Biology Chapter on DNA Structure and Function
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Questions and Answers

What induces a negative supercoil in eukaryotic DNA during chromatin assembly?

  • Binding of a histone core to form a nucleosome (correct)
  • Exposure of the DNA strand to nucleases
  • Coiling of the DNA itself
  • Interaction of DNA with RNA molecules

    • What is the length of E.coli's DNA in relation to the size of the cell?

  • It is equal to the length of the cell.
  • It is double the length of the cell.
  • It is 850 times longer than the cell. (correct)
  • It is shorter than the cell.

    • What occurs to chromosomes during the prophase of mitosis?

  • Chromosomes undergo condensation (correct)
  • Chromosomes align in pairs
  • Sister chromatids separate
  • Chromatin remains amorphous

    • What role do topoisomerases play in the maintenance of eukaryotic DNA structure?

    <p>They relax positive supercoils nearby</p> Signup and view all the answers

    Which structure organizes the circular chromosome in bacterial DNA?

    <p>SMC proteins</p> Signup and view all the answers

    How much compaction occurs to fit a DNA molecule of ~105 µm into a 5-10 µm nucleus?

    <p>10,000-fold</p> Signup and view all the answers

    What is the term for the area in prokaryotic cells that contains DNA?

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

    What is the main component of chromatin that makes up eukaryotic chromosomes?

    <p>DNA and proteins</p> Signup and view all the answers

    What happens to the DNA in a looped domain when it is cleaved?

    <p>Only the cleaved domain becomes relaxed.</p> Signup and view all the answers

    How many loops approximately form from the organization of the E.coli chromosome?

    <p>500 loops of about 10 kbp each.</p> Signup and view all the answers

    During which phase do chromatin remain amorphous in non-dividing cells?

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

    What happens to sister chromatids during anaphase?

    <p>They separate and move towards opposite poles</p> Signup and view all the answers

    What is a key feature of eukaryotic DNA during the M phase of the cell cycle?

    <p>It forms condensed chromosomes</p> Signup and view all the answers

    What are the two main types of DNA supercoiling?

    <p>Positive and negative supercoiling</p> Signup and view all the answers

    What is the primary action of topoisomerases on DNA?

    <p>Creating or relaxing supercoils</p> Signup and view all the answers

    What is the basic structural unit of chromatin?

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

    How do histone variants affect chromatin structure?

    <p>They alter the compactness and functionality of chromatin</p> Signup and view all the answers

    What is the ratio of the length of the human genome to the size of its cellular nucleus?

    <p>1:10^5</p> Signup and view all the answers

    What provides the extreme compaction of DNA within cells?

    <p>Chromatin structure and folding</p> Signup and view all the answers

    Which of the following is NOT a factor in chromatin remodeling?

    <p>Nucleotide excision repair</p> Signup and view all the answers

    What happens to chromatin structure during transcription activation?

    <p>Chromatin becomes more relaxed</p> Signup and view all the answers

    What is the primary role of DNA Topoisomerases?

    <p>To catalyze underwinding and relaxation of DNA</p> Signup and view all the answers

    Which statement is true regarding underwound DNA?

    <p>It is facilitated by the action of specific enzymes</p> Signup and view all the answers

    What characteristic of Type I topoisomerases sets them apart from Type II?

    <p>They transiently break one DNA strand only</p> Signup and view all the answers

    Why are covalent enzyme-DNA linkages important for topoisomerases?

    <p>They conserve bond energy and maintain genome integrity</p> Signup and view all the answers

    In which scenario would Type II topoisomerases be specifically required?

    <p>To separate newly formed chromosomes after replication</p> Signup and view all the answers

    How does supercoiling of DNA affect its migration during electrophoresis?

    <p>Supercoiled DNA is more compact, allowing faster migration</p> Signup and view all the answers

    What impact does DNA underwinding have on cellular functions?

    <p>It facilitates the packaging of DNA and access to genetic information</p> Signup and view all the answers

    Which factor contributes to the maintenance of DNA in an underwound state?

    <p>The closed circular form of the DNA</p> Signup and view all the answers

    What constitutes a nucleosome?

    <p>Eight histone molecules</p> Signup and view all the answers

    How many base pairs of DNA are tightly bound around the histone core in a nucleosome?

    <p>146 bp</p> Signup and view all the answers

    What role does Histone H1 serve in the nucleosome structure?

    <p>It binds to the linker DNA</p> Signup and view all the answers

    What percentage of the DNA surface in nucleosomes remains accessible for interactions with DNA-binding proteins?

    <p>75%</p> Signup and view all the answers

    Which part of the histone structure is involved in interacting with the DNA?

    <p>The histone fold</p> Signup and view all the answers

    What type of proteins are histones classified as?

