Chapter 12: Genetic Code and Transcription
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Questions and Answers

What is the characteristic of the genetic code where a given amino acid can be specified by more than one triplet codon?

  • Unambiguous
  • Degenerate (correct)
  • Commaless
  • Nonoverlapping

    • What is the term for the sequence of three ribonucleotide letters that code for an amino acid?

  • Triplet
  • Codon (correct)
  • Gene
  • mRNA

    • What is the function of 'stop' signals in the genetic code?

  • They terminate translation (correct)
  • They initiate translation
  • They code for a specific amino acid
  • They specify the start of a gene

    • What is the term for the process where an RNA molecule is intermediate between DNA and proteins?

    <p>Messenger RNA</p> Signup and view all the answers

    What is the result of a frameshift mutation?

    <p>The reading frame is shifted</p> Signup and view all the answers

    What is the characteristic of the genetic code where it is read without breaks?

    <p>Commaless</p> Signup and view all the answers

    Who suggested that an RNA molecule was intermediate between DNA and proteins?

    <p>Francois Jacob and Jacques Monod</p> Signup and view all the answers

    What is the purpose of polynucleotide phosphorylase in the experiment of Marshall Nirenberg and Heinrich Matthaei?

    <p>To degrade RNA</p> Signup and view all the answers

    What is the significance of the triplet code in the genetic code?

    <p>It is the minimal use of four letters to specify amino acids</p> Signup and view all the answers

    What is the characteristic of the genetic code where it is nearly universal across different organisms?

    <p>Nearly universal</p> Signup and view all the answers

    What is a key difference between RNA polymerase and the enzyme used in the experiment?

    <p>The enzyme used in the experiment does not require a DNA template</p> Signup and view all the answers

    What determines the probability of the insertion of a specific ribonucleotide in the experiment?

    <p>The concentration of the ribonucleotide in the reaction mixture</p> Signup and view all the answers

    What is the purpose of using homopolymers in the experiment?

    <p>To synthesize RNA with specific codons</p> Signup and view all the answers

    What is the result of using a mixed heteropolymer with a ratio of 1A:5C in the experiment?

    <p>There is a 1/6 chance for an A and a 5/6 chance for a C</p> Signup and view all the answers

    What is the technique used to determine the specific assignments of triplets?

    <p>Triple binding assay</p> Signup and view all the answers

    What is the result of the triple binding assay experiment?

    <p>The genetic code is degenerate but unambiguous</p> Signup and view all the answers

    What is the purpose of repeating copolymers in the experiment?

    <p>To decipher the genetic code</p> Signup and view all the answers

    What is the function of the poly-A tail in eukaryotic mRNA?

    <p>It provides stability and protection to mRNA from nuclease attack</p> Signup and view all the answers

    What is a characteristic of the genetic code revealed by the experiment?

    <p>The genetic code is degenerate but unambiguous</p> Signup and view all the answers

    What is the function of the TFIID complex in transcription initiation?

    <p>It binds to the TATA box and helps in the recruitment of RNA polymerase II</p> Signup and view all the answers

    What is the result of adding a specific ratio of ribonucleotides to the reaction mixture in the experiment?

    <p>The probability of the insertion of a specific ribonucleotide is proportional to the availability of that molecule</p> Signup and view all the answers

    What is the significance of the experiment in understanding the genetic code?

    <p>It deciphered the genetic code</p> Signup and view all the answers

    How does transcription termination occur in eukaryotes?

    <p>It occurs when RNAP II reaches a polyadenylation signal sequence</p> Signup and view all the answers

    What is the function of the 5' cap in eukaryotic mRNA?

    <p>It stabilizes and protects the mRNA from nuclease attack</p> Signup and view all the answers

    What is the function of RNA polymerase II in eukaryotes?

    <p>It transcribes protein-coding genes</p> Signup and view all the answers

    What is the difference between prokaryotic and eukaryotic transcription?

    <p>Eukaryotic transcription is more complex and regulated</p> Signup and view all the answers

    What is the function of the σ factor in RNA polymerase?

    <p>To recognize and bind to the promoter sequence</p> Signup and view all the answers

    What is the term for the point at which transcription actually begins?

    <p>Transcription start site</p> Signup and view all the answers

    What is the term for DNA sequences that are homologous in different genes of the same organism?

    <p>Consensus sequences</p> Signup and view all the answers

    What is the function of the core enzyme in RNA synthesis?

    <p>To synthesize RNA using DNA template</p> Signup and view all the answers

    What is the term for the process of adding a 5' cap to the mRNA and removing intervening sequences?

    <p>Processing</p> Signup and view all the answers

    What is the term for the region of the DNA molecule that is unwound to make the template strand accessible to RNA polymerase?

    <p>Transcription bubble</p> Signup and view all the answers

    What is the term for the sequence-specific cleavage of the transcript required for transcriptional termination?

    <p>Cleavage stimulation factor</p> Signup and view all the answers

    What is the term for the DNA elements that bind to trans-acting factors to influence gene expression?

