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Nucleus is the most important cell organelle found in eukaryotic cells. It is a membrane-bound structure that stores DNA. It is the place where all the information is stored for the functioning of the cell. The protein and ribosome synthesis begins in the nucleus. Apart from this, the nucleus also performs many other functions essential for the cell.
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| Table of Content |
Key Takeaways: Nucleus, Eukaryotic Cells, Chromatin, DNA, Nuclear Lamina, Nucleolus, DNA Replication, Transcription, Translation
Structure of the Nucleus
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Nucleus is a spherical body surrounded by a double-membrane. It consists of a thread-like complex structure called chromatin. The condensed form of chromatin is called DNA which is the genetic material of the cell. The nucleus is the centre of control for all the cellular activities. It is the largest organelle of the eukaryotic cells. Discussed below is the structure of the Nucleus for your reference.
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Nuclear Envelope and Nuclear Pores
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The nucleus is surrounded by a double-membrane called the nuclear envelope. The nuclear envelope is made up of two layers- inner nuclear membrane and outer nuclear membrane. This envelope acts as a separation between the genetic material and other components of the cell. The outer nuclear membrane is continuous with the endoplasmic reticulum and thus contains ribosomes. The inner and the outer nuclear membrane contains space between them called the ‘perinuclear space’.
At some points, the outer and the inner nuclear membrane fuses with each other to form nuclear pores. These pores act as a channel for the transport of water-soluble molecules. The small size of the pores prevent bigger molecules like large proteins and nucleic acids from passing out of the nucleus into the cytoplasm.
Nuclear Lamina
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Nuclear lamina is the network of proteins that provides mechanical support to the nucleus. The nuclear lamina provides the sites for chromosomes and proteins to anchor. The nuclear lamina is highly organised on the side of the inner nuclear membrane while it is less organised on the other side.
Chromatin
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Chromatin is a highly-coiled thread-like structure that contains genes. The condensed form of chromatin is called DNA. DNA is the genetic material of the cell which stores the information for protein synthesis. The chromatin is also composed of proteins called histones. Histones are the positively charged proteins that bind the negatively-charged DNA. The DNA and histones together form the nucleosome which is a subunit of chromatin. The DNA becomes more compact during the cell division and forms chromosomes.
Chromatin is further divided into two broad classifications- euchromatin and heterochromatin. Euchromatin is loosely-packed and has beads on string appearance. It is the active form of chromatin. Whereas, heterochromatin is densely-packed and remains inactive.
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Nucleolus
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Nucleolus is a dense structure that lacks membrane. It is the site for synthesis of ribosomal RNA (rRNA). It contains regions called Nucleolar Organizer Regions (NOR). The structure of nucleolus consists of three regions-
- The innermost fibrillar centres (FCs)
- Dense fibrillar component (DFC)
- Granular component (GC)
Besides these main structures, the nucleus also contains other nuclear bodies like Cajal bodies, polymorphic interphase karyosome association (PIKA), PML bodies, splicing speckles etc.
Functions of the Nucleus
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Nucleus is the site for the transcription of genes. It controls all the cellular activities by regulating gene expression and eventually protein synthesis. The main functions of the nucleus are as follows:
- Compartmentalization of Cell - The nucleus helps in the compartmentalization of cells by separating the genetic material from the other components of the cell. The presence of the double-membranous nuclear envelope ensures that the cytoplasmic content does not get mixed up with the genetic material. In this way, it also regulates the gene expression in the cell. At the same time, the nuclear pores allow for the transport of essential molecules and proteins between the cytoplasm and the nucleus.
- DNA Replication - It is the main function of the cell nucleus. DNA replication occurs inside the nucleus during the interphase of the cell cycle. The replication of DNA is an essential process for cell division.
- Gene Expression - The DNA present inside the nucleus is used as a template for the synthesis of RNA. This process of formation of RNA from DNA is called transcription and it occurs inside the nucleus of the cell. This RNA is then converted into protein by the process of translation.
