Nucleus: Definition, Types, Examples, Diagram, Questions

What is Nucleus?

At the centre of each eukaryotic cell is an extraordinary structure: the cell nucleus. A nucleus is a double-membraned eukaryotic cell organelle that stores genetic material. Being the storage facility of genetic material, the nucleus is bound by a membrane and contains DNA. This is important because it ensures genetic material in the cell is protected and organised, such that it will be passed on correctly to the subsequent generation.

Definition & Discovery (Robert Brown, 1831)

The term is derived from a Latin word meaning “kernel of a nut”. The nucleus was discovered during the formative work of early microscopists. In the 1830s, Robert Brown made the first description of a distinct structure in plant cells, to which he gave the name nucleus. Subsequent improvements in the microscope and molecular biology have now made it possible to understand the organisation of the nucleus and how crucial it is for cellular processes.

Role as Control Center of Cell

The nucleus is the largest compartment of the eukaryotic cell and is considered the control centre of most cellular activities. It is in control of gene expression, protein synthesis, and division regulation, leading to the cell's fate and activity. The nucleus essentially pulls the strings on a host of key processes central to the life and activities of a cell. The nucleus is involved in determining the destiny of the cell, from control of the genetic blueprint of the cell to regulation in a large portion of the expression of genes and control of cell division.

Structure of Nucleus

The nucleus, described in many ways as the cell's brain, exhibits complex and divisible architecture, consisting of several distinctive components, each contributing to the structure and process. Basic composition includes:

Nuclear Envelope

The nuclear envelope represents a double membrane structure constituting it, which serves to house the nucleus and to separate its contents from the cytoplasm. It consists of an outer nuclear membrane and an inner nuclear membrane, connected at nuclear pore complexes.

Nuclear Pores

It is distributed throughout the nuclear envelope are nuclear pore complexes, complex protein-containing structures that regulate large molecules' passage between the nucleus and cytoplasm. The special pores enable the selective transport of ions, proteins, and RNA molecules, providing the system with accurate control over events in nuclear-cytoplasmic communication.

Nucleoplasm

The nucleoplasm is a jelly-like material that fills the interior of the nucleus and is the approximate equivalent of the cell's cytoplasm. The nucleoplasm contains the necessary components, or tRNA, as well as significant amounts of ribosomal proteins.

Nucleolus

Embedded in the nucleoplasm is the nucleolus, a nonmembrane-bound structure composed of proteins and nucleic acids that are involved in ribosomal RNA synthesis and the formation of ribosomal subunits. It includes dense fibrillar components (DFC), granular components (GC), and fibrillar centres (FC) related to the process of ribosomal RNA (rRNA) transcription and processing.

Chromatin (Euchromatin & Heterochromatin)

Chromatin is a complex of DNA and proteins that make up the chromosomes of the cell. Generally, it comprises two types: euchromatin and heterochromatin.

Functions of Nucleus

It directs a very important process within cells, each responsible for the continued existence and performance of a cell.

Genetic Information Storage

It acts broadly as the main repository for the hereditary material of the cell in the form of Deoxyribonucleic Acid (DNA), which contains all the instructions needed for cellular activities, growth, and development of said cell.

The DNA within the nucleus is organized into chromosomes. These are made of DNA that is coiled around histone proteins. The structure is quite organised so that it allows for thorough packaging and access to genetic information and, therefore, gives the possibility for complex regulation at the level of gene expression.

Gene Expression Regulation (Transcription & RNA Processing)

The regulation of gene expression in the nucleus happens mainly through transcription and RNA processing. In transcription, a gene is copied into RNA, which is then modified to form a mature mRNA ready for protein synthesis.

Transcription factors and many other gene regulatory proteins act to mediate the proper DNA and RNA polymerases forming the bridge to synthesize the RNA. Hence, they are responsible for the regulation of rate, timing, and specificity of the gene transcription processes, which in turn control the cellular behaviour.

Cell Division (Mitosis & Meiosis)

During mitosis, it ensures that the duplication of chromosomes in the nucleus is in accurate form and that the daughter cells receive them in equal proportions, even with two copies of the chromosomes. About meiosis, it is perceived as the parameter definition that the cells adopt four gamete cells whose nuclei are haploid copies, and each has a unique combination of the DNA definition.

The nucleus is involved in the process of controlling duplication of DNA at every cell cycle in both daughter cells. Consequently, it is very strictly controlled to prevent any type of error that might affect the processes of the genome.

