Chapter 24 Genes and ChromosomesMultiple Choice Questions 1. Chromosomal elements Page: 924 Difficulty: 2 Ans: B The most precise modern definition of a gene is a segment of genetic material that: A) B) C) D) E) codes for one polypeptide. codes for one polypeptide or RNA product. determines one phenotype. determines one trait. that codes for one protein. 2. Chromosomal elements Page: 925 Difficulty: 1 Ans: A The DNA in a bacterial (prokaryotic) chromosome is best described as: A) B) C) D) E) a single circular double-helical molecule. a single linear double-helical molecule. a single linear single-stranded molecule. multiple linear double-helical molecules. multiple linear single-stranded molecules. 3. Chromosomal elements Page: 925 Difficulty: 1 Bacterial plasmids: A) B) C) D) E) Ans: E are always covalently joined to the bacterial chromosome. are composed of RNA. are never circular. cannot replicate when cells divide. often encode proteins not normally essential to the bacterium’s survival. 4. Chromosomal elements Page: 925 Difficulty: 1 Ans: B Which of these statements about nucleic acids is false? A) B) C) D) E) Mitochondria and chloroplasts contain DNA. Plasmids are genes that encode plasma proteins in mammals. The chromosome of E. coli is a closed-circular, double-helical DNA. The DNA of viruses is usually much longer than the viral particle itself. The genome of many plant viruses is RNA. Chapter 24 Genes and Chromosomes 5. chloroplasts. telomere. endomere. multiple linear double-helical molecules. 9. exon. supercoiled. Chromosomal elements Page: 928 Difficulty: 1 Introns: A) B) C) D) E) Ans: D are frequently present in prokaryotic genes but are rare in eukaryotic genes. lysosomes. double-stranded molecule with no net bending of the DNA axis on itself is: A) B) C) D) a left-handed helix. DNA supercoiling Page: 930 Difficulty: 2 Ans: C DNA in a closed-circular. 8.and left-handed helix. Chromosomal elements Page: 926 Difficulty: 2 Ans: B The DNA in a eukaryotic chromosome is best described as: A) B) C) D) E) a single circular double-helical molecule. a single linear single-stranded molecule. nuclei. a single linear double-helical molecule. are translated but not transcribed. can occur many times within a single gene. . relaxed. mitochondria. Chromosomal elements Pages: 925-927 Difficulty: 2 Functional DNA is not found in: A) B) C) D) E) 281 Ans: C bacterial nucleoids. a mixed right. multiple linear single-stranded molecules. 6. Chromosomal elements Page: 930 Difficulty: 1 Ans: A The chromosomal region that is the point of attachment of the mitotic spindle is the: A) B) C) D) E) centromere. intron. encode unusual amino acids in proteins. 7. are spliced out before transcription. DNA supercoiling Pages: 933-934 Difficulty: 3 Ans: C If the structure of a fully relaxed.5. be induced by underwinding of the double helix. DNA supercoiling Page: 933 Difficulty: 2 Ans: B For a closed-circular DNA molecule of 10. helical turns is increased by 5%. helical turns is unchanged. occur if a closed circular double-stranded DNA molecule has a nick.000. . 2. 9.05.282 Chapter 24 Genes and Chromosomes E) underwound. 10. then rejoining it. 950. 100. 11. double-stranded DNA molecule is changed by: A) B) C) D) E) breaking a strand. 13. the linking number (Lk) is about: A) B) C) D) E) 10. DNA supercoiling Page: 933 Difficulty: 2 Ans: D It is correct to say that DNA supercoiling cannot: A) B) C) D) E) be induced by strand separation.000 base pairs in the fully relaxed form. DNA supercoiling Page: 933 Difficulty: 2 Ans: B The linking number (Lk) of a closed-circular. breaking all hydrogen bonds in the DNA. helical turns is decreased by 5%. the number of: A) B) C) D) E) bases is decreased by 5%. bases is increased by 5%. closed-circular DNA molecule is changed so that the specific linking difference (σ) is –0. breaking a strand. result in compaction of the DNA structure. unwinding or rewinding the DNA. underwinding without the breaking of any phosphodiester bonds. 