Chapter 24 Genes and Chromosomes
Multiple Choice Questions
1. The most precise modern definition of a gene is a segment of genetic material that:
A) codes for one polypeptide.
B) codes for one polypeptide or RNA product.
C) determines one phenotype.
D) determines one trait.
E) that codes for one protein.
2. The DNA in a bacterial (prokaryotic) chromosome is best described as:
A) a single circular double-helical molecule.
B) a single linear double-helical molecule.
C) a single linear single-stranded molecule.
D) multiple linear double-helical molecules.
E) multiple linear single-stranded molecules.
3. Bacterial plasmids:
A) are always covalently joined to the bacterial chromosome.
B) are composed of RNA.
C) are never circular.
D) cannot replicate when cells divide.
E) often encode proteins not normally essential to the bacterium’s survival.
4. Which of these statements about nucleic acids is false?
A) Mitochondria and chloroplasts contain DNA.
B) Plasmids are genes that encode plasma proteins in mammals.
C) The chromosome of E. coli is a closed-circular, double-helical DNA.
D) The DNA of viruses is usually much longer than the viral particle itself.
E) The genome of many plant viruses is RNA.
5. Functional DNA is not found in:
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A) bacterial nucleoids.
B) chloroplasts.
C) lysosomes.
D) mitochondria.
E) nuclei.
6. The DNA in a eukaryotic chromosome is best described as:
A) a single circular double-helical molecule.
B) a single linear double-helical molecule.
C) a single linear single-stranded molecule.
D) multiple linear double-helical molecules.
E) multiple linear single-stranded molecules.
7. Introns:
A) are frequently present in prokaryotic genes but are rare in eukaryotic genes.
B) are spliced out before transcription.
C) are translated but not transcribed.
D) can occur many times within a single gene.
E) encode unusual amino acids in proteins.
8. Approximately what fraction of the human genome is tranaposable elements?
A) 1.5%
B) 5%
C) 10%
D) 45%
E) 80%
9. The chromosomal region that is the point of attachment of the mitotic spindle is the:
A) centromere.
B) endomere.
C) exon.
D) intron.
E) telomere.
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10. DNA in a closed-circular, double-stranded molecule with no net bending of the DNA axis on itself is:
A) a left-handed helix.
B) a mixed right- and left-handed helix.
C) relaxed.
D) supercoiled.
E) underwound.
11. It is correct to say that DNA supercoiling cannot:
A) be induced by strand separation.
B) be induced by underwinding of the double helix.
C) form if there is Z-DNA structure present.
D) occur if a closed circular double-stranded DNA molecule has a nick.
E) result in compaction of the DNA structure.
12. The linking number (Lk) of a closed-circular, double-stranded DNA molecule is changed by:
A) breaking a strand, then rejoining it.
B) breaking a strand, unwinding or rewinding the DNA, then rejoining it.
C) breaking all hydrogen bonds in the DNA.
D) supercoiling without the breaking of any phosphodiester bonds.
E) underwinding without the breaking of any phosphodiester bonds.
13. For a closed-circular DNA molecule of 10,000 base pairs in the fully relaxed form, the linking
number (Lk) is about:
A) 10,000.
B) 950.
C) 100.
D) 9.5.
E) 2.
14. If the structure of a fully relaxed, closed-circular DNA molecule is changed so that the specific
linking difference (
) is –0.05, the number of:
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A) bases is decreased by 5%.
B) bases is increased by 5%.
C) helical turns is decreased by 5%.
D) helical turns is increased by 5%.
E) helical turns is unchanged.
15. Topoisomerases can:
A) change the linking number (Lk) of a DNA molecule.
B) change the number of base pairs in a DNA molecule.
C) change the number of nucleotides in a DNA molecule.
D) convert D isomers of nucleotides to L isomers.
E) interconvert DNA and RNA.
16. Topoisomerases:
A) can change the linking number in increments of 1 or 2.
B) can act on single-stranded DNA circles.
C) change the degree of supercoiling of a DNA molecule but not its linking number of DNA.
D) occur in bacteria, but not in eukaryotes.
E) always require energy from ATP.
17. A commonality between type I and type II topoisomerases is that they both:
A) change the linking number in increments of 1 or 2.
B) require ATP.
C) go through a mechanism using a covalent enzyme-substrate intermediate.
D) are inhibited by quinoline antibiotics.
E) All of the above
18. Plectonemic supercoils in a negatively supercoiled DNA molecule:
A) are always left-handed.
B) are always right-handed.
C) are balanced by solenoidal supercoils
D) can be either right- or left-handed.
E) never occur.
