Protein synthesis takes place in two main stages, each in a different part of the cell. The first stage, where the gene’s instructions are copied into a portable message, happens inside the nucleus. The second stage, where that message is read and assembled into a protein, happens on ribosomes located in the cytoplasm or attached to a membrane-bound structure called the endoplasmic reticulum. In bacteria, which lack a nucleus, both stages happen in the same open space and can even occur simultaneously.
Step One: Copying the Instructions in the Nucleus
Before a protein can be built, the cell needs a working copy of the relevant gene. DNA never leaves the nucleus, so an enzyme called RNA polymerase reads the gene and produces a messenger RNA (mRNA) transcript. This process is called transcription. The mRNA is then processed, capped, and edited before it’s exported through pores in the nuclear membrane and into the cytoplasm, where the actual protein-building machinery waits.
Step Two: Building the Protein on Ribosomes
Ribosomes are the actual machines that assemble proteins, and they do so by reading the mRNA’s sequence of three-letter codes (codons) and linking amino acids together in the corresponding order. Three types of RNA work together during this process. Messenger RNA carries the sequence information. Transfer RNA (tRNA) acts as an adaptor, carrying a specific amino acid on one end and a matching anticodon on the other that pairs with the mRNA codon. Ribosomal RNA (rRNA) forms the structural and catalytic core of the ribosome itself, including the site where peptide bonds are formed between amino acids.
Each ribosome has three tRNA docking positions. The A site accepts incoming amino acid-loaded tRNAs. The P site holds the growing protein chain. The E site is the exit, where empty tRNAs leave after delivering their amino acid. The ribosome moves along the mRNA one codon at a time, and the protein chain grows until a stop codon signals the end.
Free Ribosomes vs. Membrane-Bound Ribosomes
All protein synthesis starts on ribosomes floating freely in the cytoplasm. What happens next depends on where the finished protein needs to go. Proteins destined to stay in the cytoplasm, or to be sent into the nucleus, mitochondria, or certain other compartments, are completed on these free ribosomes and released directly into the cell’s interior.
Proteins that need to be secreted from the cell, embedded in a membrane, or delivered to compartments like lysosomes take a different route. Early in translation, a short signal sequence at the front of the growing protein chain directs the entire ribosome to dock onto the surface of the endoplasmic reticulum (ER). These membrane-bound ribosomes give the rough ER its characteristic studded appearance under a microscope. The protein threads through the ER membrane as it’s being made, entering the ER interior where folding and initial modifications begin. Free and membrane-bound ribosomes are structurally identical. The only difference is which mRNA they happen to be translating.
Where Ribosomes Themselves Are Made
Ribosomes don’t just appear in the cytoplasm ready to work. Their components are manufactured in a dense region inside the nucleus called the nucleolus. Here, a large precursor RNA molecule (47S pre-rRNA) is produced and then cut and chemically modified at roughly 200 sites to yield three mature ribosomal RNAs: 18S, 5.8S, and 28S. These rRNAs combine with ribosomal proteins to form two subunits, a smaller 40S subunit and a larger 60S subunit. The subunits are exported separately to the cytoplasm, where they join together on an mRNA strand to form a complete 80S ribosome only when translation begins.
Protein Processing After Synthesis
For many proteins, synthesis on the ribosome is just the beginning. Proteins that entered the ER are packaged into small transport bubbles (vesicles) and shuttled to the Golgi apparatus, a series of flattened membrane compartments that act as a sorting and modification station. As proteins pass from one side of the Golgi to the other, sugar chains are added or trimmed, and the proteins are checked for proper folding. Only correctly folded proteins are allowed to leave the ER in the first place. The final compartment of the Golgi, the trans Golgi network, packages finished proteins into vesicles addressed to their final destination: the cell surface, lysosomes, or secretion outside the cell.
Protein Synthesis in Mitochondria and Chloroplasts
Mitochondria and chloroplasts are unique among cell compartments because they carry their own small genomes and their own ribosomes. They can transcribe and translate a limited set of genes independently from the nucleus. Mitochondrial ribosomes (mitoribosomes) are smaller than the ribosomes in the cytoplasm and look quite different, with a higher ratio of protein to RNA. Their translation machinery closely resembles that of bacteria rather than the rest of the eukaryotic cell, a reflection of the ancient bacterial ancestors these organelles evolved from. Mitochondrial translation factors even function on bacterial ribosomes in lab experiments, but not on the cell’s cytoplasmic ribosomes.
One notable quirk: mitochondria read the genetic code slightly differently. The codon UGA, which normally signals “stop” in the cytoplasm, codes for the amino acid tryptophan in most mitochondria. Translation in mitochondria also appears to occur near the inner membrane, positioning newly made proteins close to the respiratory enzyme complexes where they’ll be assembled and put to work.
How Bacteria Do It Differently
Bacterial cells have no nucleus, so their DNA sits directly in the cytoplasm. This means ribosomes can latch onto an mRNA molecule and begin translating it before transcription is even finished. This process, called coupled transcription-translation, is a defining feature of prokaryotic life. The RNA polymerase moves along the DNA producing mRNA, and a ribosome follows closely behind on the same mRNA strand, assembling protein in near-real time. The polymerase tends to pause briefly within the first 100 nucleotides after the start codon, giving the leading ribosome a chance to catch up and form a direct physical complex with the transcription machinery. This tight coupling makes bacterial gene expression remarkably fast and is one reason bacteria can respond to environmental changes within minutes.