Nucleus – Definition, Structure and Functions

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Overview

The most conspicuous organelle in the eukaryotic cell is the nucleus. And contains most of the genes that control the cell.

  • It performs two major functions. The information of the DNA of the nucleus is transcribed into RNA. This RNA is used in the synthesis of proteins by translation.
  • These proteins in the form of enzymes and other vital proteins determine the specific activities of the cells.
  • The most visible structure within the non-dividing nucleus is the nucleolus. These are specialized regions for some chromosomes and contain multiple copies of genes for ribosome synthesis.
  • The nucleus stores genetic information and transfers it during cell division from one cell to the other. They are also the source of the transfer of traits from one generation to another.
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Nucleus

The cell nucleus is a membrane-bound structure that contains the cell’s hereditary information and manages the cell’s development growth and reproduction. 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.

In general, a eukaryotic cell has only one nucleus. Nevertheless, some eukaryotic cells are enucleated cells (without a nucleus), for example, red blood cells (RBCs); whereas, some are multinucleate (includes 2 or more nuclei), for example, slime molds. The nucleus is separated from the rest of the cell or the cytoplasm by a nuclear membrane. As the nucleus regulates the stability of genes and gene expression, it is likewise described as the control center of a cell.

Structure of the Nucleus

As the organelle that contains the genetic material of a cell, the nucleus can be referred to as the command center. As such, the nucleus includes a variety of structured aspects that permit it to perform its functions. This section gives focus to the structure of the cell.

In basic, the nucleus has a round shape as you have seen in most of your biology and science books. Nevertheless, it may appear flattened, ellipsoidal, or irregular depending upon the type of cell. For example, the nucleus of columnar epithelium cells appears more lengthened compared to those of other cells. The shape of a nucleus, however, may also alter as the cell develops and matures.

Nucleus-Structure

The Nuclear Membrane

The nuclear membrane is one of the elements that differentiate eukaryotic cells from prokaryotic cells. Whereas eukaryotic cells have a nucleus bound membrane, this is not the case with prokaryotes (e.g. bacteria) that lack membrane-bound organelles. As with the other cell organelles of eukaryotic organisms, the nucleus is a membrane-bound organelle.

The nuclear membrane, like the cell membrane, is a double-layered structure that includes phospholipids (forming the lipid bilayer nucleus envelope). Present on the nuclear membrane is nuclear pores (made up of proteins) through which substances enter or leave the cell (RNA, proteins, etc.). While the lipid bi-layers are separated by a thin area in between them (perinuclear cisterna), studies have revealed them to be merged at the pores.

The outer and the inner membranes are constant at certain points leading to the formation of pores, the nuclear pores. The nuclear pores allow the exchange of products between the nucleus and the cytoplasm. The number of nuclear pores is very variable. The undifferentiated cells (such as eggs) have numerous pores (about 30,000 per nucleus), whereas differentiated cells such as erythrocytes have only 3 or 4 pores/nucleus. Each pore has a definite structure which manages the traffic of substances passing through them.

Fibrous lamina

The fibrous lamina is part of the nuclear cytoskeleton that is connected to the inner layer of the nuclear membrane. It consists of fine protein filaments and serves to supply mechanical support to the bilayer membrane.

Some of the other functions of the nuclear lamina consist of:

  • Can play a role in regulating gene expression
  • Acts as anchor sites for the pore complexes of the nuclear
  • It manages product going into or exiting the cell
  • The nuclear membrane is linked to the endoplasmic reticulum in a way that produces a connection between the nucleus and the external environment (through the lumen of the endoplasmic reticulum).
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Nucleoplasm

Also referred to as karyoplasm/nucleus sap, the nucleoplasm is a kind of protoplasm made up of enzymes, dissolved salts, and a number of organic molecules. In addition, the nucleoplasm helps cushion and therefore secure the nucleolus and chromosomes while likewise assisting keep the basic shape of the nucleus.

Nucleolus

The nucleolus is the site of the ribosome and ribosomal RNA production. On microscopy, it looks like a large dense spot within the nucleus. After a cell divides, a nucleolus is formed when chromosomes are brought together into nucleolar organizing regions. Throughout cell division, the nucleolus vanishes.

Chromatin

Chromatin describes DNA that is complexed with proteins. The primary protein parts of chromatin are histones, which are very fundamental proteins that associate easily with DNA. Histones integrated with DNA type nucleosomes, which are the subunit of chromatin. Particularly, a nucleosome describes a sector of DNA associated with 8 histone proteins. By relating to histones, DNA is more compact and able to fit into the nucleus.

Nucleus-Chromatin

Chromatin can exist as either euchromatin or heterochromatin. Euchromatin is the form of chromatin present throughout gene expression and has a characteristic ‘beads on a string’ look. It is triggered by acetylation. On the other hand, heterochromatin is the ‘non-active’ form and is largely packed. On electron microscopy, euchromatin spots lighter than heterochromatin which shows their relative densities.

