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The central dogma of molecular biology describes the directional flow of genetic information: DNA is transcribed into messenger RNA (mRNA), which is then translated into protein. This two-step process physically separates information storage (DNA in the nucleus) from protein manufacturing (ribosomes in the cytoplasm). RNA polymerase reads the template DNA strand in the 3' to 5' direction, synthesizing mRNA in the 5' to 3' direction.
Translation converts the mRNA sequence into an amino acid chain. Ribosomes read mRNA codons (three-nucleotide sequences) with the help of transfer RNAs (tRNAs), each carrying the matching amino acid. The process begins at a start codon (AUG) and ends at one of three stop codons (UAA, UAG, UGA). The resulting polypeptide folds into a functional protein. The genetic code is universal across almost all living organisms, underscoring the common ancestry of life.
Key Facts
- Transcription: DNA template strand → mRNA (in nucleus); RNA polymerase catalyzes this
- Translation: mRNA codons → amino acid chain (at ribosome); tRNA brings amino acids
- Codon: 3-nucleotide mRNA sequence encoding one amino acid (64 codons for 20 amino acids)
- Start codon: AUG (methionine); Stop codons: UAA, UAG, UGA
- mRNA is read 5' → 3'; the template DNA strand is read 3' → 5'
- One gene can produce many mRNA copies; one mRNA can be translated by many ribosomes simultaneously (polysome)
Vocabulary
- Transcription
- The synthesis of mRNA from a DNA template, catalyzed by RNA polymerase, occurring in the nucleus.
- Translation
- The synthesis of a polypeptide from an mRNA template at the ribosome, using tRNAs to match codons to amino acids.
- Codon
- A sequence of three nucleotides in mRNA that specifies a particular amino acid or a stop signal during translation.
- tRNA (transfer RNA)
- An RNA molecule with an anticodon that pairs with an mRNA codon and carries the corresponding amino acid to the ribosome.
- Promoter
- A DNA sequence upstream of a gene where RNA polymerase binds to begin transcription.
Common Mistakes to Avoid
- Confusing the template strand with the coding strand. The template strand is read by RNA polymerase; the coding strand has the same sequence as the mRNA (with T replaced by U).
- Thinking translation occurs in the nucleus. Translation happens at ribosomes in the cytoplasm (or on the rough ER). Only transcription occurs in the nucleus.
- Assuming each codon codes for a unique amino acid. The genetic code is degenerate: most amino acids are encoded by multiple codons (e.g. Leu has 6 codons).
- Forgetting that mRNA must be processed before leaving the nucleus in eukaryotes. Introns are spliced out and a 5' cap and poly-A tail are added before export.
Practice Questions
- 1 The DNA template strand reads 3'-TACGGATCC-5'. Write the mRNA sequence transcribed from it and identify the amino acids encoded.
- 2 A mutation changes codon UAC (Tyr) to UAA. How does this affect protein synthesis?
- 3 Explain why the genetic code being universal is strong evidence that all life shares a common ancestor.