Genetics Essentials: DNA, RNA, and Protein Synthesis

DNA and RNA roles

  • DNA stores genetic information; contains genes that code for proteins.
  • RNA copies the genetic information from DNA (transcription) and guides protein synthesis (translation).
  • Proteins are built from 20 amino acids; the sequence dictates shape and function.
  • Central dogma: DNA \rightarrow RNA \rightarrow Protein.

Gene structure and regulation

  • Gene: continuous nucleotide string with a promoter (start) and terminator (end).
  • Regulatory sequences can be near the promoter or distant.
  • Promoter: recognition site for RNA polymerase; controls transcription.
  • Terminator: signals transcription termination.
  • Exons: coding regions; Introns: non-coding regions removed during processing.
  • In eukaryotes, gene expression involves transcription in the nucleus and translation in the cytoplasm.

Transcription

  • Three steps: initiation, elongation, termination.
  • Initiation: RNA polymerase binds at the promoter; DNA unwinds.
  • Elongation: RNA polymerase adds nucleotides to the 3' end using base-pairing rules.
  • Termination: RNA polymerase, DNA, and RNA transcript dissociate.
  • Result: messenger RNA (mRNA) that carries the code from DNA.

RNA processing and maturation

  • Transcribed mRNA contains exons (coding) and introns (non-coding).
  • Introns must be removed and exons joined (splicing) to produce mature mRNA.
  • 5' cap and 3' poly-A tail are added for stability and export.
  • Spliceosome complex carries out intron removal and exon joining.
  • Mature mRNA exits the nucleus through a nuclear pore to the cytoplasm.

Translation and the genetic code

  • Genetic code uses codons: triplets of nucleotides in mRNA.
  • Codon identity: specifies which amino acid to add; there are 64 codons in total.
  • Start codon (initiates translation) and three stop codons regulate translation termination; hence, the four special codons: 1 start and 3 stop.
  • tRNA delivers amino acids to the ribosome; anticodon pairs with codon via base pairing.
  • Initiation: small ribosomal subunit binds mRNA upstream of start codon; initiator tRNA binds; large subunit joins to form the translation complex at the A, P, and E sites.
  • Elongation: amino acids are added one by one; peptide bond forms between amino acids; ribosome moves codon-by-codon (A site accepts, P site holds growing chain, E site exits).
  • Termination: stop codon recognized; release factor causes polypeptide release and ribosome disassembly.
  • Purpose: produce polypeptides quickly and accurately.

Post-translational modifications and protein fate

  • Newly synthesized polypeptides may be modified in the endoplasmic reticulum and Golgi.
  • Secreted or membrane proteins are directed into vesicles and transported to destinations (e.g., digestive enzymes to lumen).

Additional concepts for quick recall

  • Redundancy of the genetic code: multiple codons can code for the same amino acid; protects against mutations.
  • Alternate splicing: different exons can be included/excluded to generate protein diversity in different cell types.
  • Nutrient requirements: DNA contains instructions to make the proteins needed to process nutrients from diet.