What is a nucleus?
The human body has several organs. They are made up of several tiny building blocks called cells. The cell further contains several tiny well-defined structures called the organelles, such as the nucleus, mitochondria and endoplasmic reticulum. The nucleus (pleural nuclei) is the most prominent and largest organelle in the cell. This membrane-bound (encapsulated) organelle contains the genetic material inside it. It stores all the information that is required to reconstruct the organism. The nucleus is present in every cell of the human body except some cells, such as the red blood cells or RBCs.
What are the 3 functions of the nucleus?
The nucleus serves several important functions in the cell. The three major functions of the nucleus include
- It contains the genetic information of the cell in the form of deoxyribonucleic acid (DNA) or chromosomes and thus, controls cell growth and multiplication. It is also the site of DNA replication (formation of an identical copy of DNA).
- It regulates cell metabolism by synthesizing various enzymes.
- It is the site for the synthesis of ribonucleic acid or RNA that acts as a template for the synthesis of various proteins in the cell. It is also the site for the synthesis of the protein factories of the cell called the ribosomes.
What is the structure of the nucleus?
The nucleus is the largest organelle in the human body occupying around 25 percent of the cell volume.
The structure of the nucleus can be divided into four main parts.
- The nuclear envelope: The nucleus is bound by a double membrane layer that forms the capsule or the envelope. The two layers of this envelope stay separated from each other by a space known as the perinuclear space. The nuclear envelope separates the inner contents of the nucleus from the rest of the cell. The outer layer of the nuclear envelope is rough because of the presence of ribosomes on its surface. The outer membrane may be continuous with other organelles, such as the Golgi apparatus and the endoplasmic reticulum. The nuclear membrane has tiny gaps called pores. These pores allow the selective passage of substances between the nucleus and the cytoplasm (the matrix containing various organelles in the cell).
- The chromatin: The DNA is organized in the nucleus to form chromatin. The chromatin also contains proteins, the main proteins being histones. The chromatin further condenses to form the chromosomes. The human cell has 23 pairs of chromosomes.
- The nucleoplasm: The nucleoplasm is also called the karyoplasm or the nuclear sap. It is a semi-solid, granular substance that contains many proteins. The protein fibers form a crisscross matrix within the nucleus. This helps maintain the shape and structure of the nucleus. The nucleoplasm is the main site for enzyme activity within the nucleus. The appearance of nucleoplasm may vary during the different phases of the cell cycle. Besides proteins, the nucleoplasm also contains other substances, such as DNA, RNA and minerals.
- The nucleolus: The nucleolus is a well-defined spherical structure within the nucleus. It is the site for the synthesis and assembly of the ribosomes. The ribosomes act as the site of protein synthesis within the cell.
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Cold Harbor Springs Perspectives in Biology
Pederson T. The nucleus introduced. Cold Spring Harb Perspect Biol. 2011;3(5):a000521. Published online May 1, 2011. doi:10.1101/cshperspect.a000521