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Nucleolus, Practice Problems and FAQs

You must be aware of how important proteins are for the growth, repair and maintenance of our body. Do you remember which cell organelle is responsible for synthesis of proteins in our body? Well, these organelles are widely popular as the ‘protein factories’ of the cell. Did it ring a bell? Yes, the ribosomes.

Do you know where these ribosomes are formed? We know that proteins are synthesised in the cytoplasm or on the surface of the rough endoplasmic reticulum. So are ribosomes synthesised here too? Well, no.

There is a special region inside the nucleus where the ribosomes are formed and assembled. It is called nucleolus. While observing a cell, we can see a dark spot inside the nucleus which is the nucleolus. Since nucleus is not seen in the prokaryotic cell, nucleolus is seen only in eukaryotic cells. So in this article we are going to discuss more about the nucleolus, which can be considered as a nuclear organelle.


                               Fig: Eukaryotic cell

Table of contents:

  • Structure of nucleus
  • Nucleolus
  • Structure of nucleolus
  • Function of nucleolus
  • Practice Problems
  • FAQs

Structure of nucleus

The most essential part of the cell, which stores the genetic material of the organism is the nucleus. It is the brain of the cell. The major parts of a nucleus include the following:

  • Nuclear envelope
  • Nuclear matrix
  • Nucleoplasm
  • Chromatin reticulum
  • Nucleolus

Nuclear envelope is the membrane of the nucleus which separates the nuclear materials from the cytoplasm. Nuclear matrix is the fine criss cross fibres which forms a nuclear skeleton and provides the support and anchor to the chromatin fibres. Nucleoplasm is the transparent semi fluid substance inside the nucleus. The lightly stained thread like structures in the nucleoplasm is called chromatin reticulum and the spherical refringent body in the nucleoplasm is called nucleolus.


                                            Fig: Structure of nucleus

Nucleolus

The dense, spherical or slightly irregular body in the nucleoplasm during interphase is called nucleolus. It is a thin thread-like network of nucleo-protein fibres which is not bound by a membrane, hence it is called a naked structure. Fonatana was the one who first described the nucleolus in 1781. The detailed study on nucleolus was made by Wagner in 1840 and the term nucleolus was first used by Bowman in 1848.


                                            Fig: Nucleolus

Position of nucleolus

Some chromosomes have a nuclear organisational region or NOR. Nucleolus is formed by such chromosomes in the region of the NOR. Secondary constrictions in the chromosomes are represented by the NOR. The narrow region found at any point of a chromosome except the centromere are called secondary constrictions. However, every nucleolar chromosome does not produce nucleolus. Different chromosomes may produce nucleolus at different stages. In human somatic cells, there are five pairs of nucleolar chromosomes ( 13, 24, 15, 20 and 21) but only one nucleolus is present. Several copies of genes for 18S and 28S ribosomal RNA are present in the nucleolus.

Number of nucleolus

The number of nucleoli in cells depends on their metabolic states. Diploid cells can have one to six nucleoli. In very active cells, many nucleoli are found. In the oocyte of Xenopus we can observe 1600 nucleoli. Nucleoli are absent in reticulocytes and do not develop in blastomeres, yeast and primitive algae. Calcium is essential for the maintenance of the nucleus. During prophase of cell division, the nucleolus disappears and reappears after its formation during the telophase.

Size of nucleolus

The synthetic activity of the cell and the size of the nucleolus are directly related. Nucleolus is large in the highly active cells like the growing embryonic cells, oocytes and the cells with high protein synthesis. Nucleolus is small and ring shaped in less active cells like muscle cells, sperm cells etc and also in the cells with less protein synthesis.

Structure of nucleolus

The ultrastructure of a nucleolus has four components and they are as follows:

  • Amorphous matrix
  • Granular region
  • Fibrillar region
  • Chromatin part

Amorphous matrix

The proteinaceous, homogenous and colloidal ground substance in which the granules and fibrils are scattered is called an amorphous matrix.

Granular region

The peripheral region of the amorphous matrix where the granules are scattered is called the granular region. The size of the granules ranges from 150 Å to 200 Å. The major components of granules are proteins and RNA in the ratio 2:1 and these are thought to be the precursors of ribosomes.

Fibrillar region

The central region of the amorphous matrix where we can find a large number of small fibrils is called the fibrillar region. The length of the fibrils ranges from 50 Å to 80 Å. The components of fibrils are ribosomal RNA or rRNA and proteins and they are believed to be the precursors of granules.

Chromatin part

It is chromatin associated with nucleolus. Nucleolar chromatin is of two types - perinucleolar and intranucleolar. Perinucleolar chromatin lies around the nucleolus. It sends out projections into the nucleolus, within which lies the intra-nucleolar chromatin. Nucleolar chromatin contains DNA which contains genes for rRNAs and tRNAs.


                                            Fig: Structure of nucleolus

Functions of nucleolus

The major functions of the nucleolus are as follows:

  • The primary function of the nucleolus is the synthesis of rRNA.
  • Nucleolus facilitates the biogenesis of the subunits of ribosomes.
  • It receives ribosomal proteins from the cytoplasm for storage.
  • It plays an important role in the cell division, because it is essential for the formation of spindle fibres during the nuclear division.
  • Synthesis of tRNA and their initial processing.

