The primary organelle in which protein synthesis takes place is the ribosome. That said, without ribosomes, cells would be unable to build the enzymes, structural components, and signaling molecules necessary for survival. It is the molecular machine responsible for translating the genetic instructions encoded in messenger RNA (mRNA) into functional proteins. Understanding how this organelle operates is fundamental to grasping the very essence of cellular biology and the central dogma of molecular biology.
What Is a Ribosome?
A ribosome is a complex molecular structure found in all living cells. Unlike many other organelles that are bound by a lipid membrane, ribosomes are non-membrane-bound structures. They are composed of two main components: ribosomal RNA (rRNA) and ribosomal proteins. Because they contain both RNA and protein, ribosomes are technically classified as ribonucleoproteins.
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The ribosome is often described as the cell’s protein factory. Its primary job is to read the sequence of codons on an mRNA strand and assemble the corresponding amino acids into a polypeptide chain. This process, known as translation, is the second half of the central
dogma of molecular biology: DNA is transcribed into RNA, and RNA is translated into protein.
Structure of a Ribosome
Ribosomes are made of two subunits: a large subunit and a small subunit. Day to day, these subunits remain separate when the ribosome is not actively translating mRNA, then come together during protein synthesis. The small subunit helps read the mRNA sequence, while the large subunit catalyzes the formation of peptide bonds between amino acids.
In prokaryotic cells, ribosomes are classified as 70S ribosomes, made of a 50S large subunit and a 30S small subunit. In eukaryotic cells, ribosomes are larger and are classified as 80S ribosomes, composed of a 60S large subunit and a 40S small subunit. The “S” refers to Svedberg units, a measure of how quickly particles settle during centrifugation; these values are not simply additive.
Where Ribosomes Are Found
Ribosomes can be found in several locations within a cell. Some float freely in the cytoplasm, where they produce proteins that function within the cytosol. So others are attached to the rough endoplasmic reticulum, giving that organelle its “rough” appearance under a microscope. These bound ribosomes usually make proteins destined for secretion, insertion into membranes, or transport to organelles such as lysosomes.
Ribosomes are also found inside mitochondria and chloroplasts. This supports the endosymbiotic theory, which proposes that these organelles originated from ancient free-living bacteria. Their ribosomes resemble bacterial ribosomes more closely than the ribosomes found in the eukaryotic cytoplasm.
How Ribosomes Build Proteins
Protein synthesis occurs in three main stages: initiation, elongation, and termination.
During initiation, the small ribosomal subunit binds to the mRNA molecule and locates the start codon, usually AUG. A transfer RNA molecule carrying the amino acid methionine pairs with this start codon, and the large ribosomal subunit then joins to form a complete ribosome.
During elongation, the ribosome moves along the mRNA strand, reading its codons one at a time. Transfer RNA molecules, or tRNAs, bring the appropriate amino acids to the ribosome. Each tRNA has an anticodon that matches a specific codon on the mRNA. As the correct amino acids are brought into place, the ribosome links them together with peptide bonds, forming a growing polypeptide chain Worth knowing..
The ribosome contains three important binding sites for tRNA: the A site, P site, and E site. The A site accepts a new tRNA, the P site holds the tRNA carrying the growing polypeptide chain, and the E site is where empty tRNAs exit the ribosome.
Termination occurs when the ribosome reaches a stop codon on the mRNA, such as UAA, UAG, or UGA. These codons do not code for an