- Processing of Pre-mRNA - The RNA formed by transcription is called a pre-mRNA. The pre-mRNA undergoes post-transcriptional modifications to form the final mRNA molecule. This mRNA is converted into protein by the process of translation. The post-transcriptional modifications of pre-mRNA occurs inside the nucleus.
Evolution of Nucleus
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There are different models proposed about the origin of the nucleus. They are as follows:
- Syntrophic Model- This model proposes that the nucleus was formed due to the symbiotic relationship between the archaea and bacteria.
- Viral Eukaryogenesis- According to this, the nucleus evolved as a result of the infection of a prokaryote by a virus.
- The Exomembrane Hypothesis- According to this, a single ancestral cell developed a second outer membrane. The inner membrane became the nuclear membrane and evolved as a nucleus.
Things to Remember
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- Nucleus is called the centre of control of the cell. It is a membrane-bound spherical structure found in eukaryotic cells.
- Nucleus is bounded by a double-membrane called the nuclear envelope.
- Nucleus contains chromatin which is a highly coiled thread-like structure having genes.
- The DNA replication occurs inside the nucleus during the interphase of the cell cycle.
- Nucleus controls the regulation of gene expression inside the cell.
- The process of formation of RNA from DNA is called transcription and the formation of protein from RNA is called translation.
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| Related Topics To The fundamental unit of life | ||
|---|---|---|
| Lysosome and Ribosome | Fundamental Unit of Life | Inclusion Bodies |
| Discovery of Cells | Cyclic and Non cyclic Photophosphorylation | Gene Regulation |
| Packaging of DNA | Mitochondria and Plastids | Euchromatin and Heterochromatin |
Previous Year Questions
- A bivalent consists of… (NEET 1989)
- A cell organelle containing hydrolytic enzymes is... (NEET 2016)
- Water soluble pigments found in plant cell vacuoles are... (NEET 2016)
- Which of the following are not membrane - bound?... (NEET 2015)
- Which one of the following is not an inclusion body found in prokaryotes?... (NEET 2015)
- Flagella of prokaryotic and eukaryotic cells differ in... (NEET 2004)
- A major site for synthesis of lipids is... (NEET 2013)
- According to fluid mosaic model, plasma membrane is composed of...(NEET 1988)
- Acetabularia used in Hammerling's nucleocytoplasmic experiments is...(NEET 1988)
- Active and passive transports across cell membrane differ in...(NEET 1993)
- All types of plastids possess essentially the same structure because they...(NEET 1992)
- An outer covering membrane is absent over...(NEET 1992)
- Balbiani rings are the sites of...(NEET 1993)
- Besides giving out secretory vesicles, the Golgi apparatus is also concerned with the formation of...(NEET 1994)
- Biological organisation starts with... (NEET 2007)
- Cell recognition and adhesion occur due to biochemicals of cell membranes named...(NEET 1993)
- Cell wall shows...(NEET 1991)
- Cellular organelles with membranes are...(NEET 2015)
- Cellular totipotency is demonstrated by...(NEET 2003)
- Chlorophyll in chloroplast is located in...(NEET 2005)
Sample Question
Ques. What are the characteristics of prokaryotic cells? (3 marks) [NCERT]
Ans. Some important characteristics of prokaryotic cells are-
- Prokaryotes lack membrane bound cell organelles.
- They have 70S ribosomes.
- Nucleus is absent. DNA remains free in the cytoplasm.
- It shows asexual reproduction.
- Transcription and translation occurs in the cytoplasm.
Ques. What are nuclear pores? State their functions. (3 marks) [NCERT]
Ans. Nucleus is surrounded by a double-membrane called the nuclear envelope. The nuclear envelope is made up of two layers- the outer nuclear membrane and the inner nuclear membrane. The outer and the inner nuclear membrane fuses at some places to form the nuclear pores.
The nuclear pores serve as a channel for the movement of water-soluble molecules, RNA, proteins etc. between nucleus and the cytoplasm. The small size of the pores also prevent larger proteins from leaving the nucleus.
