The ribosome is a fundamental cellular machine present in all living cells. Its primary role involves translating genetic instructions carried by messenger RNA (mRNA) into functional proteins, enabling cells to build and maintain their structures and perform various biological tasks.
Structural Components of the Ribosome
Every ribosome is made of two main parts: a large subunit and a small subunit. These subunits are not always joined but come together only when the ribosome is actively engaged in making a protein. Each subunit is an assembly of ribosomal RNA (rRNA) molecules and numerous proteins.
The size of these ribosomal components is often described using Svedberg (S) units, which reflect how quickly a particle settles when spun in a centrifuge. This unit measures a combination of mass, density, and shape, providing a way to categorize ribosomal particles.
Prokaryotic and Eukaryotic Ribosomes
There are notable differences between the ribosomes found in prokaryotic cells (like bacteria) and eukaryotic cells (like human cells). Prokaryotic ribosomes are generally smaller, classified as 70S ribosomes, while eukaryotic ribosomes are larger, known as 80S ribosomes. This size distinction arises from differences in their rRNA molecules and protein components. For instance, the small subunit in bacteria is 30S, and the large subunit is 50S, combining to form a 70S ribosome. In contrast, human ribosomes have a 40S small subunit and a 60S large subunit, forming an 80S ribosome.
The size difference between prokaryotic and eukaryotic ribosomes has significant implications, particularly in medicine. Many antibiotics are designed to specifically target the 70S ribosomes of bacteria, disrupting their protein synthesis without harming the larger 80S ribosomes found in human cells. This selective targeting allows these medications to combat bacterial infections effectively.
The Ribosome’s Role in Protein Synthesis
The ribosome orchestrates protein synthesis by reading messenger RNA (mRNA) and assembling amino acids into a polypeptide chain. This intricate process unfolds in three main stages: initiation, elongation, and termination. The small ribosomal subunit first binds to the mRNA, locating the start signal that indicates where protein synthesis should begin.
Once the small subunit has bound the mRNA and a specific transfer RNA (tRNA) molecule carrying the first amino acid, the large ribosomal subunit joins the complex, forming a complete and functional ribosome. The large subunit contains three distinct sites: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. These sites function like stations on an assembly line, guiding the precise addition of amino acids.
During elongation, tRNA molecules, each carrying a specific amino acid, enter the A site, matching their anticodon to the mRNA codon. The ribosome then catalyzes the formation of a peptide bond between the amino acid at the A site and the growing polypeptide chain at the P site. The ribosome then moves along the mRNA, shifting the tRNAs, so the one previously in the A site moves to the P site, and the one from the P site moves to the E site, where it exits the ribosome. This sequential movement ensures the accurate and efficient addition of amino acids, building the protein one unit at a time according to the mRNA template.
The process continues until the ribosome encounters a stop codon on the mRNA, signaling the end of the protein sequence. At this point, release factors bind to the A site, causing the newly synthesized polypeptide chain to detach from the ribosome. The ribosomal subunits then separate from the mRNA and from each other, becoming available to initiate the synthesis of another protein.
Visualizing the Ribosome Model
Scientists have developed detailed models of the ribosome’s structure and function through advanced scientific techniques. Two primary methods that have significantly contributed to our understanding are X-ray crystallography and cryo-electron microscopy (cryo-EM). X-ray crystallography involves growing crystals of the ribosome and then directing X-rays through them; the diffraction patterns created by the X-rays are then analyzed to reconstruct the three-dimensional atomic structure of the ribosome.
Cryo-EM offers another powerful way to visualize the ribosome, especially in different functional states. This technique involves flash-freezing ribosome samples at extremely low temperatures, preserving their natural structure. Thousands of two-dimensional images are then captured from various angles, which are computationally combined to generate a high-resolution three-dimensional model of the ribosome. The pioneering work using these techniques, which allowed scientists to map the atomic structure of the ribosome and understand its mechanism, was recognized with the Nobel Prize in Chemistry in 2009, awarded to Venkatraman Ramakrishnan, Thomas A. Steitz, and Ada E. Yonath.