Protein synthesis is the process cells use to build proteins from genetic instructions. It is one of the most fundamental activities in biology because proteins perform much of the cell’s work: they form structures, speed up chemical reactions, transport substances, send and receive signals, and help control which genes are active.
The instructions for making a protein are stored in DNA. Cells generally do not use DNA itself as the direct template for building a protein. Instead, they copy the relevant genetic information into a molecule called messenger RNA (mRNA), and molecular machines called ribosomes then read that mRNA and assemble a chain of amino acids.
Protein synthesis therefore has two major stages: transcription, in which DNA information is copied into RNA, and translation, in which the RNA sequence is used to build a protein.
The basic flow of protein synthesis
In a typical eukaryotic cell, the information flows through this sequence:
DNA → mRNA → protein
DNA stores genes, which are stretches of DNA containing instructions for making functional products, including proteins. When a particular protein is needed, the cell uses the corresponding gene as a template to produce an RNA copy. That RNA is processed and transported out of the nucleus.
A ribosome then attaches to the mRNA and moves along it, reading its sequence three RNA bases at a time. Each three-base unit, called a codon, corresponds to a particular amino acid or signals the ribosome to start or stop protein production. Transfer RNA (tRNA) molecules help match codons with the correct amino acids.
As the ribosome proceeds, it links the amino acids together into a growing chain. That chain eventually folds into a specific three-dimensional shape, producing a functional protein or a precursor that will undergo additional processing.
Transcription: copying a gene into RNA
In eukaryotic cells, transcription occurs primarily in the nucleus, where the DNA is housed.
When a gene is going to be expressed, an enzyme called RNA polymerase binds to a region of DNA associated with that gene. The DNA strands separate locally, exposing the sequence that will serve as a template.
RNA polymerase moves along the DNA template strand and builds a complementary RNA molecule. Instead of the base thymine (T) found in DNA, RNA uses uracil (U). The resulting molecule is called messenger RNA, or mRNA, because it carries information from the DNA to the cellular machinery that makes proteins.
Transcription does not copy an entire chromosome or even an entire DNA molecule. It produces an RNA molecule corresponding to a particular gene or, in some cases, a defined portion of genetic information.
How the RNA sequence carries instructions
The sequence of bases in mRNA determines the sequence of amino acids in the resulting protein.
For example, an mRNA sequence might contain codons such as:
AUG – GCU – UAC – GGA
Each codon consists of three RNA bases. The ribosome interprets these codons in order, and tRNAs bring the amino acids specified by them.
The genetic code is redundant: several different codons can specify the same amino acid. However, each codon has a defined meaning, and the order of codons is critical. Changing that order can change the resulting protein.
RNA is processed before translation
In eukaryotic cells, the initial RNA transcript usually requires processing before it becomes mature mRNA.
The newly made RNA receives a 5′ cap at one end and a poly(A) tail at the other. These modifications help protect the RNA and are important for its transport and use by ribosomes.
The initial transcript also contains sections called introns, which generally do not remain in the final protein-coding message. Other sections, called exons, are joined together during a process called RNA splicing.
After processing, mature mRNA can leave the nucleus through nuclear pores and enter the cytoplasm, where ribosomes can translate it.
RNA processing also gives cells additional ways to regulate gene expression. For example, alternative splicing can allow different combinations of exons from the same gene to produce different mRNA molecules and, consequently, different protein products.
Translation: turning mRNA into a protein
Translation takes place on ribosomes, which are molecular machines made from ribosomal RNA (rRNA) and proteins.
Ribosomes can be found in the cytoplasm or attached to the surface of the rough endoplasmic reticulum. Their job is to read mRNA and connect amino acids in the correct order.
The process begins when a ribosome recognizes the appropriate starting region of an mRNA. A codon called AUG commonly serves as the start codon and establishes the reading frame. AUG also specifies the amino acid methionine.
Once translation begins, the ribosome moves along the mRNA one codon at a time.
tRNA delivers the amino acids
The cell uses transfer RNA, or tRNA, to connect the language of nucleic acids with the language of proteins.
Each tRNA carries a particular amino acid and contains an anticodon, a sequence of three RNA bases that can pair with a complementary codon in the mRNA.
Suppose the mRNA contains a codon specifying a particular amino acid. A tRNA with the corresponding anticodon binds to that codon while carrying the appropriate amino acid. The ribosome then incorporates that amino acid into the growing protein chain.