Ques. What are ribosomes and their functions? (3 marks)
Ans. Ribosomes are the sites of protein synthesis. They are small, rounded and are made up of ribonucleoprotein. Ribosomes are found either in the cytoplasm or on the membrane of endoplasmic reticulum. In prokaryotes, 70S ribosomes are found but in eukaryotes, ribosomes are 80S.
The main functions of ribosomes are as follows:
- Ribosomes are involved in the process of translation.
- They are also involved in the process of protein folding.
- Ribosomes also protect the mRNA from nucleases.
Ques. Differentiate between Smooth Endoplasmic Reticulum and Rough Endoplasmic Reticulum. (5 marks)
Ans. The main differences between rough and smooth endoplasmic reticulum are as follows:
| Rough Endoplasmic Reticulum | Smooth Endoplasmic Reticulum |
|---|---|
| In RER, the ribosomes are present on the surface of the endoplasmic reticulum. | In SER, the ribosomes are not found on the surface of the endoplasmic reticulum. |
| They are involved in the synthesis of proteins and enzymes. | They are involved in the synthesis of lipids, glycogen and steroids. |
| They are made up of cisternae and tubules. | They are made up of vesicles and tubules. |
| The RER is continuous with the outer nuclear membrane. | The SER is connected to plasmalemma. |
| Detoxification enzymes are absent. | Detoxification enzymes are present. |
Ques. What is a centromere? How does the position of centromere form the basis of classification of chromosomes? (5 marks) [NCERT]
Ans. The arms of chromosomes are called chromatids. The chromatids are held together in an X-shaped structure. The point of attachment of the chromatids is called centromere. It is a constricted region on the chromosome where the chromatids attach together. On one side of the centromere, a disc-shaped structure called kinetochore is present.
According to the position of the centromere, there are four types of chromosomes-
- Metacentric Chromosomes- In these chromosomes, the centromere is present in the middle resulting in two equal arms.
- Submetacentric Chromosomes- The centromere is present slightly away from the centre. This results in one arm being shorter than the other.
- Acrocentric Chromosomes- The centromere is present near the end of the chromosome. As a result, one arm is extremely short and the other is very long.
- Telocentric Chromosome- In this, the centromere is present at the terminal of the chromosome.
Ques. What are the functions of mitochondria? (3 marks)
Ans. Mitochondria are the sausage-shaped structures bounded by double-membranes. The mitochondria are called ‘powerhouses of the cell’. The main functions of the Mitochondria are-
- They help in aerobic respiration.
- They maintain the concentration of calcium ions in the cell.
- It contains essential enzymes for the synthesis of fatty acids.
- Mitochondria is also a site for the synthesis of amino acids.
- The process of ATP production takes place in the mitochondria.
Ques. What do you mean by post-transcriptional modifications? (5 marks)
Ans. Inside the nucleus, the RNA is formed from DNA by the process of transcription. This RNA is called pre-mRNA. The pre-mRNA requires modifications to be converted into a mature mRNA that forms protein. These modifications are called post-transcriptional modifications. There are 3 main types of modifications that occur after transcription-
- RNA Splicing- The pre-mRNA contains some non-coding regions called introns. These non-coding regions have to be removed in order to convert it into proteins. This is called RNA splicing. The introns are removed with the help of enzymes and the exons (coding regions) are rejoined.
- 5’ Capping- In this, a molecule of methyl guanosine triphosphate is added on the 5’ end of the pre-mRNA.
- Tailing- It occurs by the addition of 200-300 adenylate residues on the 3’ end of the pre-mRNA forming a poly-A tail.
The mature mRNA formed after the post-transcriptional modifications is transported to cytoplasm where translation takes place.
Ques: What is the structure and function of the nucleus? (2 marks)
Ans: Nucleus? is a membrane-bound structure that contains the cell's hereditary information as well as controls the cell's growth and reproduction system. It is the command center of a eukaryotic cell and is commonly the most prominent organelle in a cell accounting for about 10 percent of the cell's volume.
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