The nucleolus is responsible for the manufacturing of ribosomes within the cell. Nucleous is a small subspace within the nucleus. DNA to the nucleolus consists of ribosomal RNA (rRNA) genes. Also, after the transcription, ribosomes assemble within the nucleolus. Ribosomal RNA assemble with ribosomal proteins to produce a ribosome. Ribosomal proteins are synthesized in the cytoplasm in a process called protein synthesis. The assembled ribosomes exist the nucleus through the nuclear pores. The main function of ribosomes is to assist translation, the second step of protein synthesis.
Key Areas Covered
1. What Cellular Structure is Responsible for Manufacturing Ribosomes
– Ribosome Biogenesis
2. What is Ribosome Biogenesis
– Eukaryotic and Prokaryotic Ribosome Biogenesis
Key Terms: Large Subunit, Nucleolus, Ribosomal RNA (rRNA), Ribosomal Proteins, Ribosomes, Small Subunit
What Cellular Structure is Responsible for Manufacturing Ribosomes
The process of ribosome synthesis is known as ribosome biogenesis and it occurs in the nucleolus in eukaryotes. The nucleolus is responsible for rRNA synthesis and processing. In prokaryotes, it occurs in the cytoplasm. Both eukaryotic and prokaryotic ribosome biogenesis occur by the synthesis of two subunits of the ribosome followed by the assembly of two subunits. Eukaryotic ribosome biogenesis is a complex process when compared to that of prokaryotes.
Figure 1: Nucleolus
What is Ribosome Biogenesis
Ribosome Biogenesis in Eukaryotes
Eukaryotic ribosomes are the 80S ribosomes, consisting of 60S large subunit and 40S small subunit. The large subunit consists of three types of rRNAs (25S in plants or 28S in mammals, 5.8S, and 5S) and about 47 ribosomal proteins. The small subunit consists of one rRNA (18S) and about 33 ribosomal proteins.
Figure 2: Eukaryotic Ribosome
In eukaryotes, ribosome biogenesis occurs in two major processes. They are the synthesis of ribosomal proteins and the transcription of ribosomal RNA.
Synthesis of ribosomal proteins – Ribosomal DNA (rDNA) encodes ribosomal proteins. The translation of ribosomal protein mRNA occurs in the cytoplasm. The synthesized ribosomal proteins are imported through nuclear pores into the nucleus.
Transcription of rRNA – The nucleolus consists of 45S rRNA genes, which are transcribed in higher rates. However, 5S rRNA genes occur apart from the nucleolus.
The transcribed rRNA assembles with ribosomal proteins to form large and small ribosome subunits. Both types of subunits travel out from the nucleus into the cytosol through nuclear pores. The assembly of large and small subunits occurs in the cytosol.
Ribosome Biogenesis in Prokaryotes
70S ribosomes, which consist of 50S large and 30S small subunits, are prokaryotic ribosomes. The large subunit consists of two types of rRNAs (23S and 5S) and 33 ribosomal proteins. The small subunit consists of only 16S rRNA and 20 ribosomal proteins.
52 genes of the prokaryotic genome encode the ribosomal proteins. They belong to 20 different operons. Transcription of the ribosome gene operons occurs in the cytoplasm. The synthesis of ribosomal proteins as well as the assembly of large and small subunits of the ribosome occurs in the cytoplasm in prokaryotes.
Conclusion
The nucleolus is the cellular structure responsible for the manufacturing of ribosomes in eukaryotes. It is a structure inside the nucleus. In prokaryotes, it occurs completely in the cytoplasm. A ribosome consists of a large and small subunit. Each subunit is made up of rRNA and ribosomal proteins.
Reference:
“Ribosome.” Gale Library of Daily Life: Slavery in America, Encyclopedia.com, 2018, Available here.
Image Courtesy:
1. “Diagram human cell nucleus” By Mariana Ruiz LadyofHats (Public Domain) via Commons Wikimedia
2. “80S 2XZM 4A17 4A19” By Fvoigtsh – Own work (CC BY-SA 3.0) via Commons Wikimedia
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