Nucleolus

Introduction

The nucleolus is present within the nucleus of eukaryotic organisms. It constitutes the most significant nucleus, but they do not contain any chromosomes. The nucleolus of the nucleus is the site of ribosome biogenesis. The ribosomal RNA is then transported to the cytoplasm through the nuclear pores. The nucleoli comprise proteins, DNA, RNA. These form the nucleolar organizing region. It is estimated that they participate in the formation of signal recognition particles. These particles play an essential role in the stress response. However, some evidence also shows that these organelles regulate the cell cycle, growth , development, senescence, telomerase activity and gene silencing. 

History 

The discovery of nucleolus in eukaryotic cells was in 1830 by bright field microscopy. However, there was not much knowledge about this structure until John Gurdon and Donald Brown in 1964 gave their studies. Their study includes the clawed frog, Xenopus laevis. It states that 25% of the frog’s eggs, which lack the nucleolus, had no life. On the other hand, the leftover half eggs had one nucleolus and the other half had two nucleoli. These eggs with nucleoli proved their importance for life. Later in 1966, Max L Bernstein experiments showed that cell nucleolus codes for ribosomal RNA. 

Structure of Nucleolus

The nucleoli vary in their shape and size in different organisms. For instance, in yeast, it is less than 1 μm in diameter, whereas, in pea plants, it can be as large as 10 μm in diameter. The nucleolus membrane is dynamic and its primary function is the synthesis of ribosomal RNA. The plant nucleolus is a well-defined structure with functional units such as FC (fibrillar centers), DFC (dense fibrillar component), the GC (granular component), nuclear chromatin, nucleolonema and vacuoles. The structure comprises proteins, RNA and rRNA. The proteins are present in the maximum amount i.e. 85-90%, RNA in 5 to 10%.

The nuclear vacuole is present somewhat in the center of the nucleoli. These structures within the nucleoli are only visible through the electron microscope. The particular unique ion and dynamic activities of nucleoli are seen through the fluorescent protein tagging and fluorescent recovery after photobleaching (FRAP). A human cell usually has 10 nucleolus organizers; however, only one or two nucleons are visible. 

Ribosomal Assembly

 The nucleoli are involved in the process of ribosome biogenesis. For this process, two or three eukaryotic RNAs are needed. The functions are taking place step by step. 

The first step is the transcription of rRNA genes. These transcribe as a single unit within the nucleoli. The transcription is through the RNA polymerase enzyme. Polymerase I-associated factors and DNA-important trans-acting factors are necessary for this transcription. Humans possess a similar PIC assembled with SL1, a selective promoter factor, transcription beginning factors and upstream binding factor. These selective promoter factors are composed of TBP and TBP-associated factors. RNA polymerase transcribes 28S, 18S and 5.8 S. RNA polymerase III transcribes the 60S ribosomal subunit. This transcription produces 45S pre-rRNA. It is a long originator molecule and still holds the ITS and ETS. In further processing,18S RNA, 5.8S and 28S RNA molecules are released. 

Guiding RNA brings the RNA-update enzymes to their specific recognition sites. These guiding RNAs are members of the group of small nucleolar RNAs. These RNAs consist of proteins. To organize into large ribosomal units, an extra pair of rRNA and 5S rRNA is necessary. 

In eukaryotes like plants, the 5S rDNA is present outside the NOR (nucleolus organizer region). The transcription of them is through the RNA polymerase III within the nucleoplasm of the nucleus. Then they enter the nucleolus and help in the ribosomal assembly (assembly consists of rRNA and ribosomal proteins). Polymerase II transcribes these ribosomal proteins in the nucleoplasm. They follow the conventional pathway for protein synthesis. The r-proteins are carried back into the nucleus and then into nucleoli. 

 The 40s and 60s ribosomal units are now present due to the association of rRNA and ribosomal proteins. Nuclear pores act as the passage for transferring these proteins to the cytoplasm. Nuclear pores are the parts of a double-phospholipid nuclear membrane. The proteins can float freely in the cytoplasm or attach to the endoplasmic reticulum. The endoplasmic reticulum with protein units is the rough endoplasmic reticulum. The ER membrane remains connected to the nuclear membrane. 

Functions of Nucleolus

The nucleoli possess several functions within the nucleus. These functions are as follows:

• As discussed earlier, ribosomal biogenesis is the primary function of the cell nucleolus. Polymerase enzymes play an essential role in it
• The cell nucleoli disassemble when cells enter mitosis and reassemble during the cell division
• They also respond towards metabolic changes by altering the ribosomal protein synthesis
• Dysfunctioning of nucleoli can cause several severe diseases. As they can alter the ribosomal activities. They can cause several genetic disorders due to their encoding pattern
• Some studies reveal their importance in the maturation of cellular ribonucleoproteins (RNPs)

Difference between Nucleus and Nucleolus

The nucleolus is the largest structure present within the nucleus. Thus the nucleus is larger than the nucleoli. The nucleus consists of a double phospholipid bilayer, whereas nucleoli have no outer membrane. The nucleus consists of chromosomes as the genetic material, whereas the nucleoli do not have any trace of chromosomes. The nucleus contains the DNA (deoxyribonucleic). On the other hand, nucleoli are rich in RNA (ribonucleic acids). 

Conclusion 

Nucleoli are the largest organelle present within the nucleus. They are present in both plants and animal cells within the nucleus. Its primary function involves ribosomal biogenesis. Their structure comprises fibrillar centers, granular components, dense fibrillar components and clear spaces called nucleolar vacuoles. RNA polymerase enzymes play a vital role in the biogenesis of ribosomes. The ribosomal assembly starts with the polymerase enzyme  transcribing the pre RNA