12. then rejoining it. form if there is Z-DNA structure present. supercoiling without the breaking of any phosphodiester bonds. or left-handed. change the degree of supercoiling of a DNA molecule but not its linking number of DNA.Chapter 24 Genes and Chromosomes 14. convert D isomers of nucleotides to L isomers. The structure of chromosomes Page: 940 Difficulty: 1 Ans: D The fundamental repeating unit of organization in a eukaryotic chromosome is: A) the centrosome. B) the lysosome. but not in eukaryotes. C) the microsome. occur in bacteria. can act on single-stranded DNA circles. DNA supercoiling Page: 935 Difficulty: 2 Topoisomerases can: A) B) C) D) E) change the linking number (Lk) of a DNA molecule. change the number of nucleotides in a DNA molecule. DNA acidic proteins. RNA coenzymes derived from histidine. RNA basic proteins. A) B) C) D) E) acidic proteins. interconvert DNA and RNA. change the number of base pairs in a DNA molecule. are balanced by solenoidal supercoils can be either right. DNA supercoiling Page: 937 Difficulty: 2 Topoisomerases: A) B) C) D) E) Ans: A Ans: E always change the linking number in increments of 1. DNA supercoiling Page: 937 Difficulty: 3 Ans: B Plectonemic supercoils in a negatively supercoiled DNA molecule: A) B) C) D) E) are always left-handed. The structure of chromosomes Page: 939 Difficulty: 2 Ans: C Histones are _______ that are usually associated with _________. 17. are always right-handed. never occur. require energy from ATP. 283 . 15. 16. enzymes 18. DNA basic proteins. and are known to have all of the following properties except: A) B) C) D) E) A complete ATP binding site. are composed of proteins rich in acidic amino acids.284 Chapter 24 Genes and Chromosomes D) the nucleosome. except for: A) B) C) D) E) Core histones H2A. bind DNA and alter its supercoiling. and H4. such as Asp and Glu. The structure of chromosomes Pages: 942-943 Difficulty: 2 Ans: D A condensed eukaryotic chromosome is known to be associated with all of the following proteins. are composed of protein and RNA. E) the polysome. occur in chromatin at irregular intervals along the DNA molecule. The structure of chromosomes Page: 940 Difficulty: 2 Ans: D Nucleosomes: A) B) C) D) E) are important features of chromosome organization in eukaryotes and bacteria. A hinge region Topoisomerase activity to produce positive supercoils The ability to condense DNA Two coiled-coil domains . H2B. 19. The structure of chromosomes Page: 940 Difficulty: 2 Ans: C Which of the following contributes to the structure of nucleosomes? A) B) C) D) E) Plectonemic supercoiled DNA Relaxed closed-circular DNA Solenoidal supercoiled DNA Spacer DNA Z (left-handed) DNA 20. Histone H1 SMC proteins Topoisomerase I Topoisomerase II 22. 21. H3. The structure of chromosomes Pages: 943-944 Difficulty: 2 Ans: C The SMC proteins (for structural maintenance of chromosomes) include cohesins and condensins. 26. when fully extended are as long as the bacterial cell. The structure of chromosomes Pages: 943-944 Difficulty: 2 Bacterial chromosomes: A) B) C) D) E) 285 Ans: A are highly compacted into structures called nucleoids. Thus they interrupt the colinearity between the nucleotide sequence of the gene and the amino acid sequence of the encoded protein. are surrounded by a nuclear membrane. The DNA molecules of the organelles are putative vestiges of the chromosomes of these bacteria. Chromosomal elements Page: 929 Difficulty: 2 What is satellite DNA? Ans: Satellite DNA (also known as highly repetitive or simple-sequence DNA) consists of regions of the eukaryotic genome that are different enough in base composition from the bulk of the chromosomal DNA that they can be separated from it by density gradient centrifugation.Chapter 24 Genes and Chromosomes 23. Ans: Bacterial plasmids are generally closed-circular. may confer resistance to antibiotics. are seen in electron microscopy as “beads on a string”. They replicate