The tRNAs do not determine which amino acid they carry by themselves. Enzymes called aminoacyl-tRNA synthetases attach the correct amino acids to their corresponding tRNAs. These enzymes are essential for maintaining the accuracy of translation.
The ribosome builds the chain
A ribosome has sites that coordinate the tRNAs as translation proceeds. The growing amino acid chain is transferred and extended through the formation of peptide bonds, the chemical bonds that link amino acids together.
The ribosome repeatedly performs this basic sequence:
- A tRNA carrying the appropriate amino acid pairs with the next mRNA codon.
- The ribosome forms a peptide bond that extends the growing chain.
- The ribosome moves to the next codon.
- The empty tRNA leaves, making room for the next charged tRNA.
This continues until the ribosome encounters a stop codon.
Stop codons do not correspond to amino acids. Instead, they recruit protein factors that cause the completed chain to be released from the ribosome.
What happens after the protein is made?
Translation produces a linear chain of amino acids, but a protein’s final function depends on much more than its amino acid sequence alone.
As the chain emerges from the ribosome, it begins to fold into a particular three-dimensional structure. Interactions among its amino acids help determine that structure. Some proteins fold on their own, while others require assistance from molecular chaperones, proteins that help other proteins reach or maintain appropriate conformations.
Many proteins also undergo post-translational modifications after or during synthesis. These chemical changes can alter a protein’s activity, stability, location, or interactions with other molecules. Examples include phosphorylation, acetylation, glycosylation, and the addition of other chemical groups.
Some proteins are also cut into smaller pieces or assembled with other protein chains before becoming fully functional.
Where a protein is made affects where it goes
The location of the ribosome helps determine the eventual destination of many proteins in eukaryotic cells.
Ribosomes floating freely in the cytoplasm generally make proteins that remain in the cytosol or are directed to certain internal cellular compartments.
Other ribosomes become associated with the rough endoplasmic reticulum (ER) while translating particular mRNAs. These ribosomes make many proteins destined for secretion, insertion into cellular membranes, or transport through parts of the endomembrane system.
As such a protein is synthesized, a targeting sequence can direct the ribosome and growing protein to the ER. The protein can then enter the ER or become embedded in its membrane while translation continues.
From there, proteins may be transported to other destinations, including the Golgi apparatus, secretory vesicles, the cell surface, or outside the cell.
How cells control protein synthesis
Cells do not make every protein all the time. Protein production is tightly regulated so that cells can respond to their environment, maintain their internal conditions, and perform specialized functions.
One major control point is gene expression. A cell can regulate whether a gene is transcribed, how much RNA is produced, and how long that RNA remains available for translation.
Cells can also regulate translation itself. For example, they can alter the activity of translation factors or control which mRNAs are efficiently recruited to ribosomes.
After translation, the cell can further control proteins by modifying them, moving them to different locations, activating or inhibiting them, or marking them for destruction.
This layered regulation allows the same genome to support very different cell types. A neuron and a muscle cell contain essentially the same DNA, yet they produce different sets and amounts of proteins because they regulate gene expression differently.
Why protein synthesis is so accurate
Protein synthesis involves several checkpoints that help minimize errors.
During transcription, RNA polymerase must accurately copy the DNA template. During translation, tRNA charging enzymes help ensure that the correct amino acid is attached to each tRNA. The ribosome then checks the pairing between mRNA codons and tRNA anticodons as it selects incoming tRNAs.
Even with these safeguards, errors can occur. A change in DNA can alter an mRNA sequence and potentially change the protein it encodes. Errors in RNA processing, translation, or protein folding can also affect protein function.
Some changes have little or no functional effect, while others can substantially alter a protein’s behavior. Cells therefore have quality-control systems that identify and degrade many defective or improperly folded RNAs and proteins.
Protein synthesis is a coordinated molecular process
Protein synthesis is often described as DNA being “converted” into protein, but the process is more accurately understood as a series of linked molecular events.
A gene in DNA provides the original information. Transcription copies that information into RNA. In eukaryotic cells, the RNA is processed before leaving the nucleus. A ribosome then reads the mature mRNA during translation. tRNAs deliver amino acids according to the mRNA’s codons, and the ribosome joins those amino acids into a polypeptide chain. Finally, the chain folds, may be chemically modified, and is directed to the cellular location where it can function.
Through this system, cells continually turn genetic information into the proteins that make cellular life possible.
