Ribosomes are molecular machines found within all living cells, serving as the sites for biological protein synthesis. These complex cellular structures are fundamental to life, converting genetic information into the proteins that carry out virtually all cellular functions. Human ribosomes are composed of two distinct parts, known as subunits, which must work together precisely for every cell to function and survive.
What Are Human Ribosome Subunits
Human ribosomes are cellular components, each composed of a large subunit and a small subunit. These two parts exist separately within the cell’s cytoplasm until they are needed for protein production. The small subunit in eukaryotes is referred to as the 40S subunit, while the large subunit is known as the 60S subunit. The “S” refers to Svedberg units, a measure of sedimentation rate.
Each subunit is an assembly of specialized RNA molecules, called ribosomal RNA (rRNA), and many proteins, known as ribosomal proteins (r-proteins). For example, the human 40S small subunit contains an 18S rRNA molecule and approximately 33 proteins. The larger 60S subunit is even more complex, comprising 5S, 5.8S, and 28S rRNA molecules along with about 49 proteins. These RNA molecules provide the ribosome with its basic form and function, while proteins enhance protein synthesis.
The Role of Ribosome Subunits in Protein Production
The primary function of ribosome subunits is protein synthesis, a process also known as translation. This process begins with the small ribosomal subunit binding to a messenger RNA (mRNA) molecule. The mRNA carries the genetic instructions copied from DNA in the cell’s nucleus, dictating the specific sequence of amino acids needed to build a protein. The small subunit effectively decodes this genetic message.
Following the small subunit’s binding to the mRNA, the large ribosomal subunit then joins to form a complete ribosome. This assembly creates a molecular “workbench” where amino acids are linked together. Within this complete ribosome, there are three sites: the A (aminoacyl), P (peptidyl), and E (exit) sites, which facilitate the movement of transfer RNA (tRNA) molecules.
As the ribosome moves along the mRNA strand, it reads the genetic code in three-nucleotide units called codons. Each codon specifies a particular amino acid, and transfer RNA (tRNA) molecules, each carrying a specific amino acid and a complementary anticodon, are recruited to the ribosome. The large subunit contains the peptidyl transferase center, which catalyzes the formation of peptide bonds between adjacent amino acids, progressively building the polypeptide chain. This cooperative action ensures the accurate and efficient production of proteins.
How Ribosome Subunits Are Assembled
The process of forming new ribosome subunits, known as ribosomal biogenesis, primarily occurs within a structure inside the cell nucleus called the nucleolus. This process is coordinated, involving the transcription, processing, and modification of ribosomal RNA (rRNA) molecules, alongside their association with many ribosomal proteins. The initial step involves RNA polymerase I transcribing large precursor rRNA molecules from ribosomal DNA genes.
These nascent rRNA molecules then undergo processing and modification to yield the mature 18S, 5.8S, and 28S rRNAs. Simultaneously, ribosomal proteins, which are synthesized in the cytoplasm, are imported back into the nucleus and then into the nucleolus. These proteins associate with the maturing rRNA molecules, forming pre-ribosomal particles.
The small (pre-40S) and large (pre-60S) ribosomal subunits mature within the nucleolus. Once assembled, these pre-subunits are exported from the nucleus through nuclear pores into the cytoplasm. There, they are ready to combine and engage in protein synthesis.
When Ribosome Subunits Malfunction
Errors in the structure, assembly, or function of ribosome subunits can have consequences for human health. Such malfunctions can lead to a group of human diseases collectively termed “ribosomopathies.” These conditions arise when the cell’s ability to produce or utilize ribosomes is impaired, affecting protein synthesis.
The impact of ribosomopathies can manifest as reduced overall protein production, the creation of faulty proteins, or imbalances in the levels of specific protein types within the cell. This disruption to cellular protein factories can lead to a range of clinical issues. Examples include inherited bone marrow failure syndromes, where the production of blood cells is compromised, such as Diamond-Blackfan anemia and Shwachman-Diamond syndrome.
Ribosomopathies are also linked to various developmental disorders, affecting growth and organ development, like Treacher Collins syndrome. Additionally, some of these conditions are associated with an increased susceptibility to certain cancers, including leukemias and solid tumors. The proper functioning of ribosome subunits is connected to maintaining cellular health and human well-being.