Gene expression

Genes and Proteins

Introduction

  • Genes: Linear instructions on chromosomes for making RNA and proteins.

  • Proteins: Vital for all life processes.

  • Central Dogma: DNA -> RNA -> Protein

15.1 The Genetic Code

  • mRNA: Generated from DNA via transcription.

  • Translation: Converts mRNA into proteins; 20 amino acids form proteins from the 64 possible nucleotide triplets.

  • Codons: Triplets of nucleotides specifying amino acids; redundancy means some amino acids are encoded by multiple codons.

15.2 Prokaryotic Transcription

  • Initiation: Promoter sequences recruit RNA polymerase to start transcription.

  • Elongation: Synthesized at ~40 nucleotides per second in 5' to 3' direction.

  • Termination: Ends via Rho-dependent or hairpin formation (Rho-independent).

15.3 Eukaryotic Transcription

  • RNA Polymerases: Three types (I, II, III); each transcribes different genes (e.g., rRNA, mRNA).

  • Enhancers and Silencers: Regulate transcription efficiency; many transcription factors needed for RNA polymerase binding.

  • Termination: Varies; polymerase II synthesizes beyond gene ends, which is later cleaved.

15.4 RNA Processing in Eukaryotes

  • Pre-mRNA Modifications: 5' cap and poly-A tail added, introns removed during splicing.

  • Splicing: Catalyzed by spliceosomes; accuracy is crucial for mRNA functionality.

15.5 Ribosomes and Protein Synthesis

  • Translation Components: mRNA, ribosomes, tRNAs, and various factors.

  • Ribosome Function: Large and small subunits read mRNA and assemble amino acids into polypeptides.

  • Initiation, Elongation, and Termination: Key phases of translation; release factors recognize stop codons.

Key Terms

  • Central Dogma: Flow of genetic information (DNA -> RNA -> Protein).

  • Promoter: DNA sequence for RNA polymerase binding.

  • Codon: Sequence of three nucleotides.

  • Intron: Non-coding sequence in a gene.

  • Splicing: Process of removing introns from pre-mRNA.

  • tRNA: Transfers amino acids to the ribosome during translation.

  • Ribosome: Site of protein synthesis, composed of rRNA and proteins.

Chapter Summary

  • Genetic information flows from genes to mRNA to proteins, with coding efficiencies largely derived from the degeneracy of the genetic code.