Transcription: the beginning of the path
The biosynthesis process starts with the transcription stage. The key player here is a specialized enzyme—RNA polymerase I. This very enzyme is responsible for reading information and generating the primary product.
The result of RNA polymerase I activity is the formation of a high-molecular-weight ribosomal RNA precursor. This giant molecule is designated as 45S rRNA. At this stage, it is a single long chain that embeds the structural foundations for future parts of the ribosome. This precursor serves as a sort of framework around which all assembly events will subsequently unfold.
Formation of the primary RNP complex
As soon as the 45S rRNA molecule begins to be synthesized, it does not remain alone. As the chain elongates, this RNA immediately interacts with numerous ribosomal proteins. It is important to note that these proteins themselves are not produced locally—they come from the cytoplasm of the cell, where they were synthesized earlier.
- As a result of the active binding of RNA with proteins around the central region, a specific structure is formed—the dense fibrillar component zone.
- The outcome of this stage is the formation of a large ribonucleoprotein (RNP) complex.
- This complex has a size of 80S and is characterized by the fact that it packages exactly one molecule of the 45S rRNA precursor, tightly bound to the incoming cytoplasmic proteins.
Processing: cleavage of the 80S complex
The formed 80S complex is not capable of performing ribosomal functions in this form. It needs to undergo the processing stage—a specific cleavage during which the huge conglomerate is cut into precursors of future ribosomal subunits.
As a result of this finely tuned cleavage, two unequal parts are formed:
- Large subunit (60S): It incorporates two fragments of the cleaved 45S rRNA—these are 28S and 5.8S. However, to fully complete the assembly of the large subunit, another component is required. A third fragment joins it—5S rRNA. A crucial feature is that this 5S rRNA is synthesized completely separately—on other chromosomes, and this process proceeds independently of nucleolar organizers.
- Small subunit (40S): It has a simpler structure in terms of RNA components. After cleavage of the 80S complex, it takes only a single fragment of the original precursor—18S rRNA.
Transport of finished subunits
After processing is completed and both subunits (60S and 40S) are fully formed, their stay at the assembly site ends. The final stage begins—transport.
The finished precursors of the small and large subunits head toward the nuclear envelope. They squeeze through nuclear pores one by one and enter the cytoplasm. It is there, in the cytoplasmic space, that they will subsequently meet to assemble into a fully functioning ribosome and begin the synthesis of cellular proteins.
Briefly: answers to topical questions
On which human chromosomes are the nucleolar organizers encoding 45S rRNA located?
Human nucleolar organizers are located on chromosomes 13, 14, 15, 21, and 22.
These regions are secondary constrictions located on the short arms of acrocentric chromosomes. It is in these zones that the genes containing DNA for the synthesis of the high-molecular-weight ribosomal RNA precursor—45S RNA—are localized. During interphase, the nucleolus forms around these chromosome regions.
The composition is easy to remember: The small subunit (40S) is modest and takes only one number (18S). The large subunit (60S) is a "collector", gathering 28S, 5.8S, and also inviting a "guest" from other chromosomes—5S.
Class and exam questions
Where do the proteins for the formation of the 80S complex come from?
Ribosomal proteins are not synthesized in the nucleus. They are formed in the cell cytoplasm, after which they are specially transported to the ribosome assembly site, where they bind to 45S rRNA.
Which enzyme is responsible for the creation of 45S rRNA?
The synthesis of the high-molecular-weight precursor (45S rRNA) is carried out by the enzyme RNA polymerase I.
How is 5S rRNA unique compared to other fragments?
Unlike the 18S, 28S, and 5.8S fragments, which are produced by cutting a single 45S rRNA, the 5S rRNA molecule is synthesized on other chromosomes independently of the nucleolar organizers.
What else to study on this topic
- Mechanism of action and regulation of RNA polymerase I
- Structure of the dense fibrillar component zone
- The process of transport through nuclear pores
- Localization and role of nucleolar organizers
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