Overall Goals of the Unit

  • Information encoded within DNA directs cellular and organismal functions as well as behaviors.
  • Regulatory networks are essential for organismal function.
  • Focused areas include:
    • Genes
    • Traits
    • Evolution
    • Ecology
    • Disease & disorders.

Course Reading

  • Chapters 17.1 – 17.4, review sections 17.1 and 17.5 on gene expression

Unit Objectives (L21)

  • Understand the Central Dogma: how information translates into behavior.
  • Explore various types of RNA, crucial for interpreting DNA information.
  • Investigate the RNA world concept including RNA viruses.
  • Examine processes of transcription, translation, and protein localization.
  • Discuss differences between compartmentalization in eukaryotes and co-transcription/translation in bacteria.
  • mRNA as a transient message, subject to cellular modifications for transport and regulation.
  • Understand the propagation of mutations through DNA replication, and their impact on fitness.
  • Overview of bacterial genetics as the foundation for modern molecular biology tools.

Gene Expression

  • Definition: Gene expression is the process by which DNA directs the synthesis of proteins.
  • Output of a gene or genes results in a trait or phenotype.
    • Example: Mouse fur color is influenced by pigment produced by gene-encoded enzymes.

Genetic Information Flow

  • The flow of genetic information follows several key stages:
    1. Transcription
      • Initiation
      • Elongation
      • Termination
    2. Translation
      • Initiation
      • Elongation
      • Termination

Basic Mechanisms of Transcription and Translation in Bacteria

  • In bacteria and Archaea, information processing and protein synthesis occur simultaneously in the cytoplasm.
  • Transcription
    • Initiated at the promoter region, specifically involving the sigma factor that recognizes promoter sequences.
    • Elongation involves adding ribonucleotides to the 3' (-OH) end of the emerging mRNA strand.
    • Termination can occur through various mechanisms that release the newly formed mRNA.

RNA Types in Bacteria

  • Several RNA types are essential in the processes within bacteria:
    • mRNA (messenger RNA)
    • tRNA (transfer RNA)
    • rRNA (ribosomal RNA)
  • Not all RNA is translated into proteins; some are classified as non-coding RNA.

Role of Ribosomes

  • After transcription, ribosomes bind to mRNA to initiate translation:
    • Translation requires:
    • Ribosome binding site (RBS)
    • Initiator tRNA
  • tRNA decodes mRNA into the corresponding amino acids using codons.

Codon Interpretation

  • tRNAs ensure the correct amino acids are added based on mRNA codons, emphasizing regions important for functionality:
    • 5' end
    • 3' end
    • Anticodon sequence
    • Anticodon loop
    • Acceptor end / site for amino acid bonding

Translation Process

  • Steps of Translation:
    1. Initiation: Small ribosome subunit binds mRNA; initiator tRNA recruited to form a complex with large subunit.
    2. Elongation: Amino acids are consecutively added based on mRNA codon instructions, forming polypeptides.
    3. Termination: Occurs when ribosome encounters a stop codon (UAG, UAA, UGA), triggering the release of polypeptides and dissociation of ribosomal subunits.

Compartmentalization in Eukaryotes

  • In eukaryotic cells, transcription occurs in the nucleus, while translation transpires in the cytoplasm. Differences in control and steps of protein synthesis arise due to compartmentalization.
    • Eukaryotes possess both coding and non-coding regions in genes, requiring mRNA processing (e.g., splicing) before exiting the nucleus.

RNA Splicing

  • Splicing is the removal of introns and joining of exons, enabling mature mRNA formation for translation.

Effects of Mutations

  • DNA mutations can lead to changes in polypeptides, impacting overall gene expression:
    • Mutations may arise from:
    • Random errors during replication
    • Misalignment leading to insertions or deletions
  • Example of a mutation affecting phenotypic expression:
    1. Wild-type pigmentation gene produces functional enzyme leading to brown pigment.
    2. Mutated gene results in non-functional enzyme and therefore no pigment.

Concept Questions

  1. Given the nontemplate strand sequence 5′-TTC ACT GGT TCA -3’, determine the resulting transcript sequence for this portion. Options include:
    • a) 5′-AAG UGA CCA AGU-3’
    • b) 5′-UGA ACC AGU GAA-3’
    • c) 5′-UUC ACU GGU UCA-3’
    • d) 5′-ACU UGG UCA CTT-3’
  2. Consider the effect of injecting a drug that inhibits ribosomes into a eukaryotic cell infected with a virus. Determine the consequences on transcription, translation, and the necessary location for the drug injection.

Conclusion

  • The intricate processes of gene expression, from DNA to functional proteins, form the foundation of biological functions, with organisms adapting through evolutionary mechanisms.
  • Understanding these processes is crucial for further studies in molecular biology and genetics, especially applications tied to health and disease management.