Protein synthesis is the cellular process of creating proteins. Their formulas and the instructions on how to make them are encoded in DNA. It is helpful to refer to the process in two parts. Protein synthesis transcription copies the DNA code. Protein synthesis translation matches the code to chemical compounds in the cell, the combination of which becomes a protein.
Deoxyribonucleic acid (DNA), the master blueprint of an individual organism, is structured as a double helix. A good analogy is a long strip of twisted zipper. There are two strands made of 5-carbon sugars and phosphates. Bridging them are interlocking paired nucleotides, like the opposing teeth of a closed zipper. Adenine (A) matches with thymine (T), cytosine (C) pairs with guanine (G), and vice versa.
Protein synthesis transcription begins in the nucleus of a cell, where DNA is “unzipped” by an enzyme called helicase, resulting in two separated strands. A critical enzyme called RNA polymerase (RNAP) then attaches itself to one of the strands to begin a process called elongation. It identifies the first nucleotide on the template strand of DNA and, in doing so, attracts a free nucleotide that must be paired with it. RNAP then moves to the next nucleotide on the DNA strand, and continues on to the next, and the next, until a ribonucleic acid (RNA) chain has been assembled.
RNA is a single strand of unpaired nucleotides able to keep its structural integrity with the addition of oxygen molecules. The RNA chain that has been constructed by its polymerase agent, some with more than 2 million nucleotides, is called messenger RNA (mRNA). In theory, mRNA is purposed to be an exact duplicate of the unused single strand of DNA left behind. In practice, it is not exact, and protein synthesis transcription errors can also occur.
The mRNA is, therefore, a very long chain of only four different nucleotides. Its sequence is referred to as a transcript. An example might be AAGCAUUGAC — four letters, maybe 2 million of them, in seemingly random order. It is somewhat helpful to analogize carbon life as being a 4-bit bio-computer of very large scale. Of particular note is that, in RNA, thymine is replaced by a similar nucleotide called uracil (U).
As its name implies, messenger RNA escapes its confinement in a cell’s nucleus through pores along the nuclear membrane. Once within the cell’s cytoplasm, its destiny is to deliver the protein synthesis transcription, copied from DNA, to structures called ribosomes. Ribosomes are the cell’s protein factories and, there, the second step of protein synthesis occurs.
The encoded sequence of nucleotides must be translated. A ribosome binds to the mRNA and, in the process of reading its sequences, attracts fragments of RNA called transfer RNA (tRNA), which will have found and bonded with a free amino acid specific to its short sequence of nucleotides. If there is a match, the tRNA and its cargo bind to the ribosome. As the ribosome proceeds to read the next sequence, and the next, in a process also called elongation, a long polypeptide chain of amino acids results.
Proteins that differentiate organic tissue in form and function are the so-called “building blocks of life.” They, in turn, are built as a chain of various amino acids — the translation of DNA code as transcribed by RNA for its host cell’s most important metabolic task. There is, however, one last step remaining to complete protein synthesis that is frustrating scientific understanding. In a process called protein folding, the long chain of amino acids bends, curls, knots and otherwise compacts into its unique structure. While supercomputers have had some success in folding protein formulas into their correct three-dimensional shapes, most protein puzzles have been solved intuitively by people with a heightened sense of variable spatial dimensions.