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.
| Process | Molecule being read | Reading direction | Product growth or movement |
|---|---|---|---|
| transcription | DNA template | 3′→5′ | RNA is synthesized 5′→3′ |
| translation | mRNA | 5′→3′ | ribosome moves toward the mRNA 3′ end; polypeptide grows N-terminus→C-terminus |
Nucleotides are added to a free 3′ hydroxyl group, so every new nucleic-acid strand grows 5′→3′. The template must therefore be read antiparallel to the new strand.
A diagram arrow showing polymerase movement and an arrow showing new-strand growth refer to opposite strands; identify which molecule each arrow belongs to.
| Processing change | What happens | Main consequence |
|---|---|---|
| 5′ cap | a modified nucleotide is added to the 5′ end | protects RNA and supports export and translation initiation |
| poly-A tail | many adenine nucleotides are added to the 3′ end | increases stability and supports export and translation |
| splicing | introns are removed and exons joined | creates a continuous mature RNA sequence for translation |
pre-mRNA in nucleus → processing completed → mature mRNA exported through a nuclear pore → translation in cytoplasm
The poly-A tail is added after transcription; it is not copied from a long template-DNA sequence of thymine bases.
In alternative splicing, the same pre-mRNA is spliced in different patterns, so different exon combinations are retained in mature mRNA.
Different mature mRNAs contain different codon sequences and can therefore produce related polypeptides with different amino-acid segments, properties or tissue distributions.
Alternative splicing does not change the DNA sequence. It changes which parts of one RNA transcript remain in the message sent to the ribosome.
The small ribosomal subunit binds the mRNA and positions the start region for scanning or recognition.
The initiator tRNA carrying methionine pairs with AUG. This tRNA is placed directly in the P site, establishing the reading frame.
The large subunit joins to complete the ribosome. The A site is now open for the tRNA matching the next codon.
AUG inside an already established reading frame can code for methionine without serving as a new start. Initiation depends on context and assembly, not the triplet alone.
Preproinsulin contains an N-terminal signal peptide that directs the growing polypeptide into the endoplasmic reticulum. Removing the signal peptide produces proinsulin.
Proinsulin folds, and disulfide bonds form between regions that will become the A and B chains.
The connecting C-peptide is removed. The A and B chains remain linked by disulfide bonds, producing mature insulin.
Proteins selected for degradation are commonly marked with a chain of ubiquitin molecules.
The proteasome recognizes the tag, removes and recycles ubiquitin, unfolds the target protein and threads it into a proteolytic core.
The protein is cut into short peptides, which can be hydrolysed to amino acids for new synthesis or metabolism.
Selective degradation removes damaged or misfolded proteins and can rapidly lower the abundance of regulatory proteins. Protein concentration therefore depends on both synthesis and breakdown.