Interactive step-by-step DNA replication and transcription animation: from double helix unwinding to mRNA transcript
DNA replication is semi-conservative: each new double helix contains one parent strand and one newly synthesized strand. Replication begins at origins and proceeds bidirectionally, forming replication forks.
The leading strand is synthesized continuously 5'→3' toward the fork. The lagging strand runs in the opposite direction and must be synthesized discontinuously as Okazaki fragments (~1000-2000 bases each).
DNA polymerase has 3'→5' exonuclease proofreading activity, achieving an error rate of ~10⁻⁹. A-T pairs form 2 hydrogen bonds; G-C pairs form 3, ensuring accurate complementary base pairing.
Uses ATP hydrolysis to break hydrogen bonds between bases, separating double-stranded DNA into single strands and forming the replication fork.
Synthesizes short RNA primers (5-10 nucleotides) to provide a 3'-OH starting point for DNA polymerase, which cannot initiate synthesis de novo.
Catalyzes phosphodiester bond formation in the 5'→3' direction. The primary replication enzyme with proofreading activity for accuracy.
Seals phosphodiester bond nicks between Okazaki fragments, joining discontinuous lagging strand segments into a continuous strand.
Reads the DNA template strand and synthesizes mRNA by complementary base pairing (U replaces T). Bacteria have a single RNA polymerase type.
Proposed by Francis Crick (1958): genetic information flows DNA → RNA → Protein. Replication copies DNA, transcription converts DNA to mRNA, and translation decodes mRNA into protein at ribosomes.
RNA polymerase recognizes promoter → DNA unwinds into transcription bubble → template strand (3'→5') guides mRNA synthesis (5'→3') → termination releases mRNA. Post-transcriptional processing includes splicing, capping, and polyadenylation.
mRNA is decoded at ribosomes: every 3 bases (codon) specify one amino acid. tRNAs carry amino acids and pair via anticodons, building a polypeptide chain one residue at a time.