Chapter 16 | DNA, RNA and Proteins

  1. Figure 16.11 In eukaryotic cells, DNA and RNA synthesis occur in a separate compartment from protein synthesis. In prokaryotic cells, both processes occur together. What advantages might there be to separating the processes? What advantages might there be to having them occur together?
    Illustration shows a eukaryotic cell, which has a membrane-bound nucleus containing chromatin and a nucleolus, and a prokaryotic cell, which has DNA contained in an area of the cytoplasm called the nucleoid. The prokaryotic cell is much smaller than the eukaryotic cell.
    Figure 16.11 A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an area called the nucleoid.
  2. Figure 16.15 You isolate a cell strain in which the joining together of Okazaki fragments is impaired and suspect that a mutation has occurred in an enzyme found at the replication fork. Which enzyme is most likely to be mutated?
    Illustration shows the replication fork. Helicase unwinds the helix, and single-strand binding proteins prevent the helix from re-forming. Topoisomerase prevents the DNA from getting too tightly coiled ahead of the replication fork. DNA primase forms an RNA primer, and DNA polymerase extends the DNA strand from the RNA primer. DNA synthesis occurs only in the 5' to 3' direction. On the leading strand, DNA synthesis occurs continuously. On the lagging strand, DNA synthesis restarts many times as the helix unwinds, resulting in many short fragments called “Okazaki fragments.” DNA ligase joins the Okazaki fragments together into a single DNA molecule.
    Figure 16.15 A replication fork is formed when helicase separates the DNA strands at the origin of replication. The DNA tends to become more highly coiled ahead of the replication fork. Topoisomerase breaks and reforms DNA’s phosphate backbone ahead of the replication fork, thereby relieving the pressure that results from this supercoiling. Single-strand binding proteins bind to the single-stranded DNA to prevent the helix from re-forming. Primase synthesizes an RNA primer. DNA polymerase III uses this primer to synthesize the daughter DNA strand. On the leading strand, DNA is synthesized continuously, whereas on the lagging strand, DNA is synthesized in short stretches called Okazaki fragments. DNA polymerase I replaces the RNA primer with DNA. DNA ligase seals the gaps between the Okazaki fragments, joining the fragments into a single DNA molecule. (credit: modification of work by Mariana Ruiz Villareal)
  3. Figure 16.22 A frameshift mutation that results in the insertion of three nucleotides is often less deleterious than a mutation that results in the insertion of one nucleotide. Why?
    Illustration shows different types of point mutations that result from a single amino acid substitution. In a silent mutation, no change in the amino acid sequence occurs. In a missense mutation, one amino acid is substituted for another. In a nonsense mutation, a stop codon is substituted for an amino acid. In a frameshift mutation, one or more bases is added or deleted, resulting in a change in the reading frame.
    Figure 16.22 Mutations can lead to changes in the protein sequence encoded by the DNA.
  4. Figure 16.29 A scientist splices a eukaryotic promoter in front of a bacterial gene and inserts the gene in a bacterial chromosome. Would you expect the bacteria to transcribe the gene?
    An illustration shows that before RNA processing, there is a primary RNA transcript including five boxes labeled, left to right, as exon 1, intron, exon 2, intron, and exon 3. After RNA processing, there is a spliced RNA with these parts, left to right: a 5' cap, a 5' untranslated region, exon 1, exon 2, exon 3, a 3' untranslated region, and a poly-a tail.
    Figure 16.29 Eukaryotic mRNA contains introns that must be spliced out. A 5′ cap and 3′ poly-A tail are also added.
  5. Figure 16.31 Errors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead to splicing errors? Think of different possible outcomes if splicing errors occur.
    Illustration shows a spliceosome bound to mRNA. An intron is wrapped around snRNPs associated with the spliceosome. When the splice is complete, the exons on either side of the intron are fused together, and the intron forms a ring structure.
    Figure 16.31 Pre-mRNA splicing involves the precise removal of introns from the primary RNA transcript. The splicing process is catalyzed by protein complexes called spliceosomes that are composed of proteins and RNA molecules called snRNAs. Spliceosomes recognize sequences at the 5′ and 3′ end of the intron.
  6. Figure 16.34 Many antibiotics inhibit bacterial protein synthesis. For example, tetracycline blocks the A site on the bacterial ribosome, and chloramphenicol blocks peptidyl transfer. What specific effect would you expect each of these antibiotics to have on protein synthesis?
    1. Tetracycline would directly affect:
      1. tRNA binding to the ribosome
      2. ribosome assembly
      3. growth of the protein chain
    2.  Chloramphenicol would directly affect
      1. tRNA binding to the ribosome
      2. ribosome assembly
      3. growth of the protein chain
        Illustration shows the steps of protein synthesis. First, the initiator tRNA recognizes the sequence AUG on an mRNA that is associated with the small ribosomal subunit. The large subunit then joins the complex. Next, a second tRNA is recruited at the A site. A peptide bond is formed between the first amino acid, which is at the P site, and the second amino acid, which is at the A site. The mRNA then shifts and the first tRNA is moved to the E site, where it dissociates from the ribosome. Another tRNA binds at the A site, and the process is repeated.
        Figure 16.34 Translation begins when an initiator tRNA anticodon recognizes a codon on mRNA. The large ribosomal subunit joins the small subunit, and a second tRNA is recruited. As the mRNA moves relative to the ribosome, the polypeptide chain is formed. Entry of a release factor into the A site terminates translation and the components dissociate.

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