Nucleus Structure and Function
A cell is known as the basic structural, functional and biological unit of all living organisms. It is referred to as the smallest unit of life. Cells are known as the building blocks of living organisms. There are basically two different types of cells, prokaryotic and eukaryotic. The prokaryotic cells do not contain a nucleolus and are mostly found in bacteria. The DNA in the prokaryotic cell is present in the cytoplasm and forms smaller circular DNA strands known as plasmids. The eukaryotic cells, on the other hand, contain DNA in their nucleus.
The nucleus in the cell is an organelle which contains the genetic information of that organism. In the animal cell, the nucleus is present in the centre of the cell. However, in the plant cell, the nucleus is present towards the periphery because of the huge water-filled vacuole which is present in the centre. Let us describe the structure and function of the nucleus.
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Nucleus
To explain the structure of the nucleus, we must learn first about what is a nucleus.
The nucleus is a cell organelle which is spherical and is present in all the eukaryotic cells. Nucleus is referred to as the control centre of the eukaryotic cells. It also helps in the coordination of both the genes and the gene expression. The nucleus consists of other structures such as the nuclear membrane, nucleoplasm, nucleolus, and chromosomes.
The cell nucleus is regarded as the most prominent organelle when compared to other cell organelles because it accounts for approximately 10 per cent of the total volume of the cell. Generally, a eukaryotic cell consists of only the nucleus. However, there are some eukaryotic cells that are known as enucleate cells, meaning that they are without a nucleus, for example, the red blood cells. Some, on the other hand, are multinucleate, meaning that they contain two or more nuclei, for example, in the slime mould. The nucleus is separated from the cytoplasm or the rest of the cell by a nuclear membrane.
Structure of the Nucleus
Let us now describe the structure of the nucleus.
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The cell nucleus contains a nuclear membrane, which is referred to as the nuclear envelope, nucleolus, nucleoplasm, and chromosomes. Nucleoplasm is also known as karyoplasm and is referred to as the matrix which is present inside the cell nucleus.
The nuclear membrane differentiates the nuclear constituents from the cytoplasm. Just as the cell membrane, the nuclear envelope contains phospholipids which tend to form a lipid bilayer. The envelope helps in maintaining the shape of the nucleus of the cell and also assisting in the coordination of the flow of the molecules that go into and out of the nucleus via the nuclear pores. The nucleus of the cell consists of DNA which controls the form, growth, and function of the cell. The nucleus can be compared to the brain in its function to coordinate all the activities of the cell.
The nuclear membrane is connected to the endoplasmic reticulum in a manner that the internal portion of the nuclear membrane continues along the lumen of the endoplasmic reticulum.
Nuclear Membrane
The nuclear membrane is a double-layered membrane which encloses all the organelles of the nucleus. The outer layer of the nuclear membrane is joined with the endoplasmic reticulum of the cell. A liquid-filled space, also called the perinuclear space occurs between the two layers of the nuclear membrane. The nucleus gets through the cytoplasm or the remaining of the cell via openings known as nuclear pores. These nuclear pores are responsible for the exchange of the larger molecules between the cytoplasm and the nucleus.
Chromosomes
Chromosomes are present in the nucleus of the cell in the form of strings of protein molecules and DNA known as chromatin. The chromatin is divided into heterochromatin and euchromatin depending on their functions. The heterochromatin is known to be in a highly condensed and transcriptionally inactive form. It is usually present adjoined to the nuclear membrane. On the other hand, the euchromatin is a mild and less condensed organization of chromatin and is abundantly found in a transcribing cell.
Nucleolus
The nucleolus is a solid and spherical-shaped structure which is present inside the nucleus. Many eukaryotic organisms have a nucleus which contains up to four nucleoli. The nucleolus has an implied or indirect role in the synthesis of protein by producing ribosomes.
These ribosomes are called the cell organelles that are made up of proteins and RNA and proteins. They get transported to the cytoplasm of the cell, which is attached then to the endoplasmic reticulum.
