Question
What is the complete pathway from a gene (DNA) to a functional protein, covering transcription, RNA processing, and translation?
Solution — Step by Step
The flow of genetic information follows the Central Dogma:
In eukaryotes, there is an additional step: RNA processing (splicing) between transcription and translation.
Where: Nucleus (in eukaryotes), cytoplasm (in prokaryotes)
Enzyme: RNA polymerase
Process:
- RNA polymerase binds to the promoter region on the template (antisense) strand
- It reads the template strand 3’ to 5’ and synthesises mRNA 5’ to 3’
- The mRNA sequence is complementary to the template strand (and identical to the coding strand, except U replaces T)
- Transcription terminates at the terminator sequence
In prokaryotes, the mRNA is immediately ready for translation. In eukaryotes, it must be processed first.
The initial transcript (pre-mRNA or hnRNA) undergoes three modifications:
- 5’ capping: A methylated guanine cap is added (protects from degradation, aids ribosome binding)
- 3’ polyadenylation: A poly-A tail (200+ adenines) is added (stability, export signal)
- Splicing: Introns (non-coding) are removed, exons (coding) are joined by the spliceosome
The mature mRNA then exits the nucleus through nuclear pores.
Where: Ribosomes (in cytoplasm or on rough ER)
Key players: mRNA (message), tRNA (adapter), ribosome (factory), amino acids (building blocks)
Process:
- Initiation: Small ribosomal subunit binds to mRNA at the start codon AUG; initiator tRNA (carrying methionine) occupies the P site
- Elongation: Aminoacyl-tRNAs enter the A site, peptide bond forms, ribosome translocates; cycle repeats
- Termination: Ribosome reaches a stop codon (UAA, UAG, UGA); release factors free the polypeptide
The polypeptide then folds into its 3D shape (with help from chaperones) to become a functional protein.
graph TD
A[DNA - Gene] -->|Transcription by RNA polymerase| B[Pre-mRNA]
B -->|5 prime capping| C[Capped pre-mRNA]
C -->|Splicing - introns removed| D[Spliced mRNA]
D -->|3 prime poly-A tail added| E[Mature mRNA]
E -->|Exits nucleus through nuclear pore| F[mRNA in cytoplasm]
F -->|Translation by ribosome| G[Polypeptide chain]
G -->|Folding by chaperones| H[Functional Protein]Why This Works
Gene expression is a two-step decoding process. Transcription converts the stable DNA archive into a temporary RNA messenger. Translation reads this messenger in triplets (codons), with tRNA acting as the “dictionary” that matches each codon to its amino acid. The genetic code (64 codons mapping to 20 amino acids + 3 stop signals) makes this translation universal across almost all life forms.
NEET frequently asks about the differences between transcription and translation. Key contrasts: transcription uses RNA polymerase while translation uses ribosomes; transcription produces RNA while translation produces protein; transcription occurs in the nucleus while translation occurs in the cytoplasm (eukaryotes). Keep a comparison table ready.
Alternative Method
For quick recall, use the factory analogy:
- DNA = master blueprint (stored in the office/nucleus)
- mRNA = photocopy of the blueprint (sent to the factory floor)
- Ribosome = assembly line machine
- tRNA = workers who bring the correct parts
- Amino acids = parts being assembled
- Protein = finished product
Common Mistake
Students confuse the template strand with the coding strand during transcription. The template strand (antisense, 3’ to 5’) is the one RNA polymerase reads. The coding strand (sense, 5’ to 3’) has the same sequence as the mRNA (with T instead of U). When a question gives you the “coding strand” and asks for the mRNA, just replace T with U. When it gives the “template strand,” write the complementary sequence with U instead of T.