Questions: Central Dogma of Molecular Biology

Question 1

Which of the following correctly describes what the central dogma rules out as impossible?

• A. RNA being copied into DNA
• B. DNA being replicated before cell division
• C. Protein sequence being used to write a new DNA or RNA sequence
• D. A single gene producing multiple protein variants

Answer: (C)

The central dogma's core claim is that information cannot flow from protein back into nucleic acid. RNA-to-DNA (reverse transcription) does occur in retroviruses and was anticipated as an exception. DNA replication and alternative splicing are normal biological processes. What is forbidden is translating an amino acid sequence back into a nucleotide sequence.

Question 2

The discovery of reverse transcriptase in retroviruses disproves the central dogma of molecular biology.

• T. True
• F. False

Answer: False

Crick explicitly noted that RNA-to-DNA information flow was a theoretical possibility he called a 'special transfer.' Retroviruses confirmed this exception. What the central dogma absolutely prohibits — and what has never been observed — is protein-to-nucleic-acid information flow. The discovery of reverse transcription extended the dogma, it did not invalidate it.

Question 3

Why can't a cell use a protein's amino acid sequence to reconstruct the exact DNA sequence that encoded it?

Model answer: The genetic code is degenerate — multiple codons encode the same amino acid — so the protein sequence does not preserve which specific codon was used, making exact back-translation impossible.

For example, leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). Given a leucine in a protein, you cannot tell which codon was originally used. This degeneracy is one-directional: DNA → protein loses information, so the reverse is not recoverable from sequence alone.