Ribosomes are structures within a biological cell, dubbed “factories” for their singular role in assembling the proteins that form and define the cell’s function. Proteins are complex chemical compounds that have been called “the building blocks of life.” The instructional formulas are encoded in the genetic DNA (deoxyribonucleic acid) within a cell’s central nucleus. Protein synthesis is the field of molecular biology that studies the processes by which this DNA code is translated into functional proteins. The role of ribosomes in protein synthesis is critical.
First, in a process called transcription, DNA makes a functional copy of itself called RNA (ribonucleic acid). This is specifically named messenger RNA (mRNA) for its function of delivering the coded instructions to ribosomes. Along the way, RNA makes fragmented copies of itself named transfer RNA (tRNA). These copies are released to bind with free organic compounds within the cells called amino acids.
Meanwhile, mRNA binds to a ribosome, which begins to “read” the information in a process called translation. The information represents a sequence of many different amino acids. As it’s being read, the matching codes of tRNA are attracted to the ribosome and transfer the attached payload. One by one, a chain of amino acids is thus created until the protein is completed and mRNA signals its release from the ribosome.
The role of ribosomes in protein synthesis is akin to a linear assembly line of amino acids into finished proteins. DNA and RNA are themselves linear chains of four chemical molecules called nucleotides — adenine, cytosine, thymine in DNA or uracil in RNA, and guanine — abbreviated A, C, T or U, and G, respectively. The very long chain of these nucleotides in mRNA is akin to a ticker tape that ribosomes read and translate into the specific amino acids the code represents.
Ribosomes are themselves made from proteins and strands of RNA. They can be regarded as having two functional subunits, each binding to either messenger or transfer RNA. The ribosomes in protein synthesis begin the process of assembly when they encounter a specific series of nucleotides in mRNA, namely A-U-G. Called a codon, the sequence of nucleotides U-A-G is the instruction to stop production, and a ribosome’s two subunits separate, releasing the protein in the process.
In the final step, ribosomes in protein synthesis are not involved at all. In a process not well understood, the straight chain of amino acids that make up a protein morphs into its predetermined physical shape. Called protein folding, the factors most cited in determining how the chain compresses itself into a three-dimensional shape are: temperature, surrounding solvents such as water, the presence of salts, and the molecular attraction and interaction of all of a protein’s hydrogen bonds.