Question
Explain the central dogma of molecular biology. Describe how genetic information flows from DNA to RNA to protein through replication, transcription, and translation.
(NEET, CBSE Class 12 — Molecular Basis of Inheritance)
Solution — Step by Step
Before cell division, DNA replicates itself. DNA polymerase synthesises a new complementary strand using the existing strand as a template. The process is semi-conservative — each new DNA molecule has one old strand and one new strand (proved by Meselson and Stahl). Replication occurs in the S phase of the cell cycle.
One strand of DNA (template/antisense strand) is used to synthesise a complementary mRNA by RNA polymerase. In prokaryotes, a single RNA polymerase does all transcription. In eukaryotes, RNA Pol I makes rRNA, RNA Pol II makes mRNA, RNA Pol III makes tRNA. The mRNA undergoes processing in eukaryotes: 5’ capping, 3’ polyadenylation, and splicing (removal of introns).
mRNA is read by ribosomes in groups of three nucleotides called codons. Each codon specifies an amino acid. tRNA molecules carry amino acids to the ribosome, matching their anticodon to the mRNA codon. Amino acids are linked by peptide bonds to form a polypeptide chain. Translation occurs in the cytoplasm.
graph LR
A["DNA"] -->|"Replication<br/>DNA Polymerase"| A
A -->|"Transcription<br/>RNA Polymerase"| B["mRNA"]
B -->|"Translation<br/>Ribosome + tRNA"| C["Protein"]Why This Works
The central dogma, proposed by Francis Crick (1958), describes the unidirectional flow of genetic information: DNA to RNA to Protein. DNA stores the blueprint, mRNA carries the message to the ribosome factory, and the ribosome reads the message to build the protein.
Exceptions exist: reverse transcription (RNA to DNA, by reverse transcriptase in retroviruses like HIV) and RNA replication (in some RNA viruses). But the general flow holds true for all cellular life.
The genetic code is universal (same in almost all organisms), degenerate (multiple codons for one amino acid), non-overlapping, and comma-less (read continuously in triplets).
Alternative Method — One-Line Summary per Process
Replication: DNA to DNA (copying the library). Transcription: DNA to mRNA (writing a note from the book). Translation: mRNA to protein (building from the instructions).
For NEET, remember the key enzymes: DNA Polymerase (replication), RNA Polymerase (transcription), Peptidyl transferase in ribosome (translation). Also know: AUG is the start codon (codes for methionine), UAA/UAG/UGA are stop codons (code for no amino acid).
Common Mistake
Students confuse the template strand with the coding strand. The template strand (antisense, 3’ to 5’) is read by RNA polymerase. The coding strand (sense, 5’ to 3’) has the same sequence as mRNA (except T instead of U). When NEET gives a DNA sequence and asks for mRNA, use the template strand to find the complement — or simply copy the coding strand replacing T with U.