Trace the path from DNA to protein — transcription and translation steps

Francis Crick proposed the Central Dogma of Molecular Biology (1958):

DNA stores genetic information. Transcription converts it into messenger RNA (mRNA). Translation decodes the mRNA into a protein (chain of amino acids). Each step occurs in a different cellular compartment and uses distinct molecular machinery.

Initiation: RNA Polymerase binds to a specific DNA sequence called the promoter (upstream of the gene). In eukaryotes, transcription factors help RNA polymerase bind. The DNA double helix unwinds locally.

Elongation: RNA polymerase reads the template strand (3’→5’) and synthesises a complementary mRNA strand (5’→3’). The rule: A pairs with U (in RNA), T pairs with A, G pairs with C, C pairs with G. Note: RNA uses uracil (U) instead of thymine (T).

Termination: RNA polymerase reaches a terminator sequence on the DNA, stops, and releases the newly made mRNA.

Post-transcriptional processing (eukaryotes only):

• 5’ cap added (7-methylguanosine) — protects mRNA
• 3’ poly-A tail added — stabilises mRNA
• Splicing: Introns (non-coding sequences) removed; exons (coding sequences) joined

The processed mRNA then exits the nucleus through nuclear pores.

The codon: mRNA is read in triplets called codons (3 bases = 1 amino acid). The genetic code is the dictionary that converts each codon to an amino acid (or start/stop signals).

Key codons:

• AUG = start codon (also codes for methionine)
• UAA, UAG, UGA = stop codons (do not code for any amino acid)

Components needed:

• mRNA (template)
• Ribosomes (40S + 60S subunits in eukaryotes → 80S)
• tRNA (transfer RNA) — carries amino acids
• Amino acids + energy (GTP)
• Initiation, elongation, and termination factors

Initiation: The small ribosomal subunit binds to the 5’ cap of mRNA and scans until it reaches the AUG start codon. The initiator tRNA (carrying methionine) base-pairs with AUG at the P-site. The large subunit joins.

Elongation:

1. An aminoacyl-tRNA (carrying the next amino acid) enters the A-site, its anticodon base-pairing with the mRNA codon.
2. A peptide bond forms between the methionine (or growing chain) and the new amino acid — catalysed by peptidyl transferase activity of 23S rRNA.
3. The ribosome translocates one codon forward (3’ direction): the tRNA in A-site moves to P-site, empty tRNA in P-site moves to E-site and leaves. A new codon is exposed in A-site.

This cycle repeats for each codon.

Termination: When a stop codon (UAA, UAG, or UGA) enters the A-site, a release factor (protein) binds instead of tRNA. The release factor triggers hydrolysis of the peptide-ribosome bond. The completed polypeptide chain is released.

The newly made polypeptide chain folds into its 3D structure (spontaneously or with help from chaperone proteins like Hsp70). It may then undergo:

• Phosphorylation, glycosylation, acetylation
• Cleavage of signal peptides
• Assembly with other subunits (quaternary structure)

Only after these modifications does the protein become fully functional.