The nucleus is the most important organelle of a cell as it controls most of the cellular activities. The structure and function of nucleus will help to understand it in deatils.
What is the nucleus?
The nucleus is a membrane-bounded organelle present in both animal and plant cells. It was first discovered in 1831 by Robert Brown. The word nucleus comes from the Latin word which means “core” or “center”. Most of the activities take place inside the nucleus and it is controlling the whole cell.
The nucleus contains DNA, histone proteins, RNA, and lipids. In animal cells, the nucleus is present at the center of the cell, while in plant cells it is present at the side of the cell due to the presence of a large vacuole. The nucleus is the reserve of all the genetic material of a cell except mitochondria DNA and chloroplast DNA.
Structure of the nucleus
The nucleus is spherical and consists of a nuclear envelope, nucleoplasm, nucleoli, and chromatin material.
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| Structure of nucleus. Image created in Biorender.com |
Nuclear envelope
The nucleus is made up of a double-layer known as the nuclear envelope. The nuclear envelope encloses the entire content of the nucleus and separates it from cytoplasmic content. The nuclear envelope is impermeable, thus controls the movement of molecules. The outer nuclear envelope is connected to the endoplasmic reticulum and the inner nuclear membrane is connected to the nuclear lamina.
Nuclear lamina
The nuclear lamina is a fibrous network connected to the inner side of the nuclear envelope. It is very important to maintain the shape of the nucleus by providing mechanical support. The nuclear lamina is composed of lamin proteins which organize in a thread-like structure inside the nucleus.
Nuclear pores
A nuclear envelope is not a continuous membrane instead it has small pores known as nuclear pores. Nuclear pores are made up of a group of special proteins called nucleoporins. The diameter of nuclear pores is about 10nm and about 3000-4000 pores are present in a typical mammalian cell.
As the nuclear envelope is impermeable to large molecules, the nuclear pores make the nuclear envelope semi-permeable allowing some molecules to enter the nucleus while some molecules cannot enter the nucleus. The movement of large molecules such as proteins and mRNA is important to regulate DNA replication, cell growth, and protein synthesis.
Nucleoplasm
The internal granular part of the nucleus is called the nucleoplasm. Just like the cytoplasm, nucleoplasm contains many different substances such as DNA, proteins, and nucleotides and facilitates the proper functioning of the nucleus.
Chromatin
A thread-like structure is present inside the nucleoplasm known as chromatin. The chromatin is made up of DNA and histone proteins. DNA is a large molecule that is condensed into a small space with the help of histone proteins. A human cell contains about 3m long DNA in its nucleus.
When the cell is not dividing, the DNA is present in the form of chromatin, however, during the cell division this thread condenses and forms a chromosome. The number of chromosomes is specific in every species and stores all the genetic information. For example, a human cell contains 48 chromosomes while 8 chromosomes are present in a fruit fly.
Nucleoli
There is one or two nucleoli (nucleolus: singular) present inside the nucleoplasm. Nucleoli is dark and spherical site in the nucleus. It is the site for the synthesis of ribosomes. Ribosomal RNA is produced at nucleoli and combined to form ribosomes.
Functions of nucleus
The main functions of nucleus are:- It is involved in the protein synthesis and growth of the cell.
- It controls cell division. During the cell division, chromatin material is converted into the chromosome and these chromosomes are divided into two new daughter cells.
- It also maintains cellular metabolism by controlling the synthesis of enzymes. The ribosomes produced in nucleoli and mRNA from gene transcription work together in the nucleus to produce proteins.
- It controls the transcription of DNA. The transcription is the synthesis of mRNA from the gene.
- It stores information about hereditary traits. All the information about hereditary traits is stored in DNA which is present in the nucleus.
- DNA replication takes place in the nucleus in a localized manner.
- It separates the nuclear process from the cytoplasmic process which is important for the regulation of gene expression and metabolic processes.
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Enucleate
The cell with no nucleus is called enucleate. For example Red blood cells.
Multinucleate
The cell having more than one nucleus is called multinucleate e.g., slime mold.
Some question and answer
Q. What is the difference between cytoplasm and nuceloplasm?
A. Cytoplasm is the thick solution present inside the cell from cell membrane to the nucleus, while nucleoplasm is the liquid part present inside the nucleus from nuclear membrane to the nucleolus. The composition of both solutions is different according to their functions.
Q. State three functions of the nucleus.
A. Nucleus controls cell divison, protein synthesis and maintains cellular metabolism.
Q. Is nucleus present in prokaryotic cells?
A. No, nucleus is not present in eukaryotic cells.
Q. Why nuclear pores are present in the nuclear envelope?
A. Nuclear envelope is impermeable to large molecules such as proteins and RNA. Nuclear pores present in the nuclear envelope allow the passage of these molecules in and out of the nucleus.
Q. Can a cell contain more than one nucleus?
A. Yes, a cell can have more than one nucleus and is called multinucleated. Slime mold is an example of a multinucleated cell.
Q. How many chromosomes are present in a typical human cell?
A. A human cell contains 48 chromosomes which can be seen during cell division only.
Q. Give one example of the cell having no nucleus?
A. Red blood cell (RBC) has no nucleus. RBCs are not divided but are destroyed after 120 days in the liver, so a nucleus is not required. The absence of a nucleus allows RBCs to carry and transfer more oxygen due to the abundance of hemoglobin.
Q. Who discovered the nucleus?
A. Robert Brown was the first person who discovered the nucleus in 1831.
Q. Where ribosome are assembled?
A. nucleolus is the site where rRNA is synthesized and assembled into ribosomes.
Q. What is the location of the nucleus in animal and plant cells?
A. In animal cells, the nucleus is present at the center of the cell, while in plant cells it is present at the side of the cell due to the presence of a large vacuole.
References
- Lamond, A. I., & Earnshaw, W. C. (1998). Structure and function in the nucleus. Science, 280(5363), 547-553.
- Newport, J. W., & Forbes, D. J. (1987). The nucleus: structure, function, and dynamics. Annual review of biochemistry, 56(1), 535-565.
- Taddei, A., & Gasser, S. M. (2012). Structure and function in the budding yeast nucleus. Genetics, 192(1), 107-129.
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