19.8DNA Structure, DNA Replication, and Protein Synthesis

The idea

• Describe the double helix and semiconservative replication.
• Outline transcription and translation.

DNA is a double helix: two antiparallel strands, bases paired inward, sugar–phosphate backbone outward. Replication opens the helix and each strand is a template. New nucleotides are added so that A matches T and G matches C. Each daughter molecule has one old strand and one new strand, which is semiconservative copying. The sequence is preserved because the pairing rule, not a special new idea, does the choosing. Enzymes do the chemical work; the information is the template.

Protein synthesis has two steps. Transcription copies a gene into messenger RNA, using the same pairing idea, with U in place of T. Translation reads the mRNA in codons of three bases. Each codon matches a transfer RNA carrying a specific amino acid, and the ribosome joins those amino acids into a chain. The genetic code is the table of which codon means which amino acid, and it is nearly universal. A mutation that changes a base can change a codon, and therefore an amino acid, and therefore a fold. Not every base change does damage — some codons share an amino acid, and some regions do not code — but the mechanism of inherited molecular change is this one.

Worked path

A short DNA template strand reads TAC. What complementary DNA bases pair with it, and what mRNA codon would transcription produce if this template is transcribed?

1. Observe

   DNA pairing: T–A, A–T, C–G. RNA uses U instead of T.

Check yourself