Main Difference – Nucleolus vs Nucleus
Nucleolus is a component of the eukaryotic nucleus. The nucleolus is considered as occupying 25% of the volume of the nucleus. The nucleus is the house for cells’ genetic material. It maintains a closed environment or a compartment inside the cell. Transcription of eukaryotes occurs inside this compartment. The nucleus allows the regulation of gene expression by maintaining an asynchrony between eukaryotic transcription and translation. Eukaryotic translation occurs in the cytoplasm. On the contrary, the major function of the nucleolus is the ribosome biogenesis. Therefore, the nucleus consists of mainly DNA whereas nucleolus consists of RNA. The main difference between nucleolus and nucleus is that nucleolus is a sub-organelle located inside the nucleus whereas nucleus is a membrane-bound organelle in the cell.
This article looks at,
1. What is Nucleolus
– Definition, Structure, Function
2. What is Nucleus
– Definition, Structure, Function
3. What is the difference between Nucleolus and Nucleus
What is Nucleolus
Nucleolus is the largest structure in the cell nucleus. The nucleolus is responsible for the production of ribosomes. This process is referred to as the ribosome biogenesis. The nucleolus also has two other roles: assembling the signal recognition particles and generating the cells’ response to stress. The nucleolus is formed around specific chromosomal regions and it is made up of DNA, RNA and associated proteins. The malfunctioning of nucleoli causes illnesses, diseases, disorders and syndromes in humans. The nucleolus can be observed under the electron microscope as a part of the nucleus.
Nucleolus Structure
The nucleolus is composed of three components: the dense fibrillar component (DFC), the fibrillar center (FC) and the granular component (GC). Newly transcribed rRNAs that are bound with the ribosomal proteins are contained in the DFC. GC contains ribosomal proteins bound with RNA. These ribosomal proteins are assembled into immature ribosomes. The nucleolus can be seen only in higher eukaryotes. The evolution of the nucleolus occurred from bipartite organization with the transition of anamniotes to amniotes. The original fibrillar component is separated into FC and DFC due to the substantial increase in the DNA intergenic region. In plant nucleoli, nuclear vacuole can be identified as a clear area at the center of the nucleolus. The nucleolus in the nucleus is shown in figure 1.
Figure 1: Nucleolus in the nucleus
Function of Nucleolus
During the ribosome biogenesis, RNA polymerase I transcribes rRNA genes responsible for 28S, 18S, and 5.8S rRNA transcripts within the nucleus. The 5S rRNA is transcribed by RNA polymerase III. The genes responsible for ribosomal proteins are transcribed by RNA polymerase II. Ribosomal proteins are translated in the cytoplasm during the conventional pathway and imported back into the nucleolus. After the maturation and association of rRNA and ribosomal proteins, they produce 40S and 60S subunits of the 80S ribosome in eukaryotes. Other than ribosomal biogenesis, nucleolus captures proteins and immobilize them in a process known as nucleolar detention.
What is Nucleus
The nucleus is a membrane-bound organelle found only in eukaryotic cells. Most eukaryotic cells contain a single nucleus. Human muscle cells contain more than one nucleus and red blood cells contain no nucleus. The nucleus holds most of the genetic material of the cell. This genetic material is organized into linear chromosomes associated with histones. The integrity of genes is maintained by the nucleus. It also controls the gene expression.
Nucleus Structure
The nucleus is made up of nuclear envelope which is a double-membrane structure. The outer membrane of the nucleus is continuous with the rough endoplasmic reticulum. The aqueous channels within the nuclear membrane are the nuclear pores. Nucleoplasm is the viscous liquid enclosed by the nuclear envelope. The network within the nucleus is called the nuclear matrix or the nuclear lamina. It provides the mechanical support to the nucleus. In addition, chromosomes are also present in the nucleus. Chromosomes exist as DNA-protein complex called chromatin. Two types of chromatin can be identified within the nucleus: euchromatin and the heterochromatin. Euchromatin is the lesser packed chromatin type which consists of frequently expressing genes. Heterochromatin is a more compact form consisting of infrequently transcribing genes. The nucleolus is also a component of the nucleus. Subnuclear bodies like Cajal bodies and gems, PML bodies and splicing speckles are present besides the nucleolus.
Figure 2: Human cell nucleus
Function of Nucleus
The nucleus holds most of the genetic material in eukaryotic cells organized as DNA with proteins in the form of chromosomes. A separate compartment is provided by the nucleus for the transcription of genetic material other than the cytoplasm where the translation occurs. Primary transcript of the mRNA is evolved inside the nucleus and before it is exported to the cytoplasm, post-transcriptional modifications such as 5’ end capping, addition of the 3’ poly A tail and splicing out of introns occur in the nucleus itself. This allows the regulation of gene expression. Thus, the main function of the nucleus is to control the gene expression. DNA replication is also mediated by the nucleus during the cell cycle.
Difference Between Nucleolus and Nucleus
Definition
Nucleolus: The nucleolus is a sub-organelle in the nucleus.
Nucleus: The nucleus is a membrane-enclosed, large spherical organelle found in the eukaryotic cells.
Structure
Nucleolus: The nucleolus is composed of dense fibrillar component (DFC), fibrillar center (FC), granular component (GC) and nuclear vacuole.
Nucleus: The nucleus is composed of a nuclear envelope, nuclear pores, nucleoplasm, nuclear lamina, chromosomes, nucleolus and other subnuclear bodies.
Enclosure
Nucleolus: There is no limiting membrane.
Nucleus: This is enclosed by the nuclear envelop.
Chromosomes
Nucleolus: This does not process any chromosomes but it is organized on one chromosome, the nucleolar organizer.
Nucleus: The nucleus consist of chromosomes.
DNA/RNA
Nucleolus: Nucleolus is rich in RNA.
Nucleus: Nucleus is rich in DNA.
Function
Nucleolus: Its main function is ribosome biogenesis, nucleolar detention as a response to cells’ stress and assembling of signal recognition particles.
Nucleus: It’s main function is controlling the gene expression and DNA replication.
Conclusion
During the life time of a cell, some cell nuclei are broken down in the process of nuclear division. Before nuclear division, cellular DNA is replicated. Then, structural components of the nucleus such as nuclear envelope and lamina are degraded systematically and the nucleolus disappears. Next, sister chromatids are separated into opposite poles. After completing the cell division, all the components of the nucleus are regenerated including the nucleolus. Therefore the key difference between nucleolus and nucleus is their organization inside the cell.
Reference:
1. “Nucleolus”. Wikipedia, the free encyclopedia, 2017. Accessed 27 Feb 2017
2. “Cell nucleus”. Wikipedia, the free encyclopedia, 2017. Accessed 27 Feb 2017
Image Courtesy:
1. “0318 Nucleus”By OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Diagram human cell nucleus” By Mariana Ruiz LadyofHats -( Public Domain) via Commons Wikimedia
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