Download Chapter 24: Genes and Chromosomes

Chapter 24 Genes and Chromosomes
Multiple Choice Questions
1. Chromosomal elements
Pages: 947-948
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: 949 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: 949 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.
are circular
4. Chromosomal elements
Pages: 948-951
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 double-helical molecule.
a single linear single-stranded molecule.
multiple linear double-helical molecules.
multiple linear single-stranded molecules.
5. Chromosomal elements
Page: 952 Difficulty: 1
Introns:
A)
B)
C)
D)
E)
Ans: D
are frequently present in prokaryotic genes but are rare in eukaryotic genes.
are spliced out before transcription.
are translated but not transcribed.
can occur many times within a single gene.
encode unusual amino acids in proteins.
6. Chromosomal elements
Page: 953 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.
endomere.
exon.
intron.
telomere.
7. DNA supercoiling
Page: 953 Difficulty: 2 Ans: C
DNA in a closed-circular, double-stranded molecule with no net bending of the DNA axis on itself is:
A)
B)
C)
D)
E)
a left-handed helix.
a mixed right- and left-handed helix.
relaxed.
supercoiled.
underwound.
8. DNA supercoiling
Pages: 956-957
Difficulty: 2 Ans: B
The linking number (Lk) of a closed-circular, double-stranded DNA molecule is changed by:
A)
B)
C)
D)
E)
breaking a strand, then rejoining it.
breaking a strand, unwinding or rewinding the DNA, then rejoining it.
breaking all hydrogen bonds in the DNA.
supercoiling without the breaking of any phosphodiester bonds.
underwinding without the breaking of any phosphodiester bonds.
9. DNA supercoiling
Pages: 958-959
Difficulty: 2
Topoisomerases can:
A)
B)
C)
D)
Ans: A
change the linking number (Lk) of a DNA molecule.
change the number of base pairs in a DNA molecule.
change the number of nucleotides in a DNA molecule.
convert D isomers of nucleotides to L isomers.
E) interconvert DNA and RNA.
10. DNA supercoiling
Pages: 958-960
Difficulty: 2
Topoisomerases:
A)
B)
C)
D)
E)
Ans: E
always change the linking number in increments of 1.
can act on single-stranded DNA circles.
change the degree of supercoiling of a DNA molecule but not its linking number of DNA.
occur in bacteria, but not in eukaryotes.
require energy from ATP.
11. The structure of chromosomes
Page: 963 Difficulty: 2 Ans: C
Histones are _______ that are usually associated with _________.
A)
B)
C)
D)
E)
acidic proteins; DNA
acidic proteins; RNA
basic proteins; DNA
basic proteins; RNA
coenzymes derived from histidine; enzymes
12. The structure of chromosomes
Pages: 964-965
Difficulty: 1 Ans: D
The fundamental repeating unit of organization in a eukaryotic chromosome is:
A)
B)
C)
D)
E)
the centrosome.
the lysosome.
the microsome.
the nucleosome.
the polysome.
13. The structure of chromosomes
Pages: 964-965
Difficulty: 2
Nucleosomes:
A)
B)
C)
D)
E)
Ans: D
are important features of chromosome organization in eukaryotes and bacteria.
are composed of proteins rich in acidic amino acids, such as Asp and Glu.
are composed of protein and RNA.
bind DNA and alter its supercoiling.
occur in chromatin at irregular intervals along the DNA molecule.
14. The structure of chromosomes
Pages: 968-970
Difficulty: 2
Bacterial chromosomes:
Ans: A
A) are highly compacted into structures called nucleoids.
B)
C)
D)
E)
are seen in electron microscopy as “beads on a string”.
are surrounded by a nuclear membrane.
contain large numbers of nucleosomes.
when fully extended are as long as the bacterial cell.
Short Answer Questions
15. Chromosomal elements
Page: 949 Difficulty: 2
Describe the structure and function of a typical bacterial plasmid.
Ans: Bacterial plasmids are generally closed-circular, double-stranded DNA molecules that are much
smaller than the bacterial chromosome itself. They replicate autonomously and may encode proteins
which, although not normally essential to bacterial survival, may confer resistance to antibiotics.
16. Chromosomal elements
Pages: 949-951
Difficulty: 2
Describe a current hypothesis to explain the presence of functional DNA in mitochondria and
chloroplasts.
Ans: These organelles are thought to have originated from aerobic bacteria and photosynthetic
bacteria, which took up endosymbiotic residence within primitive eukaryotic cells. The DNA
molecules of the organelles are putative vestiges of the chromosomes of these bacteria.
17. Chromosomal elements
Page: 952 Difficulty: 2
What are introns?
Ans: Introns are regions of genes (primarily eukaryotic) that in mRNA are transcribed but are not
translated. They do not code for amino acid sequences within the protein that is coded by the gene.
Thus they interrupt the colinearity between the nucleotide sequence of the gene and the amino acid
sequence of the encoded protein.
18. DNA supercoiling
Page: 954 Difficulty: 2
Describe two functions of DNA supercoiling.
Ans: Supercoiling allows for the extreme compaction required for DNA to fit in a cell. Negative
supercoiling also facilitates the unwinding of the strands of the double helical DNA that is required
for its transcription and replication.
19. DNA supercoiling
Pages: 956-958, 964-965 Difficulty: 2
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.
___ In a nucleosome, eukaryotic DNA is wrapped around histone proteins.
Ans: T; F; T; T
20. DNA supercoiling
Pages: 958-961
Difficulty: 2
Define topoisomerase, and explain the difference between type I and type II topoisomerases.
Ans: Topoisomerases are enzymes that change the linking number in a closed-circular, doublestranded DNA molecule by breaking one or two strands, adding or removing twists, and rejoining the
strand(s). Type 1 topoisomerases break and rejoin only one strand, changing the linking number in
increments of 1. Type 2 topoisomerases break and rejoin both strands, changing the linking number
in increments of 2.
21. DNA supercoiling
Pages: 958-961
Difficulty: 2
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. This
enzyme is classified as a type (b) ____________, which affects the linking number in steps of (c)
___________.
Ans: (a) E. coli topoisomerase II or DNA gyrase; (b) type II topoisomerase; (c) two
22. The structure of chromosomes
Pages: 963-964
Difficulty: 2
What are histones and what is their principal role in chromatin structure?
Ans: Histones are small basic proteins that bind to DNA. There are five main types, four of which
interact with each other to form the core of the nucleosome around which the DNA is wrapped. The
fifth (H1) is believed to be involved in compaction of nucleosomes to form a higher-order structure.
23. The structure of chromosomes
Pages: 964-965
Difficulty: 2
Describe the composition and structure of a nucleosome.
Ans: A nucleosome consists of 146 base pairs of double-stranded DNA wrapped in a solenoidal
supercoil around a core of histones (small, basic proteins). This core contains two copies each of
histones H2A, H2B, H3, and H4.