Protein synthesis is the fundamental process by which individual cells build specific proteins from instructions stored in their genetic code. Proteins perform essential roles in every organism: structural support, chemical catalysis, signaling, and transport.
Phase 1: Transcription (Reading the Master Blueprint)
In eukaryotic cells, DNA stays safely inside the nucleus. To preserve the master blueprint, cells copy the needed gene into a portable messenger RNA (mRNA) molecule that can travel to the cytoplasm.
Initiation
An enzyme called RNA polymerase binds to a promoter sequence on the DNA and unwinds the double helix. This exposes the gene's nucleotide sequence so the enzyme can read it.
Elongation
RNA polymerase moves along the DNA template strand and builds a complementary pre-mRNA strand. In RNA, uracil (U) pairs with adenine (A), replacing the thymine (T) found in DNA.
Termination
When RNA polymerase reaches a stop signal, it releases the newly formed pre-mRNA and the DNA strands rewind.
Processing
Before mRNA leaves the nucleus, it is processed: non-coding introns are removed, coding exons are joined, and a protective cap and tail are added. The mature mRNA then exits the nucleus through a nuclear pore.
Phase 2: Translation (Building the Protein)
In the cytoplasm, the mRNA sequence is read by a ribosome, the cell's protein-building machinery. The ribosome reads the mRNA three bases at a time, with each codon specifying one amino acid.
Initiation
The ribosome binds near the start codon, typically AUG. A transfer RNA (tRNA) brings the matching amino acid, methionine, and its anticodon pairs with the mRNA codon.
Elongation
As the ribosome moves along the mRNA, each new codon calls for a specific tRNA. The ribosome links the amino acids together with peptide bonds, growing the polypeptide chain.
Termination
Translation ends when the ribosome reaches a stop codon (UAA, UAG, or UGA). No tRNA matches these codons, so a release factor triggers the finished polypeptide to detach.
Post-Translational Folding and Processing
The newly synthesized amino acid chain is not yet a functional protein. It must fold into a precise three-dimensional shape with the help of chaperone proteins.
Many proteins also undergo further modifications in the endoplasmic reticulum and Golgi apparatus, such as adding carbohydrate or lipid groups. These modifications help direct each protein to its final cellular destination.