The Nucleus of the Cell
The word nucleus is derived from the Latin word, which means "kernel of a nut". A nucleus is a large double membrane-bound organelle that contains the cell's chromosomes, it is responsible for regulating almost all forms of cellular activities. Every cell is categorized based on the presence or absence of the nucleus. If the nucleus is present in a cell it is called a Eukaryotic cell. If the nucleus is absent in a cell it is called a prokaryotic cell.
Structure of Nucleus
The nucleus is the most important and functional part of the cell.
The nucleus is enclosed by the nuclear membranes.
It is covered by a structure called the nuclear envelope.
The membrane distinguish the cytoplasm from the contents of the nucleus
The cell’s chromosomes are also cramped within the nucleus.
DNA is present within the chromosomes, it provides the genetic information for the creation of different cell components including reproductive cells.
Compartmentalization of the Cell
The nuclear envelope gives a definite structure to the eukaryotic nucleus and separates the contents from the rest of the cytoplasm that encloses the nucleus. The nuclear envelope prevents the translation of unspliced mRNA by separating the nuclear process from the cytoplasmic process.
Gene Expression
DNA transcription occurs in the nucleus. DNA transcription involves the unwinding of DNA double helix, synthesis of RNA, the coiling of DNA, and the actual transcription process.
Nuclear Transport
Transporting various molecules in and out of the nucleus through the nuclear pores is an important function of the nucleus. Important proteins are carried into the nucleus with the help of importins. Molecules like the RNA are transported out of the nucleus with the help of exports.
Assembly and Disassembly
During cell division, the nucleus is integrated and disintegrated to divide the contents and the genetic material. The cell nucleus is also associated with programmed cell death or apoptosis. The nuclear lamina and the nuclear envelope dissolve and then form again during the cell cycle. Duplication of the DNA and RNA replication takes place in the nucleus. Thus, the nucleus integrates and disintegrates various structures during different phases of the cell cycle.
Pre-mRNA Processing
Before getting transported to the cytoplasm, the primary transcript, or the newly synthesized mRNA go through post-transcriptional modification. This happens inside the nucleus and involves a series of biological processes.
Ribosome Biogenesis
A part of ribosome biogenesis takes place in the nucleus. Over 200 different proteins work in coordination to synthesize the four eukaryotic rRNAs, which then together with the ribosomal proteins synthesize the pre-40S subunit and the pre- 60S subunit. These subunits are then transported to the cytoplasm for maturation.
Functions of Nucleus
Following are the important nucleus function:
It contains the genetic information of the cell and controls the growth and reproduction of the cell.
It not only stores DNA but also acts as a home of some important cellular processes.
One's DNA in the nucleus can be duplicated. This process is called DNA Replication and It can produce DNA's identical copy.
The first step of cell division is the Production of two identical copies of the body, where every new cell will get its own instructions.
Secondly, the nucleus is the site of transcription that creates various types of RNA from DNA. Transcription is similar to creating copies of the human body’s instructions which are moved out and read by the other cells.
The most important rule of biology states that DNA is copied to RNA, and then proteins.
FAQs on Eukaryotic Nucleus
1. What is a eukaryotic nucleus and what is its primary function?
The eukaryotic nucleus is a large, membrane-bound organelle found in eukaryotic cells. Its primary function is to act as the control centre of the cell. It achieves this by housing the cell's genetic material (DNA) and coordinating essential cellular activities like growth, metabolism, protein synthesis, and reproduction (cell division).
2. What are the key components that make up the structure of a eukaryotic nucleus?
The structure of a eukaryotic nucleus, as per the CBSE syllabus for the 2025-26 session, consists of four main components:
Nuclear Envelope: A double membrane that separates the nucleus from the cytoplasm.
Nuclear Pores: Channels within the nuclear envelope that regulate the passage of molecules between the nucleus and cytoplasm.
Nucleoplasm: The jelly-like substance within the nucleus that contains the chromatin and nucleolus.
Chromatin Material: A complex of DNA and proteins (mainly histones) that condenses to form chromosomes during cell division.
Nucleolus: A dense, non-membranous structure involved in ribosome synthesis.
3. What is the difference between chromatin and chromosomes?
Chromatin and chromosomes are essentially the same material (DNA and histone proteins) but exist in different states of condensation. Chromatin is the uncondensed, thread-like, and tangled mass of genetic material found within the nucleus during the cell's non-dividing phase (interphase). In contrast, a chromosome is the highly condensed, organised, and compact structure that chromatin forms into, becoming visible only during cell division (mitosis or meiosis).
4. How does the nucleus of a eukaryotic cell differ from the nucleoid in a prokaryotic cell?
The primary difference lies in their structure and organisation. A eukaryotic nucleus is a true, well-defined organelle enclosed by a double membrane (the nuclear envelope). A prokaryotic nucleoid, on the other hand, is not a true nucleus; it is simply a region within the cytoplasm where the genetic material (a single, circular DNA molecule) is concentrated, without any surrounding membrane.
5. What is the importance of the nucleolus found inside the nucleus?
The nucleolus is crucial for protein synthesis. It is the primary site for the synthesis of ribosomal RNA (rRNA) and the assembly of rRNA and proteins to form ribosomal subunits. These subunits are then exported to the cytoplasm to form functional ribosomes, which are the cell's protein factories.
6. Why is the nucleus often called the 'brain' or 'control centre' of the cell?
The nucleus earns this title because it contains the cell's entire set of genetic instructions in the form of DNA. By controlling which genes are expressed (a process called transcription), the nucleus dictates which proteins the cell produces. These proteins, in turn, regulate all metabolic activities, determine the cell's structure, and manage its growth and reproduction, effectively controlling all cellular functions.
7. How is the extremely long DNA molecule packed into the tiny space of a nucleus?
The immense length of DNA is efficiently packed through a multi-level coiling system. The negatively charged DNA strand wraps around a group of positively charged proteins called histones, forming a structure known as a nucleosome. These nucleosomes, often described as 'beads on a string', are then further coiled and condensed into a more compact fibre called chromatin, which supercoils to form the highly condensed chromosome visible during cell division.
8. Do all eukaryotic cells have a single nucleus? Provide examples.
No, not all eukaryotic cells have a single nucleus. While most do, there are important exceptions:
Anucleated cells: Some cells, like mature mammalian Red Blood Cells (RBCs) and sieve tube cells in plants, lose their nucleus upon maturation to maximise space for their primary function (e.g., oxygen transport).
Multinucleated cells: Some cells, such as skeletal muscle cells and the slime mould Physarum, contain multiple nuclei within a single cytoplasm. This condition is known as syncytium or coenocyte.
9. What is the significance of the nuclear pores in the nuclear envelope?
Nuclear pores are vital for the cell's survival as they act as selective gates. They regulate the two-way transport of molecules between the nucleus and the cytoplasm. They allow essential molecules like proteins (e.g., histones, DNA polymerase) to enter the nucleus, while permitting the exit of molecules like messenger RNA (mRNA) and ribosomal subunits, which are necessary for protein synthesis in the cytoplasm.
10. What would be the immediate consequences for a typical animal cell if its nucleus were experimentally removed?
If the nucleus were removed from a typical animal cell, it would lead to its eventual death. The immediate consequences would be the cessation of all genetically controlled activities. The cell would lose its ability to:
Synthesise new proteins, as the mRNA blueprints could no longer be produced through transcription.
Repair itself or respond to environmental changes.
Undergo cell division (mitosis), as the chromosomes required for replication and distribution would be absent.