autonomously and may encode proteins which. Ans: These organelles are thought to have originated from aerobic bacteria and photosynthetic bacteria. although not normally essential to bacterial survival. double-stranded DNA molecules that are much smaller than the bacterial chromosome itself. . Short Answer Questions 24. Chromosomal elements Page: 928 Difficulty: 2 What are introns? Ans: Introns are regions of genes (primarily eukaryotic) that in mRNA are transcribed but are not translated. Satellite DNA is often associated with centromeres and telomeres and consists of short sequences (5 to 10 base pairs) that are repeated millions of times per cell. Chromosomal elements Pages: 927-928 Difficulty: 2 Describe a current hypothesis to explain the presence of functional DNA in mitochondria and chloroplasts. 25. They do not code for amino acid sequences within the protein that is coded by the gene. 27. contain large numbers of nucleosomes. Chromosomal elements Page: 925 Difficulty: 2 Describe the structure and function of a typical bacterial plasmid. which took up endosymbiotic residence within primitive eukaryotic cells. Negative supercoiling also facilitates the unwinding of the strands of the double helical DNA that is required for its transcription and replication. 30. also called superhelical density. ___ DNA of all organisms is overwound (i. ___ The linking number (Lk) of a closed-circular DNA molecule can be changed only by breaking one or both strands.000 base pairs (for simplicity. eukaryotic DNA is wrapped around histone proteins. ___ In a nucleosome. assume there are 10 base pairs per turn in the relaxed DNA). (b) 180. DNA supercoiling Page: 933 Difficulty: 2 Define. Ans: The specific linking difference (σ) is a measure of the number of turns removed (∆Lk) relative to the linking number of the fully relaxed form (Lk0): σ = ∆Lk/Lk0. 29. DNA supercoiling Page: 933 Difficulty: 3 Define “specific linking difference” (σ). Ans: Supercoiling allows for the extreme compaction required for DNA to fit in a cell. T. positively supercoiled). DNA supercoiling Pages: 933-937.e.025 . F. 31.286 Chapter 24 Genes and Chromosomes 28.. ___ Topoisomerase I relaxes DNA that is highly negatively supercoiled. in the context of DNA structure. but that cannot be physically separated because they cross over each other like links in a chain. DNA supercoiling Pages: 933-935. 940 Difficulty: 2 Indicate whether the following statements are true (T) or false (F). (e) –0.1. DNA supercoiling Page: 930 Difficulty: 2 Describe two functions of DNA supercoiling. “topological bond. 940 Difficulty: 3 Calculate values for the following topological properties of a closed-circular DNA molecule containing 2. Ans: T.” Ans: Topological bonds are links between two molecules that are not joined by specific chemical bonds. (c) 195 (d) –0. (a) The linking number when the DNA is relaxed (b) The linking number when the DNA has been underwound by 10 enzymatic turnovers of DNA gyrase (+ATP) (c) The linking number when the DNA has been underwound by binding five nucleosomes followed by complete relaxation by a eukaryotic topoisomerase (d) The superhelical density of the DNA molecule in (b) (e) The superhelical density of the DNA molecule in (c) Ans: (a) 200. T 32. The structure of chromosomes Page: 939 Difficulty: 2 Briefly describe the changes in eukaryotic chromosome structure during the cell cycle. negatively supercoiled) relative to B-form DNA. doublestranded DNA molecule by breaking one or two strands. This enzyme would (f) ____________ (decrease/increase/not change) this linking number when acting upon this DNA molecule in the presence of the above energy source. This circular DNA molecule of 4. (See Fig. DNA supercoiling Pages: 937-938 Difficulty: 2 Explain the difference between plectonemic and solenoidal supercoiling of DNA. the condensed chromosomes line up along a plane halfway between the spindle poles (to which they are connected by microtubules). After . “topoisomers. (c) type II topoisomerase. 