Transcription and splicing

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Transcript of Transcription and splicing

  • Genetics: Analysis and Principles Robert J. Brooker Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display CHAPTER 12 GENE TRANSCRIPTION AND RNA MODIFICATION (processing)
  • The central dogma of genetics Figure 12.1 12-5
    • A key concept is that DNA base sequences define the beginning and end of a gene and regulate the level of RNA synthesis
    • Gene expression is the overall process by which the information within a gene is used to produce a functional product which can determine a trait in play with the environment
    12.1 OVERVIEW OF TRANSCRIPTION Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12-6
  • Figure 12.2 12-7 Signals the end of protein synthesis
  • Gene Expression Requires Base Sequences Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • The strand that is actually transcribed (used as the template) is termed the template strand
    • The opposite strand is called the coding strand or the sense strand
      • The base sequence is identical to the RNA transcript
        • Except for the substitution of uracil in RNA for thymine in DNA
    • Transcription factors recognize the promoter and regulatory sequences to control transcription
    • mRNA sequences such as the ribosomal-binding site and codons direct translation
    12-8
  • The Stages of Transcription Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • Transcription occurs in three stages
      • Initiation
      • Elongation
      • Termination
    • These steps involve protein-DNA interactions
      • Proteins such as RNA polymerase interact with DNA sequences
      • Transcription factors that control transcription bind directly or indirectly to DNA
    12-9
  • 12-10 Figure 12.3
  • RNA Transcripts Have Different Functions Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • Once they are made, RNA transcripts play different functional roles
    • Well over 90% of all genes are structural genes producing mRNA
    • The other RNA molecules are never translated: This collection appears much greater that initially believed; Some RNAs are 20-25 nts long that have important functions!
    12-11
  • RNA Transcripts Have Different Functions Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • The RNA transcripts from nonstructural genes are not translated
      • They do have various important cellular functions
      • They can still confer traits
      • In some cases, the RNA transcript becomes part of a complex that contains protein subunits
        • For example
          • Ribosomes
          • Spliceosomes
          • Signal recognition particles
    12-12
    • Our molecular understanding of gene transcription came from studies involving bacteria and bacteriophages
    • Indeed, much of our knowledge comes from studies of a single bacterium
      • E. coli , of course
    • In this section we will examine the three steps of transcription as they occur in bacteria
    12.2 TRANSCRIPTION IN BACTERIA Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12-14
  • Promoters Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • Promoters are DNA sequences that promote gene expression: Events at this piece of DNA are needed to initiate RNA synthesis/transcription
      • More precisely, they direct the exact location for the initiation of transcription and determine when and how frequently a gene is transcribed.
    • Promoters are typically located just upstream of the site where transcription of a gene actually begins
      • The bases in a promoter sequence are numbered in relation to the transcription start site
    12-15
  • Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12-16 Figure 12.4 The conventional numbering system of promoters Bases preceding this are numbered in a negative direction There is no base numbered 0 Bases to the right are numbered in a positive direction Most of the promoter region is labeled with negative numbers
  • Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12-17 Figure 12.4 The conventional numbering system of promoters The promoter may span a large region, but specific short sequence elements are particularly critical for promoter recognition and activity level Sometimes termed the Pribnow box, after its discoverer Sequence elements that play a key role in transcription
  • Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12-18 Figure 12.5 Examples of 35 and 10 sequences within a variety of bacterial promoters The most commonly occurring bases For many bacterial genes, there is a good correlation between the rate of RNA transcription and the degree of agreement with the consensus sequences
  • Initiation of Bacterial Transcription Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • RNA polymerase is the enzyme that catalyzes the synthesis of RNA
    • In E. coli , the RNA polymerase holoenzyme is composed of
      • Core enzyme
        • Five subunits = 2
      • Sigma factor
        • One subunit =
      • These subunits play distinct functional roles
    12-19
  • Initiation of Bacterial Transcription Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • The RNA polymerase holoenzyme binds loosely to the DNA
    • It then scans along the DNA, until it encounters a promoter region
      • When it does, the sigma factor recognizes both the 35 and 10 regions
        • A region within the sigma factor that contains a helix-turn-helix structure is involved in a tighter binding to the DNA
      • Refer to Figure 12.6
    12-20
  • Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12-21 Figure 12.6 Amino acids within the helices hydrogen bond with bases in the promoter sequence elements
  • 12-23 Figure 12.7
  • 12-26 Similar to the synthesis of DNA via DNA polymerase Figure 12.8 On average, the rate of RNA synthesis is about 43 nucleotides per second!
  • Termination of Bacterial Transcription Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
    • Termination is the end of RNA synthesis
      • It occurs when the short RNA-DNA hybrid of the open complex is forced to separate
        • This releases the newly made RNA as well as the RNA polymerase
    • E. coli has two different mechanisms for termination
      • 1. rho-dependent termination
        • Requires a protein known as (rho)
      • 2. rho-independent termination
        • Does not require
    12-27
  • 12-28 r ho ut ilization site Rho protein is a helicase -dependent termination Figure 12.10
  • 12-29 -dependent termination Figure 12.10
    • -independent termination is facilitated by two sequences in the RNA
      • 1. A uracil-