Protein synthesis links DNA information to functional proteins through transcription, RNA processing, translation, genetic-code reading and post-translational modification in cells.
RNA polymerase binds near the gene and opens a short region of the DNA double helix, exposing the template strand.
Free RNA nucleotides align by complementary base pairing: DNA A pairs with RNA U, DNA T with RNA A, and C with G.
RNA polymerase joins the RNA nucleotides into a sugar–phosphate backbone. The mRNA separates, and the DNA strands pair again.
| Strand | Relationship to the new mRNA |
|---|---|
| DNA template | antiparallel and complementary |
| DNA coding strand | same base order as mRNA, except DNA has T where RNA has U |
DNA template: 3′–TAC CTT GCG–5′
mRNA: 5′–AUG GAA CGC–3′
Codon tables are read from mRNA 5′→3′. Do not look up a DNA template triplet directly.
Hydrogen bonds between complementary bases can break and re-form. This lets a short DNA region open temporarily without breaking the covalent sugar–phosphate backbones.
After the mRNA leaves, the original complementary DNA strands pair again. The gene's base sequence is conserved and can be transcribed repeatedly.
Transcription copies one gene region into RNA; it does not duplicate the whole DNA molecule and does not consume the template strand.
If a DNA template is supplied, write the antiparallel complementary mRNA, replacing DNA A with RNA U.
Write the mRNA 5′→3′, locate the stated or biologically relevant start, and divide the sequence into consecutive triplets from that frame.
Look up each mRNA codon, record amino acids in order and stop when a stop codon is reached.
Template 3′–TAC CTT GCG ACT–5′ gives mRNA 5′–AUG GAA CGC UGA–3′, which translates as Met–Glu–Arg–Stop.
A charged tRNA with a complementary anticodon enters the A site beside the tRNA carrying the growing chain in the P site.
The ribosome catalyses a peptide bond, transferring the growing polypeptide to the amino acid on the A-site tRNA.
The ribosome moves one codon along the mRNA. The peptidyl-tRNA shifts to P, the empty tRNA shifts to E and exits, and the A site becomes available again.
The entry–bond–movement cycle repeats for successive codons. When a stop codon reaches the A site, no tRNA enters; a release factor releases the completed polypeptide and the ribosome dissociates.
A polysome is one mRNA being translated simultaneously by several ribosomes. Each ribosome starts near the 5′ end and moves toward the 3′ end.
All ribosomes read the same codon sequence, so they produce polypeptides with the same amino-acid order. Simultaneous translation increases the rate of protein production from that mRNA.
The ribosomes do not cooperate to make one long polypeptide. Each ribosome makes a separate copy.
