Nucleus And Nucleolus? 7 Facts You Should Know

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Nucleus and nucleolus are two cell organelles of living organisms found inside the cell surrounded by a nuclear membrane. Let us have a detailed look at nucleus and nucleolus.

Nucleus is the double membranous structure that carries out all the functional activities of the cell while nucleoli are the structures suspended within the nucleus embedded in nucleoplasm having ribosomal RNA involved in synthesising ribosomes.

The human cell does not possess a nucleus (enucleated), paramoecium has two nuclei i.e. macronuclei and micronuclei while some fungi are multinucleate. Nucleus possesses chromosomes laced with DNA which undergoes replication to double its number. But nucleolus is a small compartment inside the nucleus. They both are absent in bacteria and all the procaryotes.

Where is nucleolus found?

Nucleolus is another cell organelle like all other organelles found in a eucaryotic cell. Let us see where nucleolus is found inside a cell.

Nucleolus is found inside the nucleus. They are small spherical structures absent in procaryotes. Bacterial cells can synthesise their ribosomes through nucleolus. It is the largest subsection of a cell body.

Nucleolus forms a specific region on chromosomes. The site for ribosomal biogenesis and transcription is named as nucleolar organization region (NORs).

How nucleolus is related to the nucleus

Nucleus and nucleolus are related to each other in many ways. Let us see some reason behind this.

Listed below are the facts that shows the relation between nucleus and nucleolus.

  • Nucleolus is related to the nucleus as they are the site for transcription and translation with 100% fidelity.
  • Both nucleus and nucleolus involves in proof-reading & coding of transcribed messenger RNA
  • They also lead to the polymerization of amino acids followed by post-translational modifications.
  • They both have some precursors for ribosomal biogenesis.
  • They both are essential cell organelles as they both play a vital role in the process of central dogma and transmission of heredity.
  • They also get simultaneous in action during karyokinesis as well as in cytokinesis. 

Difference between nucleus and nucleolus

Despite of their similarities, nucleus and nucleolus are different in many aspects. Some are mentioned below.

On the basis of- NUCLEUS NUCLEOLUS 
Location  The well-defined structure inside the cell A dark spot within the nucleus 
Membrane Double membrane Lack boundaries 
Chromosome Present Absent 
Size Largest organelle Small section 
Nucleic acid Rich in DNA Rich in RNA 
Space occupied Almost 10% of the cell ~0.02% of the nucleus 
Difference beween nucleus and nucleolus

Function of nucleolus within the nucleus

Nucleus and nucleolus complement each other in terms of functions performed. Let us see the functions performed by nucleolus within the nucleus.

  • Nucleolus is involved in ribosomal synthesis as they require the assembly of ribosomal precursor RNA and rRNA, within the nucleus.
  • The chromatin found inside the nucleolus possesses the encoded genes for the maturation and processing of cytoplasmic ribosomes.
  • After the formation of ribosomes inside the nucleus, they get released from nucleoporins of the nucleus and adhere to the walls of the endoplasmic reticulum, forming a rough endoplasmic reticulum where proteins are synthesised.
  • Nucleolus also maintains integrity and regulates the genomic expression within the nucleus.
  • Nucleolus plays an important role in response to stress, and the processing of small interfering RNA (siRNA).

Structure and composition of nucleus

Nucleus is the storehouse of hereditary characters in the form of DNA. Let us see in detail the structure and composition of the nucleus.

The structure and composition of nucleus is listed below:

  1. Nucleoplasm: Condensed and colloidal solution stored in the nucleus is termed nucleoplasm. Beside nucleolus, some other compartments are also found i.e., Cajal bodies, Gemini of coiled bodies and interchromatin granule clusters.
  2. Nuclear envelope: The outer and inner layers of the nucleus which surround the genomic contents. The space between the two nuclear membranes is known as “perinuclear space” which provides stiffness and rigidity to the nucleus. The inner one is around the chromatin while the outer membrane is linked with the endoplasmic reticulum.
  3. Nucleoporins: These are the channels present in between the nuclear membranes. They connect the cytosol with the inner matrix to transport the ions and other large molecules in and from directions.
  4. Nuclear lamina: It is the dense network of intermediate filaments to provide mechanical support to the nucleus. ‘Lamins” are the proteins produced in the cytoplasm and present at the cytosolic face of the envelope to support the structure by interconnected with the cytoskeleton.
  5. Chromosomes: It’s the genetic material having deoxyribonucleic acids and plays a major role in the functioning of the central dogma. It is divided during the cell division of an organism.
nucleus and nucleolus
Structure of Nucleus
Image credit: Wikimedia commons

Structure and composition of nucleolus

Nucleolus is divided into three most prominent compartments. Let us learn about the structure of nucleolus in detail.

Nucleolus are composed of the fibrillar centres and the granular centres. The fibrillar centres (FC) are surrounded by thick fibrillar part and the granular part (GC), is the part in which the FC and DFC are embedded.

  • Fibrillar centres(FC)- These contain unengaged RNA polymerase I transcription factors.
  • Dense fibrillar components(DFC)- Mostly pre-RNA processing factors are present. It also contains fibrillarin and nucleolin which facilitates the ribosomal assembly.
  • Granular components(GC)- It contains a protein, nucleophosmin which helps in the transcription of ribosomal DNA and leads to maturation and assembly. Some guide RNAs are also present near the boundaries of FC and DFC and are known as small-nucleolar ribonucleoproteins (snoRNPs) help in further processing.

Conclusion

In this article, we have provided some facts related to nucleus and nucleolus as they both are absent in prokaryotic cell and regulates major functions in cellular responses. They control the growth and development and nucleolus carries the steps of transcription and protein synthesis.

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