Ribosome Synthesis (Role of Nucleolus)

The nucleolus is a subnuclear structure involved in the synthesis and assembly of ribosomes—the cellular machinery for making proteins. It possesses the genetic material that transcribes rRNA until it reaches the mature state and is ready to be further processed and assembled.

Here, rRNA genes are transcribed and the resultant products are processed in the production of ribosomal RNA molecules. The products then bind to proteins to form the ribosomal subunits, which constitute the protein factories and are located in the cytoplasm for further assembly.

Dynamic Nature of Nucleus

The nucleus is highly dynamic, constantly reorganizing its structure, while still maintaining control over genetic material and cellular activities.

Nuclear Transport

The nucleus actively controls molecular trafficking between the nucleus and cytoplasm through nuclear pores. In the double membrane, there are pores that act as gateways for the selective transfer of proteins, the RNA and other macromolecules through the import of proteins like DNA polymerases, associated with replication, transcription, and repair and the export of RNA molecules such as mRNA, tRNA, and the subunits of ribosomal complexes. Nuclear pore structures control this pathway for any molecule to pass with proper tagging, thereby maintaining the cellular health of the genome.

Nuclear Reorganisation

The nucleus also undergoes reorganisation throughout the cell cycle and development. The nuclear envelope disassembles and then reassembles during the cell cycle, so that the chromosomes can be properly segregated. This dynamic remodelling results in DNA replication and partitioning into the daughter cells.

Moreover, the nuclear architecture remodels during development and cell differentiation, which is indicative of a change in gene expression. This reorganisation manifests as morphological transformations for the transitions between the euchromatin and heterochromatin states of chromatin, enabling specified genes to be selectively transcribed or silenced with developmental and environmental cues.

Nucleus vs Cytoplasm (Comparison Table)

The structure and function of the nucleus and cytoplasm are remarkably dissimilar, but they closely cooperate to perform actions within the cell.

Applications and Importance of Nucleus

Importance and applications of nucleus are:

Medical Applications

• Knowledge of the nucleus and its functions is fundamental in medicine for both disease diagnosis and treatment.

• A full understanding of nuclear processes, such as DNA replication, transcription and repair, gives a clear understanding of mutations and genetic abnormalities such as cancer, Huntington's, or cystic fibrosis disease.

• Such understanding leads to targeted gene therapies, where gene correction or replacement of a defective gene might provide cures for hitherto incurable ailments.

• Also, the nucleus is a target for therapies, including antivirals, which inhibit viral replication in the host nucleus.

Nucleus NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Discovery of Nucleus

• Structure of Nucleus

• Functions of Nucleus

Practice Questions for NEET

Q1. A darkly stained body within a nucleus without any prominent membranous boundary is

1. Vacuole

2. Nucleolus

3. Ribosomes

4. Mitochondria

Correct answer: 2) Nucleolus

Explanation:

Structure of Nucleus -The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibers called chromatin, nucleus matrix, and one or more spherical bodies called nucleoli. The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.

Hence, the correct answer is option 2) Nucleolus.

Q2. Protoplasm which is found inside the nucleus is termed as?

1. Nucleoplasm

2. Amyloplast

3. Elaioplast

4. Cytoplasm

Correct answer: 1) Nucleoplasm

Explanation:

As we learned

1. Robert Hooke (1665): Introduced the term "cells" to describe hollow cavities observed in a cork section under a microscope.

2. Félix Dujardin (1835): Described the internal contents of cells and called it sarcode.

3. J.E. Purkinje (1839) and Hugo von Mohl (1838): Renamed Dujardin's sarcode as protoplasm, meaning "primary substance," emphasizing its essential role in cellular function.

4. Nucleoplasm: Refers to the protoplasm specific to the nucleus, differentiating it from the protoplasm in the cytoplasm.

This historical progression highlights the evolution of our understanding of cell biology and the terminology that laid the foundation for modern cytology.

Hence, the correct answer is option 1) Nucleoplasm.

Q3. Find out the wrong statement about heterochromatin:

1. It is densely packed

2. It stains dark

3. It is transcriptionally active

4. It is late replicating

Correct answer: 3) It is transcriptionally active

Explanation:

Heterochromatin - The chromatin that is more densely packed and stains dark is called heterochromatic. It is said to be transcriptionally active. Heterochromatin is typically found in regions of the genome that are not actively transcribed, such as centromeres and telomeres, although some forms, like euchromatin-associated heterochromatin, can participate in gene regulation.

Hence, the correct option is 3) It is transcriptionally active.

Also Read:

• MCQs on Nucleus

• Endoplasmic Reticulum

• Golgi Apparatus

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