What is Nucleus? | Class 11 Biology

The cell is the basic structural and functional unit of living organisms. In biology, the cell has the same central position as that of an atom in the physical sciences. The bodies of living organisms are made up of microscopic units called cells. Cells have evolved a variety of different lifestyles. Many organisms, such as bacteria (both archaebacteria and eubacteria), protozoa(e.g., amoeba), and yeasts consist of single cells (called unicellular organisms) that have the ability to perform a function like self-replication. More complex organisms, called multicellular organisms, those consists of collections of cells that perform particular functions.

Term cell was discovered by Robert Hook in his book Micrographia. The size of the cell is 10-16 micrometers. The shape of the cell may be polygonal, disc-like amoebic, thread-like, cuboid or irregular, oval, hexagonal, circular, branched, elongated, etc. The cells are too small to be seen with naked eyes. So, it is studied with the help of microscopes.

Nucleus

Nucleus as a cell organelle was first discovered 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 nucleus is a double membrane-bound protoplasmic body containing all the genetic information. That the nucleus is a storehouse of hereditary information was proved by Danish biologist Joachim Hammerling on the basis of his studies in Acetabularia. The interphase nucleus is a nucleus in the non-dividing or metabolic phase.

It is the largest of the cell organelles. It is present in all living eukaryotic cells except mature sieve cells of vascular plants and red blood corpuscles of mammals, which have a nucleus only during the early stages of their development.

• Number-Commonly, cells are uninucleate, that is, they possess a single nucleus. Binucleate cells, e.g., Paramecium, has two nuclei. Multinucleate or poly nucleate cells have many nuclei, e.g., some cells of bone marrow, striated muscles, latex vessels, several fungi, Ascaris, and algae. Multinucleate animal or Protista cells are called syncytial cells, while multinucleate plant and fungal cells are known as coenocytic cells.
• Position-It is found in the region of maximum metabolic activity in the cytoplasm. Commonly, it is situated in the geometric center of the cell. The nucleus is peripheral in fat-storing cells, in plant cells due to the development of a large central vacuole, and basal in glandular cells. In Spirogyra, it is suspended in the central vacuole by cytoplasmic strands.
• Shape-The nuclei are generally round in outline, but they appear oval or elliptical in plant cells having large central vacuoles, disc-shaped nuclei in the cells of squamous epithelium, lobed in white blood corpuscles, and irregularly branched in silk spinning cells of insects.
• Biochemical Analysis. DNA—9-12%. RNA—5%. Lipids—3%. Basic proteins—15%. Acid proteins, neutral proteins, and enzymes—65%. Traces of minerals like Calcium, Magnesium, Potassium, and Sodium. Phosphorus is a constituent of DNA, RNA and acid proteins.

Ultrastructure

A typical interphase nucleus is 5-25 mm in diameter and is differentiated into the following parts-nuclear envelope, nucleoplasm, nuclear matrix, chromatin, and nucleolus.

