Present in all living cells, from bacteria to humans, ribosomes are molecular machines that translate the genetic code from messenger RNA into functional proteins. A single human cell can contain up to 10 million of these structures. This process is fundamental to life, as proteins perform a vast array of functions, including catalyzing metabolic reactions, replicating DNA, and providing structural support to cells.
The Structure of a Ribosome
A ribosome is a complex structure composed of ribosomal RNA (rRNA) and proteins, making it a ribonucleoprotein. Its structure consists of two distinct subunits, one large and one small, which come together to form a functional unit. The small subunit is responsible for binding and decoding the messenger RNA (mRNA) molecule, while the large subunit is where amino acids are linked together to form a protein chain.
The size of ribosomes differs between prokaryotic organisms like bacteria and eukaryotic organisms like humans. In prokaryotes, ribosomes are referred to as 70S ribosomes, with the “S” representing a Svedberg unit, a measure of sedimentation rate. Eukaryotic ribosomes are larger, designated as 80S. These structural differences are significant and have been exploited in the development of antibiotics that specifically target bacterial ribosomes.
The Process of Protein Synthesis (Translation)
The synthesis of proteins, known as translation, is a multi-step process orchestrated by the ribosome. It begins with initiation, where the small ribosomal subunit binds to an mRNA molecule. This is followed by the binding of the large ribosomal subunit, forming a complete ribosome ready for protein production. This assembly occurs at a specific start sequence on the mRNA, ensuring the protein is built correctly from the beginning.
Once assembled, the ribosome enters the elongation phase, moving along the mRNA and reading its genetic code in three-nucleotide segments called codons. Each codon specifies a particular amino acid, which is brought to the ribosome by a transfer RNA (tRNA) molecule. The tRNA has a corresponding three-nucleotide anticodon that pairs with the mRNA codon, ensuring the correct amino acid is added to the growing protein chain.
The ribosome facilitates the formation of a peptide bond between the newly arrived amino acid and the growing polypeptide chain. This reaction is catalyzed by the ribosomal RNA (rRNA) within the large subunit. The ribosome then moves to the next codon on the mRNA, a process called translocation, and the cycle of tRNA binding and peptide bond formation repeats.
The final stage of translation is termination. This occurs when the ribosome encounters a stop codon on the mRNA sequence. Release factor proteins recognize this signal and prompt the ribosome to release the newly synthesized polypeptide chain. The ribosomal subunits then separate from the mRNA and each other, ready to be used again.
Location and Specialization of Ribosomes
Ribosomes are found in two main locations within a eukaryotic cell, and this placement determines the destination of the proteins they synthesize. Free ribosomes are found suspended in the cytoplasm, the fluid-filled space within the cell. These ribosomes produce proteins that will function within the cytoplasm itself or in other organelles like the mitochondria and nucleus.
Other ribosomes are bound to the surface of the endoplasmic reticulum, a network of membranes within the cell, creating a region known as the rough endoplasmic reticulum. The proteins made by these bound ribosomes are destined for different locations. They can be inserted into the cell’s membranes, secreted out of the cell to act as hormones or enzymes, or delivered to specific organelles like the lysosome.
The determination of whether a ribosome remains free or becomes bound to the endoplasmic reticulum depends on the protein it is synthesizing. A short signal sequence on the growing polypeptide chain directs the ribosome to the endoplasmic reticulum. This ensures that proteins are sorted and delivered to their correct functional locations within or outside the cell.
Ribosomes in Health and Disease
Proper ribosome function is necessary for health, and defects in these molecular machines can lead to a class of genetic disorders known as ribosomopathies. These conditions arise from mutations in genes that code for ribosomal proteins or rRNA, or in factors involved in ribosome assembly. The resulting dysfunctional ribosomes can lead to a range of symptoms and diseases.
The structural differences between prokaryotic and eukaryotic ribosomes are a primary target for many antibiotics. Drugs such as streptomycin are designed to specifically inhibit the function of bacterial 70S ribosomes, halting protein synthesis and preventing the bacteria from growing. This selective targeting allows these medicines to combat bacterial infections without harming the patient’s own 80S ribosomes.
Conversely, some toxins and viruses have evolved to exploit the ribosome’s function for their own purposes. For example, the polio virus can hijack the host cell’s translational machinery to produce its own viral proteins. This leads to the replication of the virus and the progression of the disease.