The Unit of Life: Nucleus

In biological terms, a nucleus is a specialized structure occurring in most cells separated from the rest of the cell. This separation is done by a double layer film called the nuclear membrane. A nucleus is an important organelle that categorizes the type of cell as either a prokaryotic or eukaryotic cell based on the absence and presence of the nucleus. It is known as the director of the cell, which directs and controls cellular functions and contains genetic material. Robert Brown was the first to note the nucleus in 1831. The nucleus plays a determinative role in cells and organisms, and this was experimentally demonstrated by Hammerling (1934) upon conducting experiments with green marine uni celled algae Acetabularia. 

Structure of Nucleus 

Membranes completely border the nucleus. The envelope of the nucleus is made up of two lipid bilayer membranes and encases the genetic material. The membrane separates the cytoplasm from the contents of the nucleus. The chromosomes of the cell are also present in it. DNA is present in the Chromosomes, responsible for providing the genetic makeup. This genetic material enables the creation of different cell components and the reproduction of life.

Occurrence

The nucleus occurs with eukaryotes and contains a well defined nuclear membrane and a linear chromosome. The exception occurs in mature mammalian RBCs, tracheids, sieve tube cells of phloem and vessels of xylem. The prokaryotes have an incipient nucleus, called the nucleoid. It is also known as prokaryote or genopore or false nucleus.

Types based on the number of nuclei in a cell

Usually, a cell contains a single nucleus, also called a mononuclear condition. For example, the Acetabularia, a genus of green algae.

1. Anucleate: A cell without a nucleus such as RBCs of mammals, phloem sieve tube, tracheid, and xylem vessels.
2. Binucleate: A cell having two nuclei. For example, Ciliate, Protozoans like Paramecium.
3. Polynucleated: A cell having many nuclei. For instance, the fungal hyphae of Rhizopus, Vaucheria – a genus of yellow-green algae. This may occur due to the fusion of several cells. i.e. coconut endosperm, syncytium, or by free nuclear divisions without cytokinesis, i.e., coenocytes.

Size, Shape and appearance

The size is dependent on the cell’s metabolic activity and is directly proportional to the number of chromosomes. The diameter of the cell nucleus is normally around 5-10 μm in many multicellular organisms. The smallest nuclei are approximately one μm in diameter and are found in yeast cells. Generally, the Shape of the Nucleus is spherical or elliptical. The nucleus is found in different shapes as well, such as cuboidal (columnar cells of the intestine), bean-shaped (paramecium), horseshoe-shaped (vorticella) and long and beaded form (stentor), ovoid (lateral geniculate nucleus).

Chemical composition

The nucleus mainly contains nucleoproteins composed of nucleic acid and proteins, besides enzymes and inorganic salts. The following is the composition:

• Proteins = 80% (65% acidic, neutral and enzymatic proteins; 15% basic proteins-histones)
• DNA = 12%
• RNA = 5%
• Lipids = 3%

Ultrastructure of the Nucleus

The nucleus is composed of the following structure:

1. The nuclear membrane. 
2. The nucleolus.
3. The nuclear sap or nucleoplasm.
4. The chromatin fibres.

Nuclear membrane: Also called the nuclear envelope and as its name suggests surrounds the nucleus, which distinguishes it from the cell’s cytoplasm. It is a double membrane in which each membrane is a phospholipid bilayer containing proteins. These membranes are divided by 20 to 40 nm of space. The membranes are often called or referred to as the inner and outer nuclear membranes. The outer membrane is known as ecto karyotheca (with ribosome), and the inner membrane is known as endo karyotheca (without ribosome). Two membranes are separated by fluid-filled intramembranous perinuclear space. The space between the inner and outer nuclear membranes then links to the lumen of the endoplasmic reticulum as the outer membrane is continuous with the cell’s endoplasmic reticulum. Similar to the endoplasmic reticulum, the outer nuclear membrane has ribosomes attached to its body. The inner membrane of the envelope is attached to Nucleus specific proteins that aren’t found anywhere else. 

Functions: The basic function of the nuclear membrane is to maintain the Shape of the Nucleus. It regulates the nucleo-cytoplasmic interactions and allows the passage of small organic molecules, inorganic ions, RNAs, ribosomal subunits and proteins through nuclear pores.

Nucleolus: Nucleolus is the most significant structure within the nucleus. While there is only a single nucleolus present in most cases, sometimes some nuclei have multiple nucleoli. It is a mass of granules and fibres attached to chromatin. The nucleolus is mainly formed of RNA and non-histone acidic proteins. It is a storehouse of rRNA.

Function: The nucleolus is important because it is the site or place of ribosomal RNA (rRNA) production. Inside the nucleolus, rRNA molecules are amalgamated with proteins to produce ribosomes. The nucleolus is involved in pre-rRNA processing, RNA transcription, and assembly of ribosome subunits. The nucleolus isn’t surrounded by a membrane but has a dense portion that separates it from the surrounding nucleoplasm. The nucleolus has other roles, as it contains a number of proteins unrelated to rRNA and ribosome synthesis and is involved in ribosomal biogenesis. It is thought to play a role in cell cycle regulation, DNA damage repair and RNA editing.

Nucleoplasm: Also called the karyolymph, it is transparent, semifluid, homogeneous, colloidal and a ground substance present inside the nuclear membrane. It contains Nucleic acid (DNA and RNA), Enzyme (DNA and RNA polymerase, NAD synthetase etc.) Proteins (Basic proteins and acidic protein), Minerals (K, Na, Ca, Mg etc.) and Ribonucleoproteins.

Function: Nuclear bodies can be found in the nucleoplasm, and these include structures such as Cajal bodies, Gemini bodies, and Polycomb bodies. The nucleoplasm helps maintain the shape of the nucleus formation of the spindle protein of NAD, ATP, DNA, RNAs and ribosomal subunits. Plasmosomes and karyosomes are combinedly known as the “amphi nucleoli”.

Chromatin fibres /Nuclear chromatin: The nucleoplasm contains many coiled, thread-like and super-elongated structures that readily take basic stains such as “basic fuchsin”. These thread-like structures are called nuclear chromatin or chromatin fibre. They are evenly distributed in the nucleoplasm. Therefore, they are observed or are found only in the “interphase stage”.

Main functions of the nucleus

The nucleus is a very important part of the cell structure. It contains genetic material, coordinates and synchronizes cell activities like cell division and protein synthesis. Anatomically, the nucleus is made up of multiple components: nuclear envelope, chromatin fibres, chromosomes, nucleolus, nucleoplasm are some of the components present 

All the components work together for the nucleus to accomplish its functions. The functions are

• Control and coordination of the genetic information of the cell and thus the hereditary characteristics of an organism
• Control of the enzyme and protein synthesis
• Control of cell growth and cell division
• Storage of RNA, DNA and the ribosome
• Regulation of the transcription of the mRNA to protein
• Production of ribosomes

Conclusion

The nucleus contains the cell’s hereditary information and controls the cell’s growth and reproduction.

The nucleus is a membrane-bound structure that comprises the genetic material of a cell. It is the control centre as it regulates metabolic activities and cell division. The nucleus is engulfed by a nuclear membrane, consisting of outer and inner membranes that run parallel to one another and are separated by the perinuclear space. The outer membrane of the nuclear envelope is connected with the endoplasmic reticulum.