Genetic basis: nucleolar organizers
The formation of the nucleolus does not occur in random locations of the nucleus, but at strictly defined chromosomal regions—nucleolar organizers (NORs). Essentially, these are clusters consisting of multiple copies of genes encoding ribosomal RNAs.
In humans, there are exactly 10 nucleolar organizers. They are located in the region of the secondary constrictions of five chromosome pairs: 13th, 14th, 15th, 21st, and 22nd.
- Genetic content: These zones concentrate the genes for three types of rRNA (28S, 18S, and 5.8S). They are grouped into a single cluster and are always transcribed together. It is important to note that the gene for the fourth type of rRNA (5S) is located and transcribed completely separately.
- Copy number: According to various estimates, there are anywhere from 200 copies per gene to values exceeding this figure by an order of magnitude.
- Structural dynamics: Despite having 10 organizers, one rarely sees exactly 10 separate nucleoli in the nucleus. Depending on the degree of aggregation (clustering) of these regions, they can fuse into a single massive nucleolus or distribute into several groups. This is why the visible number of nucleoli is variable.
Functional activity: from transcription to assembly
The nucleolus functions as a conveyor belt for producing ribosomal components. This process can be divided into three consecutive stages:
- Transcription. On the gene clusters within the NOR region, continuous and very active synthesis of a single long precursor molecule—pre-rRNA—takes place. This molecule is unique in containing nucleotide sequences for three future types of ribosomal RNA simultaneously.
- Processing (maturation). Right inside the nucleolus, the single precursor chain is cleaved into individual functional rRNA molecules, which also undergo necessary chemical modification. Simultaneously, the formation and maturation of a separate pre-5S-rRNA molecule take place.
- Subunit assembly. Mature rRNA molecules combine with special ribosomal proteins. These proteins are not synthesized in the nucleus—they arrive here from the cytoplasm. As a result of this interaction, finished ribosomal subunits are formed, which then leave the nucleus and head back to the cytoplasm to participate in translation.
Ultrastructure of the nucleolus
When studying the nucleolus using electron microscopy, it becomes evident that its morphology is strictly subordinate to its functions. Three key structural components are distinguished:
- Amorphous component: Reflects the genetic base. These are directly the chromatin regions (DNA of the nucleolar organizers) physically associated with the nucleolus.
- Fibrillar component: Reflects the transcription process. It consists of the finest RNA filaments—molecules of synthesized pre-rRNA and already mature rRNAs.
- Granular component: Reflects the assembly stage. It appears as an accumulation of small grains or granules—these are nothing other than the maturing subunits of future ribosomes.
Identification in light microscopy
Nucleoli have high optical density, making them easy to spot even when using routine histological stains.
However, for specific identification, a special histochemical method is used—Brachet's reaction. This method aims at the selective staining of RNA. Because the concentration of ribosomal RNA and its precursors is extremely high in the nucleolus, against the background of weakly stained remaining nuclear components, the nucleolus acquires a rich, intense crimson color.
Briefly: answers to topical questions
Where exactly and on which chromosome is the 5S-rRNA gene transcribed?
The 5S-rRNA gene is located and transcribed separately from the gene cluster of the main ribosomal RNA types (28S, 18S, 5.8S). The exact chromosome and specific transcription site in the cell are not specified. It is only known that parallel to the processing of the single precursor of other rRNAs, the formation and maturation of pre-5S-rRNA take place.
In which phases of the cell cycle (mitosis) does the nucleolus disappear and reappear?
The nucleolus disappears in prophase of the mitotic cycle and reappears in telophase. These structures are transient formations whose dynamics are strictly tied to the cell division phases:
- Prophase — nucleoli decrease in size, fragment (break down), and finally disappear at the end of the period.
- Telophase — the process of restoration and formation of new nucleoli takes place.
Which specific proteins and RNP complexes carry out pre-rRNA processing in the nucleolus?
Sources explicitly name fibrillarin as a marker protein of the fibrillar component involved in rRNA processing. It is also indicated that the fibrillar component contains rRNA and ribonucleoproteins, and among the nucleolar proteins are ribosomal proteins, RNA polymerase I, transcription factors, and nucleases. Specific RNP complexes carrying out pre-rRNA processing are not named in the sources.
To memorize the numbers of chromosome pairs with nucleolar organizers (13, 14, 15, 21, 22), note their sequence: three consecutive numbers from 13 to 15, and two consecutive numbers from 21 to 22.
Class and exam questions
Is the nucleolus a membrane-bound organelle?
No, the nucleolus is not an independent organelle at all. It is a dense derivative of chromatin lacking its own membrane.
Why can cells of the same type have a variable number of nucleoli?
There are 10 nucleolar organizers in total within the nucleus. They can fuse with each other into a single large cluster or form several groups, which causes the number of nucleoli visible under a microscope to constantly change.
Where do the proteins for ribosome assembly in the nucleolus come from?
Ribosomal proteins are not synthesized inside the nucleus. They are produced in the cell cytoplasm and then transported into the nucleolus to combine with mature rRNAs.
What does Brachet's reaction stain and what color does it produce?
Brachet's reaction specifically detects RNA. Due to the huge amount of rRNA, nucleoli stain a bright crimson color.
What else to study on this topic
- Molecular mechanisms of pre-rRNA transcription
- Stages of processing and chemical modification of ribosomal RNA
- Transport of ribosomal subunits through nuclear pores into the cytoplasm
- Structure and functions of 5S-rRNA
- Correlation of nucleolar activity with the intensity of protein synthesis in the cell
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