    <p>Cis-acting elements</p> Signup and view all the answers

    What is the term for the complex that binds to the TATA box and determines the transcription start site?

    <p>TATA-binding protein</p> Signup and view all the answers

    What is the term for the process of unwinding the DNA double helix to make the template strand accessible to RNA polymerase?

    <p>Unwinding</p> Signup and view all the answers

    What is the significance of the Wobble Hypothesis?

    <p>It states that the first two base pairings are more critical.</p> Signup and view all the answers

    What is the significance of the ordered nature of the genetic code?

    <p>It reduces the effect of mutations on protein function.</p> Signup and view all the answers

    What is the function of methionine (AUG) in protein synthesis?

    <p>It is the initiator codon for protein synthesis.</p> Signup and view all the answers

    What is the result of a mutation that produces a terminator codon (UAG, UAA, or UGA) in a gene?

    <p>Only a partial polypeptide is made.</p> Signup and view all the answers

    What is the significance of the discovery of exceptions to the universal genetic code?

    <p>It reveals that the genetic code is not universal.</p> Signup and view all the answers

    What is the role of RNA polymerase in transcription?

    <p>It synthesizes RNA from DNA.</p> Signup and view all the answers

    What is the result of transcription?

    <p>An mRNA molecule is produced.</p> Signup and view all the answers

    What is the substrate for the enzyme RNA polymerase?

    <p>NTPs</p> Signup and view all the answers

    Study Notes

    Ribonucleosides and RNA Synthesis

    • Ribohomopolymer synthesis uses a single type of ribonucleoside diphosphate.
    • Addition of ribonucleotides is random without a DNA template, influenced by their relative concentrations.
    • The probability of specific ribonucleoside insertion reflects its availability.

    Mixed Heteropolymers

    • Mixed heteropolymers consist of two or more ribonucleoside diphosphates.
    • The ratio of ribonucleotides can predict the frequency of specific codons in synthetic mRNA.
    • Nirenberg & Matthaei established that ribonucleoside insertion depends on relative concentrations in the mixture.

    Triple Binding Assay

    • Developed by Nirenberg & Leder to assign specific triplet codons to amino acids.
    • Ribosomes bind to short RNA sequences and attract the correct charged tRNA.
    • Two key conclusions:
      • The genetic code is degenerate (one amino acid can be coded by multiple triplets).
      • The genetic code is unambiguous (a single triplet specifies one amino acid).

    Repeating Copolymers

    • Khorana's work involved synthesizing long RNA molecules with repeated sequences (e.g., AAG).
    • Involves di-, tri-, and tetranucleotides transformed into RNA copolymers.

    RNA Polymerase Function

    • Directs RNA synthesis using a DNA template.
    • Requires no primer for initiation.
    • Transcription starts when RNA polymerase binds to the promoter using the sigma factor.

    Transcription Process

    • Begins with template strand unwinding, allowing RNA polymerase access to initiate RNA synthesis.
    • The interaction between promoter sequences and RNA polymerase significantly regulates transcription efficiency.

    Transcription Start Sites and Promoters

    • Transcription initiation sites are determined by specific promoter sequences.
    • E. coli has consensus sequences at -35 and -10 regions (e.g., TTGACA, TATAAT).
    • Promoters influence RNA polymerase binding and transcription initiation.

    Eukaryotic Transcription

    • Eukaryotic transcription occurs in the nucleus; mRNA requires further processing before leaving for translation.
    • Three forms of RNA polymerase (e.g., RNAP II) are complex and regulated differently.
    • Involves chromatin remodeling, transcription factors, and regulatory elements like enhancers and silencers.

    Post-Transcriptional Modifications

    • Eukaryotic mRNAs undergo alterations such as 5' capping, poly-A tail addition, and splicing.
    • Capping stabilizes mRNA against degradation, while the poly-A tail facilitates nuclear export and translational efficiency.

    Genetic Code Characteristics

    • The genetic code is linear, comprised of triplet codons that are unambiguous and degenerate.
    • Codons direct the initiation and termination of translation (start codon: AUG; stop codons: UAG, UAA, UGA).
    • The code is nearly universal across species, with some exceptions noted in mitochondrial DNA.

    Wobble Hypothesis and Degeneracy

    • The wobble hypothesis permits flexibility in the third position of the codon, leading to degeneracy.
    • Chemically similar amino acids can share one or two bases within their codons, providing robustness against mutations.

    Mutations and Frame Shifts

    • Frame shifts result from insertions or deletions of nucleotides, potentially altering the reading frame of the codon sequence.
    • Inserting three nucleotides reestablishes the reading frame, supporting the triplet nature of the code.

    Conclusion

    • mRNA serves as the carrier of genetic information, translating DNA's instructions into proteins.
    • RNA polymerase facilitates the synthesis of RNA from DNA templates and plays a critical role in gene expression regulation.

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    Description

    This quiz covers the characteristics of the genetic code, including its linear form, codon structure, and properties such as unambiguity and degeneracy. It also explores the role of start and stop signals in transcription.

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