In Eukaryotic cells, the nucleus can be of two types:

• Uninucleate: that has one nucleus.
• Multinucleate: that has many nuclei.
• Interphase nuclei are nuclei of undivided cells.

Mammalian Red blood cells, however, lack a nucleus as it is lost secondarily.

In 1831, while working on orchid root cells, Robert Brown discovered the nucleus as an areola. Robert Brown himself also coined the term nucleus.

What is nuclei and nucleus?

The plural form of Nucleus is Nuclei.

Size of nucleus:

The nucleus occupies 15% to 20% of the cell mass. There are two main factors upon which the size of the nucleus is dependent; they are

1. Cell maturity
2. Cell function

The size of the nucleus usually varies from 5 to 25 micrometres. The size occupied by the nucleus in various cells is as follows:

• Thymus cells have 60% of the cell mass occupied by the nucleus.
• Mature parenchyma cells have 10% of the nucleus in their cell mass.
• Dormant cells have small-sized nuclei.

Shape of nucleus:

The shapes of the cells vary in all cells, and hence a wide variety of shapes are seen.

In young plant cells, the nucleus is round, whereas it is cuboidal and sometimes polygonal in animal cells.

The table below list the structure of nucleus in various mammalian cells:

Position of nucleus:

Mostly, the nucleus is found lined up near the geometric centre of the cell since it is responsible for controlling the cellular activities. Due to a central vacuole in matured plants, the nucleus is found in the peripheral cytoplasm. Due to fat in the centre, adipocytes have a nucleus located in the periphery known as the peripheral nucleus.

Chemical composition of the nucleus:

The nucleus consists of various other components like

• Histones that are the basic proteins
• DNA, the genetic materials
• RNA
• Enzymes
• Acid and Neutral proteins
• Lipids and minerals.

Ultrastructure parts of the Nucleus:

The ultrastructure of the nucleus can be divided into five parts, namely,

• The nuclear envelope is the double membrane covering the nucleus. The critical function of the nuclear envelope is to separate the contents of the nucleus from the cytoplasm; the nuclear envelope contains small perforations referred to as nuclear pore, which is a large complex of proteins, and its purpose is transportation of molecules.
• Nucleoplasm is the storehouse of the materials required for the formation of RNA, DNA and hosts enzymes required to synthesise both DNA and RNA. It is a semifluid transparent colloidal complex that forms the ground substance of the nucleus. They are involved in the formation of Gemini bodies, Cajal bodies, etc. They are mainly responsible for providing turgidity to the nucleus. 
• Nuclear matrix provides chromatin anchorage and maintains the shape of the nucleus
• Chromatin contains genetic information since they are made up of DNA histone complexes. It can be differentiated into two as:

1. Heterochromatin -darkly stained 
2. Euchromatin- lightly stained

• Nucleolus is attached to the chromatin at a site known as the nucleolar organiser region.

Functions of nucleus:

1. The primary function of the nucleus is that it contains genetic information in its chromatin.

2. The nucleus is responsible for passing genetic information from the parent to the offspring.
3. The nucleus also functions in cell division, growth, and differentiation.
4. Nucleus functions by controlling the cellular metabolism of the cell.
5. The nucleus controls the formation of Ribosomes. 

Conclusion: 

The nucleus is a dense protoplasmic double-membrane structure that is the largest cell organelle controlling cellular metabolism and containing DNA that is the genetic information transmitted to the next generation. It can be concluded that the two principal functions of the nucleus (plural-nuclei) are

• Containing genetic information (DNA, RNA) that are replicated and passed on from the parent to the offspring
• It is also required for reproduction, development, cellular metabolism
•  Direct synthesis of protein and enzymes required for DNA and RNA synthesis

From the above, it can be concluded that the nucleus is the largest and most conspicuous cell organelle.