Question
Explain the central dogma of molecular biology. Describe the flow of genetic information from DNA to RNA to Protein.
Solution — Step by Step
flowchart LR
A[DNA] -->|Replication| A
A -->|Transcription| B[mRNA]
B -->|Translation| C[Protein]
D[Reverse Transcriptase] -->|Reverse Transcription| A
D -.->|Exception: Retroviruses| BDNA replicates itself during cell division. The double helix unwinds, and each strand serves as a template for a new complementary strand. Enzyme: DNA polymerase. Result: two identical DNA molecules. This is semiconservative — each new molecule has one old strand and one new strand.
One strand of DNA (the template strand) is used to synthesise a complementary mRNA molecule. Enzyme: RNA polymerase. This occurs in the nucleus. The mRNA carries the genetic code from the nucleus to the cytoplasm. In eukaryotes, the primary transcript undergoes splicing (removing introns, joining exons) before leaving the nucleus.
The mRNA sequence is read by ribosomes in the cytoplasm. Each set of three nucleotides (a codon) specifies one amino acid. tRNA molecules bring the correct amino acids, matching their anticodon to the mRNA codon. Amino acids are joined by peptide bonds to form a polypeptide chain (protein).
Why This Works
The central dogma (proposed by Francis Crick, 1958) describes the unidirectional flow of genetic information: DNA serves as the permanent information store, mRNA is the temporary working copy, and proteins are the functional products. This explains how genes control cellular functions — the DNA sequence determines the protein sequence, and proteins carry out virtually all cellular work.
Exceptions to the central dogma: Retroviruses (like HIV) use reverse transcriptase to make DNA from RNA (reverse transcription). Some RNA viruses can replicate RNA from RNA. Prions involve protein-to-protein transmission without nucleic acid involvement.
Common Mistake
Students write that “both DNA strands are transcribed.” Only ONE strand (the template/antisense strand) is transcribed for any given gene. The other strand (coding/sense strand) has the same sequence as the mRNA (except T instead of U). Also, different genes may use different strands as templates.
The genetic code is universal (same codons = same amino acids across almost all organisms), degenerate (multiple codons can code for the same amino acid), and has start (AUG) and stop (UAA, UAG, UGA) codons. These properties are guaranteed NEET questions.