Practice Problems

1. Which is the spherical refringent body in the nucleoplasm?

  1. Nuclear matrix
  2. Nuclear envelope
  3. Chromatin reticulum
  4. Nucleolus

Solution: The most essential part of the cell, which stores the genetic material of the organism is called nucleus. It is the brain of the cell. The major parts of a nucleus include the nuclear envelope, nuclear matrix, nucleoplasm, chromatin reticulum and nucleolus. Nuclear envelope is the membrane of the nucleus which separates the nuclear materials from the cytoplasm. Nuclear matrix is the fine criss cross fibres which forms a nuclear skeleton and provides the support and anchor to the chromatin fibres. Nucleoplasm is the transparent semi fluid substance inside the nucleus. The lightly stained thread like structures in the nucleoplasm is called chromatin reticulum and the spherical refringent body in the nucleoplasm is called nucleolus. Hence the correct option is d.

2. Which of the following is/ are the correct statements about nucleolus?

I) It is a thin thread-like network of nucleo-protein fibres which is not bound by a membrane.
II) The term nucleolus was first used by Bowman in 1848.
III) Nucleolus is attached with the NOR of particular chromosomes.
IV) In each diploid cell, only one nucleolus can be present.
V) The synthetic activity of the cell and the size of the nucleolus are directly related.

  1. I, II, III, IV, V
  2. I, II, III, V
  3. I, II and III
  4. I, III and V

Solution: The dense, spherical or slightly irregular body in the nucleoplasm during interphase is called nucleolus. It is a thin thread-like network of nucleo-protein fibres which is not bound by a membrane, hence it is called a naked structure.

Fonatana was the one who first described the nucleolus in 1781. The detailed study on nucleolus was made by Wagner in 1840 and the term nucleolus was first used by Bowman in 1848.

Chromosomes have a nuclear organisational region or NOR. Nucleolus is attached with the NOR of particular chromosomes. Secondary constrictions in the chromosomes are represented by the nucleolus.

The number of nucleoli in cells depends on their metabolic states. Diploid cells can have one to six nucleoli. In very active cells, many nucleoli are found. In the oocyte of Xenopus we can observe 1600 nucleoli.

The synthetic activity of the cell and the size of the nucleolus are directly related. Nucleolus is large in the highly active cells like the growing embryonic cells, oocytes and the cells with high protein synthesis. Nucleolus is small and ring shaped in less active cells like muscle cells, sperm cells etc and also in the cells with less protein synthesis. Hence the correct option is b.

3. Which of the following can be seen in the peripheral region of the amorphous matrix?

  1. Granular region
  2. Fibrillar region
  3. Chromatin part
  4. All the above

Solution: The ultrastructure of a nucleolus has four components and they are amorphous matrix, granular region, fibrillar region and chromatin part. The proteinaceous and homogenous ground substance in which the granules and fibrils are scattered is called an amorphous matrix. The peripheral region of the amorphous matrix where the granules are scattered is called the granular region. The central region of the amorphous matrix where we can find a large number of small fibrils is called the fibrillar region. The chromatin part is the chromatin associated with nucleolus. Nucleolar chromatin is of two types - perinucleolar and intranucleolar. Perinucleolar chromatin lies around the nucleolus. It sends out projections into the nucleolus, within which lies the intra-nucleolar chromatin. Hence the correct option is b.

4. Assertion: Nucleolus plays an important role in the cell division.

Reason: Nucleolus is essential for the formation of spindle fibres.

Which of the following statements are correct about the assertion and reason given above?

  1. Both assertion and reason are correct and reason is the correct explanation of the assertion.
  2. Both assertion and reason are correct and reason is not the correct explanation of the assertion.
  3. Assertion is correct, but the reason is wrong.
  4. Both assertion and reason are wrong.

Solution: The dense, spherical or slightly irregular body in the nucleoplasm during interphase is called nucleolus. It is a thin thread-like network of nucleo-protein fibres which is not bound by a membrane, hence it is called a naked structure. The major function of the nucleolus is the synthesis of rRNA. It also plays an important role in the cell division, because it is essential for the formation of spindle fibres during the nuclear division. Hence the correct option is a.

FAQs

1. Do every cell have a nucleolus?
Answer:
All the eukaryotic cells have nucleolus present inside the nucleus. But prokaryotic cells lack a well defined nucleus and nucleolus.

2. What are the differences between nucleus and nucleolus?
Answer:
Nucleolus are present inside the nucleus and they have different structures and functions. The major differences between a nucleus and nucleolus are as follows:

Nucleus

Nucleolus

Larger size than nucleolus

Smaller size than nucleus

Surrounded by a nuclear envelope

Envelope is absent

Chromosomes are present

Chromosomes are absent

It is rich in DNA

It is rich in RNA

3. How nucleolus is formed?
Answer:
Nucleolus is formed through the transcription of 45S pre-rRNA which guides the fusion of small pre-nucleolar bodies which contains the processing factors and the other components needed for the formation of the nucleolus. Initially there will be many nucleoli and then later fuse to form a single nucleolus.

4. Why is the nucleolus dark coloured?
Answer:
While staining with the DAPI (4,6- diamino-2-phenylindole) DNA stain, the nucleolus appears as a dark region with low DNA density. This is due to the abundance of ribosomal RNA and the proteins that assemble throughout the nucleolus.

YOUTUBE LINK: https://www.youtube.com/watch?v=tdoS1Iglj38 

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