Ribosomes are known as the protein-producing organelles of the cell. Nucleolus gets disappeared whenever there is a cell division and it gets reformed once the cell division is completed.
FAQs on Structure of Nucleus
1. What are the main components that make up the structure of a eukaryotic nucleus?
The eukaryotic nucleus is a highly organised organelle with four main components, each with a specific function:
Nuclear Envelope: A double-membrane barrier that separates the nucleus from the cytoplasm, regulating the passage of molecules through nuclear pores.
Nucleoplasm: The jelly-like substance (matrix) within the nucleus that suspends all its contents, including chromatin and the nucleolus.
Chromatin Material: A complex of DNA and proteins (primarily histones) that condenses to form chromosomes during cell division. It contains the cell's genetic information.
Nucleolus: A dense, non-membranous structure within the nucleoplasm responsible for synthesising ribosomal RNA (rRNA) and assembling ribosomes.
2. What is the importance of the nucleolus in a cell?
The primary importance of the nucleolus is its role as the 'ribosome factory' of the cell. It is the specific site for the transcription of ribosomal RNA (rRNA) and its assembly with proteins to form ribosomal subunits. These subunits are then exported to the cytoplasm to form functional ribosomes, which are essential for protein synthesis.
3. How does the nuclear envelope regulate what enters and exits the nucleus?
The nuclear envelope regulates molecular traffic through specialised channels called Nuclear Pore Complexes (NPCs). This double-membrane structure acts as a selective barrier. Small molecules and ions can pass through freely, but the passage of larger molecules, such as proteins and RNA, is a tightly controlled, active process. This ensures that nuclear processes like DNA replication and transcription are separated and protected from the cytoplasm.
4. What is the fundamental difference between chromatin and a chromosome?
The fundamental difference between chromatin and a chromosome is their level of condensation and the cell cycle stage in which they appear. Chromatin is the less condensed, thread-like complex of DNA and histone proteins found within the nucleus of a non-dividing (interphase) cell. A chromosome is the highly condensed and organised form of chromatin that becomes visible during cell division (mitosis or meiosis), making the segregation of genetic material manageable.
5. Why is the nucleus often called the 'control centre' of the cell?
The nucleus is called the 'control centre' because it houses the cell's genetic material, DNA, organised into chromosomes. This DNA contains the instructions for synthesising all the proteins and functional RNA molecules a cell needs. By controlling gene expression—deciding which genes are turned on or off—the nucleus directs all cellular activities, including metabolism, growth, differentiation, and reproduction.
6. How does the vast length of DNA fit inside the tiny nucleus?
DNA fits inside the tiny nucleus through a remarkable process of compaction. The long DNA strand is wrapped around a set of eight protein molecules called histones, forming a structure known as a nucleosome. This 'beads on a string' structure is then coiled and folded into progressively more compact structures, ultimately forming the dense chromatin fibre that can be efficiently stored within the nuclear volume.
7. Compare the genetic material arrangement in a eukaryotic nucleus with that in a prokaryotic nucleoid.
The key differences in genetic material arrangement are:
Location & Membrane: In eukaryotes, DNA is contained within a true, double-membraned nucleus. In prokaryotes, the genetic material is located in a region of the cytoplasm called the nucleoid, which is not enclosed by a membrane.
DNA Structure: Eukaryotic DNA is linear and complexed with histone proteins to form chromatin. Prokaryotic DNA is typically a single, circular molecule and is not associated with histones.
8. What happens to the structure of the nucleus during cell division?
During cell division (mitosis), the nucleus undergoes dramatic structural changes. In the early stages (prophase), the nuclear envelope and nucleolus disintegrate and disappear. The chromatin condenses tightly to form visible chromosomes. After the chromosomes have been segregated, in the final stage (telophase), the nuclear envelope and nucleolus re-form around each set of separated chromosomes, resulting in two new daughter nuclei.