938.000 base pairs has a linking number of approximately (e) ____________ when it is closed and relaxed.e.” Ans: Topoisomers are different forms of a closed-circular DNA molecule that differ only in a topological property such as their linking number (Lk). (f) decrease 36. the double helix is intertwined such that it crosses over itself. This enzyme is classified as a type (c) ____________. use diagrams to help in your explanation.. changing the linking number in increments of 2. Ans: Starting with prophase. the sister chromatids separate. and explain the difference between type I and type II topoisomerases. (d) two. and rejoining the strand(s). Ans: Topoisomerases are enzymes that change the linking number in a closed-circular. 35. As the cell moves into anaphase. Ans: (a) E. Type 1 topoisomerases break and rejoin only one strand.000. the DNA undergoes condensation (employing cohesins and condensins). an enzyme called (a) ____________ introduces negative supertwists into DNA using (b) ____________ as a source of energy. DNA supercoiling Pages: 935-937 Difficulty: 2 The DNA of virtually every cell is underwound (i. in the context of DNA structure. In solenoidal supercoiling. the double helix forms a spiral resembling the helix formed by each of the strands. which affects the linking number in steps of (d) ___________. coli topoisomerase II or DNA gyrase. During the subsequent metaphase. The usual substrate for this enzyme within an E. adding or removing twists. changing the linking number in increments of 1. DNA supercoiling Pages: 935-937 Difficulty: 2 Define topoisomerase. Ans: In plectonemic supercoiling. (b) ATP. 24-24a. each drawn towards its respective spindle pole.700. Type 2 topoisomerases break and rejoin both strands. coli cell is the bacterial chromosome.Chapter 24 Genes and Chromosomes 287 33. p. 34.) 37. (e) 450. In bacteria. DNA supercoiling Page: 934 Difficulty: 1 Define. histone H1. H2B. which compacts about ________ -fold overall. The structure of chromosomes Pages: 940-941 Difficulty: 2 Describe the composition and structure of a nucleosome. The condensins are essential for DNA condensation as the cell enters mitosis.000. 40. The structure of chromosomes Pages: 943-944 Difficulty: 2 SMC proteins facilitate the structural maintenance of chromosomes. topoisomerase II (last two in either order). four of which interact with each other to form the core of the nucleosome around which the DNA is wrapped. and H4. scaffold. the chromosomes decondense. Ans: The cohesins help link sister chromatids together after replication and keep them together as they condense. 24-25. and remain so during interphase. 944). . which contains large amounts of ________________ and ___________________ . The fifth (H1) is believed to be involved in compaction of nucleosomes to form a higher-order structure. This core contains two copies each of histones H2A. 41. There are five main types. (See Fig. basic proteins). 38. 939). Next is the 30 nm fiber. Higher order folding involves association of the DNA with a nuclear ______________________ . describe the roles of the two main classes. The structure of chromosomes Page: 942 Difficulty: 2 The overall compaction of a eukaryotic chromosome is greater than ____________ -fold.288 Chapter 24 Genes and Chromosomes cell division is complete. (See Fig. 100. which is essential for proper segregation. and their binding is thought to create positive supercoiling. The structure of chromosomes Pages: 939-941 Difficulty: 2 What are histones and what is their principal role in chromatin structure? Ans: Histones are small basic proteins that bind to DNA. p. The first level is nucleosome formation. p. 39. Ans: 10. Ans: A nucleosome consists of 146 base pairs of double-stranded DNA wrapped in a solenoidal supercoil around a core of histones (small. 7. which compacts about _________ -fold. 24-25. H3.
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