Nucleus chemical composition
  • 9-12 percent DNA
  • 15 percent histone
  • 65 percent of enzymes, neutral proteins, and acid proteins
  • 5 percent RNA
  • 3 percent lipids
Functions of Nucleus

The information above can be simplified into 3 key functions:

  • Cell compartmentalization: The existence of a selectively permeable nuclear envelope separates the contents of the nucleus from that of the cytoplasm.
  • Gene expression: Gene expression initially needs transcription, which is the process by which DNA is transcribed into mRNA. As the nucleus is the site of transcription, proteins within the nucleus play a key function in managing the process.
  • Processing of pre-mRNA: Freshly synthesized mRNA particles are referred to as pre-mRNA. Before they exit the nucleus, they undergo a process referred to as post-transcriptional modification where particles are included or eliminated from the structure.
Comparison of Eukaryotic and Prokaryotic Nucleus

The primary distinction between these two kinds of organisms is that eukaryotic cells have a membrane-bound nucleus and prokaryotic cells do not. The nucleus is where eukaryotes store their hereditary information. In prokaryotes, DNA is bundled together in the nucleoid area, however, it is not stored within a membrane-bound nucleus.

Nucleus-Eukaryotic-Prokaryotic

Multiple-Choice Questions (MCQs) – Nucleus: Definition, Structure, and Functions

  1. What is the primary function of the nucleus in a eukaryotic cell?
    • a) Energy production
    • b) Protein synthesis
    • c) Lipid storage
    • d) Waste elimination

    Answer: b) Protein synthesis

  2. Which organelle is commonly considered the command center of a eukaryotic cell?

    Answer: c) Nucleus

  3. What is the most visible structure within the non-dividing nucleus?
    • a) Chromatin
    • b) Nucleoplasm
    • c) Nucleolus
    • d) Nuclear membrane

    Answer: c) Nucleolus

  4. What is the primary function of the nucleolus?
    • a) Protein synthesis
    • b) Energy production
    • c) Ribosome and ribosomal RNA production
    • d) Lipid storage

    Answer: c) Ribosome and ribosomal RNA production

  5. What component of the nuclear cytoskeleton provides mechanical support to the nuclear membrane?
    • a) Chromatin
    • b) Nucleoplasm
    • c) Fibrous lamina
    • d) Nuclear pores

    Answer: c) Fibrous lamina

  6. Which term refers to the protoplasm within the nucleus composed of enzymes, dissolved salts, and organic molecules?
    • a) Chromatin
    • b) Nucleoplasm
    • c) Nucleolus
    • d) Nuclear membrane

    Answer: b) Nucleoplasm

  7. What is the function of the nuclear pores in the nuclear membrane?
    • a) Mechanical support
    • b) Substance exchange between the nucleus and cytoplasm
    • c) Ribosome production
    • d) DNA transcription

    Answer: b) Substance exchange between the nucleus and cytoplasm

  8. Which type of chromatin is present during gene expression and has a ‘beads on a string’ appearance?
    • a) Euchromatin
    • b) Heterochromatin
    • c) Nucleosomes
    • d) Histones

    Answer: a) Euchromatin

  9. What is the primary chemical composition of the nucleus?
    • a) 15% DNA, 65% histone, 9-12% RNA
    • b) 65% DNA, 15% histone, 9-12% RNA
    • c) 9-12% DNA, 15% histone, 65% RNA
    • d) 9-12% DNA, 15% histone, 65% enzymes, neutral proteins, and acid proteins, 5% RNA, 3% lipids

    Answer: d) 9-12% DNA, 15% histone, 65% enzymes, neutral proteins, and acid proteins, 5% RNA, 3% lipids

  10. What does the nucleus contribute to during gene expression?
    • a) DNA replication
    • b) RNA transcription
    • c) Protein synthesis
    • d) Lipid metabolism

    Answer: b) RNA transcription

  11. What function is associated with the nuclear membrane’s connection to the endoplasmic reticulum?
    • a) Protein synthesis
    • b) Gene regulation
    • c) Substance exchange
    • d) Lipid production

    Answer: c) Substance exchange

  12. What is the primary role of the nuclear membrane in cell compartmentalization?
    • a) DNA replication
    • b) Substance exchange
    • c) Mechanical support
    • d) Selective permeability

    Answer: d) Selective permeability

  13. Which type of cell lacks a nucleus?
    • a) Muscle cell
    • b) Nerve cell
    • c) Red blood cell (RBC)
    • d) Liver cell

    Answer: c) Red blood cell (RBC)

  14. What is the primary site for the synthesis of proteins by translation in the eukaryotic cell?
    • a) Endoplasmic reticulum
    • b) Golgi apparatus
    • c) Nucleus
    • d) Mitochondria

    Answer: a) Endoplasmic reticulum

  15. Which region within the nucleus contains multiple copies of genes for ribosome synthesis?
    • a) Chromatin
    • b) Nucleoplasm
    • c) Nuclear membrane
    • d) Nucleolus

    Answer: d) Nucleolus

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FAQs (Frequently Asked Questions)