    <p>Basic proteins</p> Signup and view all the answers

    What structural feature is central to the interaction between histones and DNA?

    <p>Histone fold</p> Signup and view all the answers

    What describes the arrangement of chromosomes as they relate to nucleosomes?

    <p>Beads-on-a-string</p> Signup and view all the answers

    What is the primary role of histones in eukaryotic cells?

    <p>They package and order DNA into nucleosomes.</p> Signup and view all the answers

    Which of the following correctly describes nucleosomes?

    <p>They represent the primary structural unit of chromatin.</p> Signup and view all the answers

    How does chromatin behave under low ion concentrations?

    <p>It unfolds into a 'beads-on-a-string' structure.</p> Signup and view all the answers

    Which proteins are NOT considered histones?

    <p>Transcription factors (TFs)</p> Signup and view all the answers

    What is the function of structural maintenance of chromosomes (SMC) proteins?

    <p>They are involved in chromosome segregation.</p> Signup and view all the answers

    Which of the following statements about chromatin is true?

    <p>Chromatin can unfold into 'beads-on-a-string' at low ion concentrations.</p> Signup and view all the answers

    What effect does treating chromatin with a nonspecific nuclease have?

    <p>It cuts DNA where it is not associated with proteins.</p> Signup and view all the answers

    In what form do histones primarily exist when interacting with DNA?

    <p>As part of the nucleosome core.</p> Signup and view all the answers

    What do topoisomerases primarily function to do?

    <p>They maintain DNA structure during replication.</p> Signup and view all the answers

    What percentage of chromatin is composed of histone proteins?

    <p>70-80%</p> Signup and view all the answers

    Study Notes

    Chromatin Structure

    • Chromosomes, the carriers of genetic information, are significantly longer than the cellular or viral packages that contain them.
    • DNA molecules are extremely compacted inside cells, but the information in DNA must remain accessible for replication and transcription.
    • DNA compaction is achieved through a high degree of structural organization.
    • Fundamental to this organization are the neutralization of negative charges in the DNA backbone by cations and polyamines, and supercoiling.
    • Supercoiling is a change in the DNA's structure creating additional coils.

    DNA Supercoiling

    • Define types of DNA supercoiling and explain their significance in biological processes.
    • Describe how DNA topoisomerases affect DNA supercoiling. Critically assess the different types of supercoiling.

    Chromatin Structure

    • Describe the structure of chromatin in detail, from fundamental units to higher-order structures.
    • Assess the roles of histone variants and modifications in chromatin structure and gene expression.

    Organization of Bacterial DNA

    • Bacteria lack nucleosomes, but their DNA is still compacted into a nucleoid, a distinct region within the prokaryotic cell.
    • Bacterial DNA is organized into loops that are attached to a chromosomal scaffold.
    • The structure of looped DNA domains is dynamically arranged, shifting along the DNA during replication.

    Eukaryotic Chromatin Assembly and Organization

    • Eukaryotic DNA is underwound compared to the established B-form structure.
    • Underwinding is maintained because of the stabilization by proteins and the presence of a closed circle.
    • Describe how the binding of histones forms a nucleosome and how this induces negative supercoils and helps organize chromatin.
    • Describe how Relaxation of positive supercoils is performed by topoisomerases.
    • Explain how histone tails are necessary for dynamic regulation of chromatin structure. Identify different histone variant types.
    • Describe the role of nucleosomes in chromatin organization.
    • Describe higher-order chromatin structure: chromatin loops, compartments, and chromosome territories and how these structures contribute to the organization of the genome within the nucleus of the cell.
    • Assess the importance of SMC proteins (structural maintenance of chromosomes) in higher-order chromatin organization and in different cellular processes.

    DNA Topoisomerases

    • Topoisomerases are enzymes that alter DNA supercoiling by transiently breaking and rejoining DNA strands.
    • Type I topoisomerases break one strand at a time.
    • Type II topoisomerases break both strands, allowing the passage of another DNA segment through the break.
    • Assess the roles of topoisomerases in different cellular processes, such as replication, transcription and DNA packaging.
    • Explain how the activity of topoisomerases can also make them a target for cellular toxins, leading to DNA damage, cell death, and other cellular issues.

    Histones

    • Histones are proteins that effectively package DNA into nucleosomes.
    • Understand the composition of core histones and their role in DNA packaging
    • Recognize how histone binding is not random, but rather depends on the abundance of base pairs within different areas of DNA

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    Description

    This quiz covers key concepts related to DNA structure and function, particularly in eukaryotic and prokaryotic cells. Topics include supercoiling, chromatin assembly, and the organization of DNA within cells. Test your understanding of chromosome behavior during mitosis and the roles of topoisomerases.

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