1. Nuclear Envelope or Karyotheca-It separates the nucleus from the cytoplasm. The nuclear envelope is made up of two lipoprotein and trilaminar membranes. The inner membrane is smooth. The outer membrane may be smooth, or its cytoplasmic membrane may bear ribosomes like the rough endoplasmic reticulum. Perinuclear space (100-700 Angstrom wide) is an electron transparent space that separates the two membranes of the nuclear envelope. The outer membrane is often connected to the endoplasmic reticulum.
2. Nucleoplasm-It is a transparent, semifluid, and colloidal substance filling the nucleus which contains nucleoside and a number of enzymes (e.g., DNA polymerase, RNA polymerase, nucleoside phosphorylase) required for the synthesis and functioning of DNA, RNA, nucleoproteins, etc. Some proteins present in nucleoplasm are essential for spindle formation.
3. Nuclear Matrix-It is a network of the fine fibrils of acid proteins which function as a scaffold for chromatin. It forms a dense fibrous layer called nuclear lamina below the nuclear envelope, on the periphery. In this lamina, terminal ends of chromatin fibers are embedded.
4. Chromatin-Chromatin is a hereditary DNA-protein fibrillar complex that contains DNA, RNA, and proteins. It has the ability to get stained with basic dyes. It occurs in the form of fine overlapping and coiled fibers which produce a network called chromatin reticulum. 
5. Nucleolus-A nucleolus is a naked, round, or slightly irregular structure that is attached to the chromatin at a specific region called the nucleolar organizer region (NOR). About 1-4 nucleoli are found commonly in a nucleus. A covering membrane is absent around the nucleolus.
6. Nuclear lamina-The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of the lamina, which is also present in the nuclear interior, and lamin-associated proteins. Its function is to maintain the stability of the nucleus, organize chromatin and anchor nuclear pore complexes.
7. Nuclear bodies-Nuclear bodies (also known as nuclear domains, or nuclear dots) are membrane-less structures found in the cell nuclei of eukaryotic cells. Nuclear bodies include Cajal bodies, the nucleolus, and promyelocytic leukemia protein (PML) nuclear bodies (also called PML oncogenic dots).
8. Nuclear pores-The nuclear pore is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm. In eukaryotic cells, the nucleus is separated from the cytoplasm and surrounded by a nuclear envelope. This envelope safeguards the DNA contained in the nucleus.

Function of Nucleus

• The nucleus is known as the controller of a cell and its metabolic activities. If the nucleus is removed from a cell, the protoplasm will dry up and dies ultimately.
• The nucleus is responsible for the cell cycle, it regulates it.
• The nucleus plays an important role in transmitting hereditary traits from parent to offspring.
• The nucleus controls the protein and enzyme synthesis.
• The nucleus is the storehouse of DNA, RNA, and ribosomes.
• Replication of the nucleus is essential for cell replication.
• The nucleus directs cell differentiation by allowing certain particular sets of genes to operate.
• Ribosomes are formed in the nucleolus part of the nucleus.

Chromosomes

The interphase nucleus has a loose and distinct network of nucleoprotein fibers 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 an approximately 2-meter-long thread of DNA distributed among its forty-six (twenty-three pairs) chromosomes.

Differences between nucleus and nucleolus.

1. The nucleus represents the whole eukaryotic complex that contains genetic information, while the nucleolus is a component of the nucleus.
2. The nucleus is covered by a two-membrane envelope, whereas the nucleolus does not have a covering membrane.
3. The nucleus controls the structure and working of cells, and the nucleolus synthesizes ribosomal subunits.

FAQs on Nucleus

Question 1: Who discovered the nucleus?

Answer:

Nucleus as a cell organelle was first discovered by Robert Brown as early as 1831. 

Question 2: What is the function of nucleus?

Answer:

The nucleus is known as the controller of a cell and its metabolic activities. If the nucleus is removed from a cell, the protoplasm will dry up and dies ultimately and the nucleus is responsible for the cell cycle, it regulates it.

Question 3: What is the position of the nucleus inside a cell?

Answer:

It is found in the region of maximum metabolic activity in the cytoplasm. Commonly, it is situated in the geometric center of the cell. The nucleus is peripheral in fat-storing cells, in plant cells due to the development of a large central vacuole, and basal in glandular cells. In Spirogyra, it is suspended in the central vacuole by cytoplasmic strands.

Question 4: Define the structure of the nucleus.

Answer:

A typical interphase nucleus is 5-25 mm in diameter and is differentiated into five parts-nuclear envelop, nucleoplasm, nuclear matrix, chromatin, and nucleolus.

Question 5: What do you mean by nucleolus?

Answer:

A nucleolus is a naked, round, or slightly irregular structure that is attached to the chromatin at a specific region called the nuclear organizer region (NOR). About 1-4 nucleoli are found commonly in a nucleus. A covering membrane is absent around the nucleolus.