  1. What is the nucleus, and why is it considered the most conspicuous organelle in eukaryotic cells?
    • The nucleus is a membrane-bound structure that contains the cell’s hereditary information. It is considered conspicuous as it houses most of the genes that control the cell.
  2. What are the two major functions performed by the nucleus in a eukaryotic cell?
    • The nucleus transcribes the information of DNA into RNA, which is then used in the synthesis of proteins through translation. These proteins, including enzymes, dictate the specific activities of the cells.
  3. Which structure is most visible within a non-dividing nucleus, and what is its function?
    • The nucleolus is the most visible structure, serving as specialized regions for certain chromosomes and containing multiple copies of genes for ribosome synthesis.
  4. How does the nucleus contribute to the transfer of genetic information during cell division?
    • The nucleus stores genetic information and transfers it during cell division from one cell to another, playing a crucial role in passing traits from one generation to the next.
  5. Describe the structure of the cell nucleus.
    • The nucleus is a membrane-bound organelle with a round shape, though it may appear flattened, ellipsoidal, or irregular. It contains a nuclear membrane, nucleoplasm, nucleolus, chromatin, and nuclear pores.
  6. What distinguishes the nuclear membrane in eukaryotic cells from prokaryotic cells?
    • Eukaryotic cells have a membrane-bound nucleus, while prokaryotic cells lack membrane-bound organelles. The nuclear membrane in eukaryotic cells is double-layered and contains nuclear pores for substance exchange.
  7. What is the function of the fibrous lamina in the nuclear cytoskeleton?
    • The fibrous lamina provides mechanical support to the nuclear membrane, regulates gene expression, acts as anchor sites for pore complexes, and manages the entry or exit of cellular products.
  8. What is nucleoplasm, and what role does it play in the nucleus?
    • Nucleoplasm, also known as karyoplasm, is a protoplasm within the nucleus composed of enzymes, dissolved salts, and organic molecules. It cushions and protects the nucleolus and chromosomes while helping maintain the nucleus’s shape.
  9. What is the primary function of the nucleolus?
    • The nucleolus is the site of ribosome and ribosomal RNA production. It appears as a large dense spot within the nucleus and forms during cell division when chromosomes are brought together.
  10. Explain the composition of chromatin and its role in the nucleus.
    • Chromatin is DNA complexed with proteins, primarily histones. It forms nucleosomes, aiding in DNA compaction. Chromatin can exist as euchromatin (active) or heterochromatin (inactive) during gene expression.
  11. What is the chemical composition of the nucleus, and how is it distributed?
    • The nucleus is composed of 9-12% DNA, 15% histone, 65% enzymes, neutral proteins, and acid proteins, 5% RNA, and 3% lipids. This composition plays a vital role in various cellular functions.
  12. Name the three key functions of the nucleus.
    • The nucleus functions in cell compartmentalization, gene expression through transcription, and the processing of pre-mRNA before its exit from the nucleus.
  13. How does the nucleus contribute to the compartmentalization of a eukaryotic cell?
    • The presence of a selectively permeable nuclear envelope separates the contents of the nucleus from the cytoplasm, contributing to cell compartmentalization.
  14. What distinguishes eukaryotic and prokaryotic nuclei, and where is genetic information stored in each?
    • Eukaryotic cells have a membrane-bound nucleus, while prokaryotic cells lack one. In eukaryotes, genetic information is stored in the nucleus, whereas in prokaryotes, it is in the nucleoid region.
  15. Name an example of a cell type without a nucleus, and how does it differ from multinucleate cells?
    • Red blood cells (RBCs) are examples of cells without a nucleus (enucleated). Multinucleate cells, like slime molds, have two or more nuclei, distinguishing them from typical eukaryotic cells.
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Summary: Nucleus – Definition, Structure, and Functions

The nucleus, a vital organelle in eukaryotic cells, serves as the cell’s command center, managing hereditary information, growth, and reproduction. Most genes controlling the cell are housed within this conspicuous structure, comprising approximately 10% of the cell’s volume. Key functions include transcribing DNA into RNA, facilitating protein synthesis, and determining cellular activities.

The nucleus exhibits a unique structure, with a round shape that can vary based on cell type. It is surrounded by a nuclear membrane containing pores for substance exchange. The fibrous lamina, part of the nuclear cytoskeleton, provides mechanical support and regulates gene expression. The nucleoplasm, or nucleus sap, cushions the nucleolus and chromosomes.

The nucleolus, a dense spot within the nucleus, is crucial for ribosome and ribosomal RNA production. Chromatin, consisting of DNA and histones, forms nucleosomes, impacting DNA compactness. The nucleus’s chemical composition includes DNA, histone, enzymes, RNA, and lipids in varying proportions.

Three key functions of the nucleus involve cell compartmentalization, gene expression through transcription, and the processing of pre-mRNA before exit. A comparison between eukaryotic and prokaryotic nuclei highlights the membrane-bound nature of the former, storing genetic information, while prokaryotes bundle DNA in the nucleoid area without a membrane-bound nucleus.

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