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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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.
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.
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 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.
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.
1. Explain the Structure and Function of the Nucleus.
Nucleus is responsible for the hereditary characteristics of organisms.
It is also responsible for the synthesis of protein, cell division, growth and cell differentiation.
The nucleolus stores proteins and RNA.
Transcription takes place in the nucleus wherein the messenger RNA is produced for the synthesis of proteins.
Nucleus is regarded as the control centre for an organism since it regulates genes and gene expression.
It helps in the production of ribosomes inside the nucleolus.
2. What is the Structure of the Cell Nucleus?
Nucleus consists of a double-membrane organelle which is referred to as the nuclear envelope or the nuclear membrane which encircles it.
Inside the nucleus is the nucleolus, which occupies around 25 per cent of the nuclear volume.
Chromatin is dense thread-like structures which are found inside the nucleus and contain DNA and protein.
The nuclear matrix, which is a network of filaments and fibres, is responsible for the mechanical strength of the nucleus. It has the same function as that of a cytoskeleton.