Question
State the central dogma of molecular biology. Where does transcription occur in eukaryotes? Where does translation occur? What is the role of mRNA in this process?
(CBSE 2024 Board Exam — 3 marks)
Solution — Step by Step
The central dogma, proposed by Francis Crick in 1958, describes the flow of genetic information:
DNA is the master blueprint — it never leaves the nucleus (in eukaryotes). Instead, it sends a messenger copy out.
Transcription occurs in the nucleus (in eukaryotes). The enzyme RNA polymerase reads the template strand of DNA (3’→5’) and synthesises a complementary mRNA strand (5’→3’).
The product is pre-mRNA (or hnRNA), which undergoes processing — capping, poly-A tail addition, and splicing of introns — before becoming mature mRNA.
The mature mRNA passes out through nuclear pores into the cytoplasm. This is the critical handoff — the information is now outside the nucleus where the protein-making machinery lives.
Think of DNA as the original answer key locked in the principal’s office. mRNA is the photocopy that the teacher carries to the classroom.
Translation occurs in the cytoplasm, on ribosomes (either free in cytoplasm or attached to rough ER). The ribosome reads mRNA codons (triplets of bases) in the 5’→3’ direction.
tRNA molecules carry specific amino acids and match their anticodons to mRNA codons. The ribosome links amino acids via peptide bonds to build the polypeptide chain.
| Process | Location (Eukaryotes) | Key Enzyme |
|---|---|---|
| Transcription | Nucleus | RNA Polymerase |
| mRNA processing | Nucleus | Spliceosomes, etc. |
| Translation | Cytoplasm (ribosomes) | Peptidyl transferase |
The central dogma flow: DNA → (transcription, nucleus) → mRNA → (translation, cytoplasm) → Protein.
Why This Works
The spatial separation of transcription and translation in eukaryotes exists because eukaryotic cells have a nuclear membrane — a luxury prokaryotes lack. In prokaryotes (like E. coli), both processes happen simultaneously in the cytoplasm, which is why they’re called coupled transcription-translation.
This separation also allows eukaryotes to edit their mRNA before translation. Introns (non-coding sequences) are spliced out; exons (coding sequences) are joined. The resulting mature mRNA carries only the protein-coding sequence — a quality control step prokaryotes skip entirely.
The mRNA is temporary by design. It carries information, gets read multiple times by many ribosomes (forming a polysome), and is then degraded. This gives the cell fine control over how much of each protein is made.
Alternative Method
For a 1-mark or short-answer version, CBSE often accepts this compressed answer:
“The central dogma states that genetic information flows from DNA → RNA → Protein. In eukaryotes, transcription occurs in the nucleus and translation occurs in the cytoplasm on ribosomes.”
For a 3-mark answer, always mention RNA polymerase, mRNA processing, and ribosome involvement — those are the mark-fetching details examiners look for.
In NEET, a common question type asks: “In which of the following does transcription and translation occur at the same location?” — the answer is prokaryotes (and also mitochondria/chloroplasts in eukaryotes, since they have prokaryotic-type machinery). This distinction appeared in NEET 2023.
Common Mistake
Most students write “transcription occurs in the cytoplasm” — mixing it up with translation. Remember: Trans-CRIP-tion happens where the CRIPT (script/DNA) is kept — the nucleus. Translation happens where the ribosomes are — the cytoplasm. The mRNA travels from nucleus to cytoplasm; it does NOT carry DNA out with it.
A second common error: writing that DNA directly participates in translation. DNA never leaves the nucleus in eukaryotes. Only mRNA does the commute.