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19. Histones are _______ that are usually associated with _________.
A) acidic proteins; DNA
B) acidic proteins; RNA
C) basic proteins; DNA
D) basic proteins; RNA
E) coenzymes derived from histidine; enzymes
20. Which of the following does not contribute to the octameric histone core?
A) H1
B) H2A
C) H2B
D) H3
E) H4
21. The fundamental repeating unit of organization in a eukaryotic chromosome is the:
A) centrosome.
B) lysosome.
C) microsome.
D) nucleosome.
E) polysome.
22. Which of the following contributes to the structure of nucleosomes?
A) Plectonemic supercoiled DNA
B) Relaxed closed-circular DNA
C) Solenoidal supercoiled DNA
D) Spacer DNA
E) Z (left-handed) DNA
23. Nucleosomes:
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A) are important features of chromosome organization in eukaryotes and bacteria.
B) are composed of proteins rich in acidic amino acids, such as Asp and Glu.
C) are composed of protein and RNA.
D) bind DNA and alter its supercoiling.
E) occur in chromatin at irregular intervals along the DNA molecule.
24. A condensed eukaryotic chromosome is known to be associated with all of the following proteins,
except for:
A) core histones H2A, H2B, H3, and H4.
B) histone H1.
C) SMC proteins.
D) topoisomerase I.
E) topoisomerase II.
25. The SMC proteins (for structural maintenance of chromosomes) include cohesins and condensins, and
are known to have all of the following properties except:
A) a complete ATP binding site.
B) a hinge region.
C) topoisomerase activity to produce positive supercoils.
D) the ability to condense DNA.
E) two coiled-coil domains.
26. Bacterial chromosomes:
A) are highly compacted into structures called nucleoids.
B) are seen in electron microscopy as “beads on a string.”
C) are surrounded by a nuclear membrane.
D) contain large numbers of nucleosomes.
E) when fully extended are as long as the bacterial cell.
Short Answer Questions
27. Describe the structure and function of a typical bacterial plasmid.
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28. The genome of the bacterium E. coli is 4,639,675 bp long and consists of 4435 genes; the human
genome is 3,070,128,600 bp long and consists of roughly 29,000 genes. Calculate the average gene
size in each organism and provide an explanation for the difference.
29. Describe a current hypothesis to explain the presence of functional DNA in mitochondria and
chloroplasts.
30. What are introns?
31. What is satellite DNA?
32. Describe two functions of DNA supercoiling.
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33. Define, in the context of DNA structure, “topological bonds.”
34. Define “specific linking difference” (
), also called superhelical density.
35. Indicate whether the following statements are true (T) or false (F).
___ The linking number (Lk) of a closed-circular DNA molecule can be changed only by
breaking one or both strands.
___ DNA of all organisms is overwound (i.e., positively supercoiled).
___ Topoisomerase I relaxes DNA that is highly negatively supercoiled.
36. Calculate values for the following topological properties of a closed-circular DNA molecule
containing 2000 base pairs (for simplicity, assume there are 10 base pairs per turn in the relaxed
DNA).
(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)
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37. Define, in the context of DNA structure, “topoisomers.”
38. Define topoisomerase, and explain the difference between type I and type II topoisomerases.
39. The DNA of virtually every cell is underwound (i.e., negatively supercoiled) relative to B-form DNA.
In bacteria, an enzyme called (a) ____________ introduces negative supertwists into DNA using (b)
____________ as a source of energy. This enzyme is classified as a type (c) ____________, which
affects the linking number in steps of (d) ___________. The usual substrate for this enzyme within
an E. coli cell is the bacterial chromosome. This circular DNA molecule of 4,700,000 base pairs has
a linking number of approximately (e) ____________ when it is closed and relaxed. 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.
40. Explain how inhibitors of topoisomerase would inhibit the growth of tumors. Would these drugs be
expected to inhibit the growth of normal cells as well?
41. Explain the difference between plectonemic and solenoidal supercoiling of DNA; use diagrams to
help in your explanation.
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42. Briefly describe the changes in eukaryotic chromosome structure during the cell cycle.
43. What are histones and what is their principal role in chromatin structure?
44. Describe the composition and structure of a nucleosome.
45. The overall compaction of a eukaryotic chromosome is greater than ____________ -fold. The first
level is nucleosome formation, which compacts about _________ -fold. Next is the 30 nm fiber,
which compacts about ________ -fold overall. Higher order folding involves association of the DNA
with a nuclear ______________________, which contains large amounts of ________________ and
___________________.
46. SMC proteins facilitate the structural maintenance of chromosomes; describe the roles of the two
main classes.
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