Central Dogma: Transcription and Translation — Quick Reference

Transcription: DNA to RNA

  • DNA serves as the template; RNA polymerase synthesizes an RNA strand complementary to the DNA template.
  • In RNA, adenine pairs with uracil (U) instead of thymine; the RNA transcript is produced and shipped out of the nucleus for translation.
  • Not every gene is transcribed in every cell; all cells share the same DNA, but transcription factors regulate which genes are expressed.
  • Transcription factors bind DNA at promoter regions to recruit RNA polymerase and initiate transcription.

Regulation of Transcription in Cells

  • The same genome is present in all cells, but gene expression profiles differ due to which genes are transcribed.
  • Transcription factors control promoter activity and RNA polymerase access, shaping cell identity and function.

Translation: RNA to Protein

  • Translation occurs at ribosomes, which have large and small subunits and are composed of proteins and rRNA.
  • mRNA codons (three-nucleotide words) are translated into amino acids via the genetic code, which is nearly universally conserved.
  • Codon length is 33 nucleotides.
  • Start codon AUG signals where translation begins and encodes Methionine as the first amino acid.
  • Stop codons UGA, UAA, and UAG terminate translation; they do not code for an amino acid.
  • Example codons: CCU → Proline; AUG → Methionine; UUU → Phenylalanine; CGA → Arginine.

The Genetic Code and Codons

  • The genetic code maps each codon to a specific amino acid or a stop signal; translation uses this code to build proteins.
  • The table used to translate codons to amino acids is largely universal across organisms, supporting a common ancestry.

Initiation, Elongation, and Termination of Translation

  • Initiation: the ribosome (large + small subunits) binds the mRNA at the start codon; the initiator tRNA carries Methionine with anticodon UAC.
  • Elongation: a new tRNA binds its complementary codon; ribosome shifts by one codon (\$\$1\$ codon) along the mRNA; amino acids are linked to form a growing polypeptide.
    • Example sequence: AUG (Met) followed by UUU (Phe) followed by CGA (Arg); anticodons pair with codons to bring in the corresponding amino acids.
  • Termination: readthrough of a stop codon by release factors; polypeptide is released; ribosome dissociates and tRNAs are recycled.

Key Takeaways

  • Central dogma: DNA → RNA (transcription) → Protein (translation).
  • RNA polymerase makes RNA; RNA uses Uracil instead of Thymine.
  • Transcription is regulated by transcription factors that act at promoters.
  • Translation converts the RNA code into a protein sequence, guided by the genetic code.
  • Start codon: AUG → Methionine; Stop codons: UGA, UAA, UAG; Codons are read in sets of three to specify amino acids.
  • The genetic code’s universality supports a single common origin of life.