Genetics- Chapter 13 The Genetic Code and Transcription

Chapter 13: The Genetic Code and Transcription

Learning Objectives

• 13.1: The Genetic Code Uses Ribonucleotide Bases as "Letters"

• 13.2: Early Studies Established the Basic Operational Patterns of the Code

• 13.3: Studies by Nirenberg, Matthaei, and Others Led to Deciphering of the Code

• 13.4: The Coding Dictionary Reveals Several Interesting Patterns among the 64 Codons

• 13.5: The Genetic Code Has Been Confirmed in Studies of Phage MS2

• 13.6: The Genetic Code Is Nearly Universal

• 13.7: Different Initiation Points Create Overlapping Genes

• 13.8: Transcription Synthesizes RNA on a DNA Template

• 13.9: RNA Polymerase Directs RNA Synthesis

• 13.10: Transcription in Eukaryotes Differs from Bacterial Transcription in Several Ways

• 13.11: The Coding Regions of Eukaryotic Genes Are Interrupted by Intervening Sequences Called Introns

• 13.12: RNA Editing May Modify the Final Transcript

• 13.13: Transcription Has Been Visualized by Electron Microscopy

Section 13.1: Introduction

• Central Dogma of Molecular Genetics: Flow of genetic information from DNA -> RNA -> Protein

Section 13.8: Transcription

• Transcription Defined: RNA synthesized on DNA template

  • Transfers genetic information from DNA to RNA

  • Serves as an intermediary between DNA and proteins

  • Each triplet codon on mRNA is complementary to an anticodon of tRNA

Section 13.9: RNA Polymerase

• RNA Polymerase: Enzyme responsible for RNA synthesis using a DNA template

  • Nucleotides contain ribose, not deoxyribose

  • No primer needed for initiation

Section 13.9: Promoters

• Transcription: Results in ssRNA (single-stranded RNA)

  • Begins with RNA polymerase binding to promoter sequences in the region upstream from transcription initiation point

  • Promoters assist in recognizing the initiation of transcription

Section 13.9: Transcription Start Site

• Initiation Process: DNA is unwound at the transcription start site

  • This exposes the template strand for RNA polymerase

  • The promoter's interaction with RNA polymerase regulates transcription efficiency

Section 13.9: Consensus Sequences

• Consensus Sequences: Homologous DNA sequences in different genes

  • E. coli promoters typically contain two consensus sequences: TTGACA and TATAAT

  • These are positioned relative to the transcription initiation site

Section 13.9: Chain Elongation

• Chain Elongation: When transcription progresses, RNA grows as nucleotides are added to the nascent RNA chain

  • The sigma (σ) subunit of RNA polymerase dissociates after initiating transcription

Section 13.9: Termination

• Termination Process: RNA polymerase stops transcription upon encountering a termination sequence

  • In bacteria, a hairpin loop structure in RNA signals termination

  • Rho-dependent termination involves the rho factor breaking RNA-DNA interactions

Section 13.10: Eukaryotic Transcription

• Eukaryotic Transcription: Occurs in the nucleus with mRNA needing to exit for protein synthesis

  • Chromatin must uncoil for RNA polymerase accessibility

  • Involves transcription factors and regulatory sequences like enhancers and silencers

Section 13.10: RNA Polymerases in Eukaryotes

• Types of RNA Polymerases: Three forms exist in eukaryotes, each transcribing different genes:

  • RNA Pol I: Synthesizes rRNA (nucleolus)

  • RNA Pol II: Synthesizes mRNA, snRNA (nucleoplasm)

  • RNA Pol III: Synthesizes tRNA, 5SrRNA (nucleoplasm)

Section 13.10: TATA Box and Regulatory Sequences

• TATA Box: Essential for transcription initiation

  • Binds TATA-binding protein (TBP) of transcription factor TFIID

  • Enhancers and silencers regulate transcription efficiency

Section 13.10: Transcription Factors

• Role of Transcription Factors: Facilitate RNA polymerase binding and transcription initiation

  • Include general transcription factors and specific activators/repressors

Section 13.10: mRNA Processing

• Posttranscriptional Modifications: Required to produce mature mRNAs

  • Include capping (7-mG cap), tailing (poly-A tail), and intron excision

Section 13.11: Introns and Exons

• Introns: Non-coding regions in the initial RNA transcript

  • Exons are the coding sequences retained in mature mRNA

• Splicing: Introns are removed to create the final mature mRNA structure

Section 13.13: Transcription Visualization

• Electron Microscopy Findings: Reveals multiple transcription events occurring simultaneously along DNA in prokaryotes

• Polyribosomes: Observed in both prokaryotes and eukaryotes, highlighting simultaneous translation and transcription.