Chapter 12—The Eukaryotic Chromosome: An Organelle for Packaging and Managing
DNA
Fill in the Blank
1. In many human tumor cells, abnormal activation of the _________________ gene
seems to bestow the capacity to divide indefinitely.
Ans: telomerase
Difficulty: 2
2. Specialized elements that ensure the accurate duplication and segregation of
chromosomes include telomeres, origins of replication, and _________________.
Ans: centromeres
Difficulty: 2
3. The components of eukaryotic chromosomes include DNA, _________________, and
nonhistone proteins.
Ans: histones
Difficulty: 2
4. _________________ is the generic term for any complex of DNA and protein found in
a cell's nucleus.
Ans: Chromatin
Difficulty: 2
5. The most rudimentary DNA packaging unit which involves four types of histones is the
_________________.
Ans: nucleosome
Difficulty: 2
6. The largest class of nonhistone proteins foster or regulate _________________ and
RNA processing during gene expression.
Ans: transcription
Difficulty: 2
7. Packaging into nucleosomes condenses naked DNA about _________________.
Ans: 7-fold
Difficulty: 3
8. Two small acrocentric chromosomes in the great apes fused to form
_________________ chromosome 2 in humans.
Ans: metacentric
Difficulty: 2
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9. The duplication of chromosomal DNA depends on origins of replication and
________________.
Ans: telomeres
Difficulty: 2
10. In a typical human cell, DNA polymerase replicates about _________________
nucleotides per second.
Ans: 50
Difficulty: 2
11. The DNA running both ways from one origin of replication to the endpoints where it
merges with DNA from adjoining replication forks is called a replication unit, or
_________________.
Ans: replicon
Difficulty: 3
12. The telomeres of all mammals and most of the other higher animals contain the same
repetitive DNA sequence. The bases in this sequence are _________________.
Ans: TTAGGG
Difficulty: 3
13. Even though telomeres normally carry no genes, they contain essential information for
the duplication, segregation, and stability of _________________.
Ans: chromosomes
Difficulty: 2
14. The _________________ of eukaryotic chromosomes assure their precise distribution
during cell division.
Ans: centromeres
Difficulty: 2
15. Centromeres are contained within blocks of repetitive, simple noncoding sequences
known as _________________ DNAs.
Ans: satellite
Difficulty: 2
16. The _________________ is the specialized structure composed of DNA and proteins
that is the site at which chromosomes attach to the spindle fibers.
Ans: kinetochore
Difficulty: 2
17. Cells express their genes mainly during _________________ when the chromosomes
have decondensed, or decompacted.
Ans: interphase
Difficulty: 2
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18. When viewed under the light microscope, certain regions of chromosomes that have
been stained with certain DNA-binding chemicals appear lighter than others. These
regions are called _________________.
Ans: euchromatin
Difficulty: 2
19. Chromosomal regions that remain condensed in heterochromatin at most times in all
cells are known as _________________ heterochromatin.
Ans: constitutive
Difficulty: 2
20. Regions of chromosomes that are heterochromatic in some cells and euchromatic in
other cells of the same organism are known as _________________ heterochromatin.
Ans: facultative
Difficulty: 2
21. Inactive X chromosomes that appear in interphase cells as darkly stained
heterochromatin masses are called _________________.
Ans: Barr bodies
Difficulty: 2
22. Drosophila salivary glands contain giant chromosomes called _________________
chromosomes consisting of many identical chromatids lying in parallel register.
Ans: polytene
Difficulty: 2
23. The centromeres of polytene chromosomes fuse to form a dense heterochromatic mass
known as the _________________.
Ans: chromocenter
Difficulty: 2
24. When observing the interphase nucleus of a eukaryotic cell, a certain large sphereshaped structure may be seen clearly. This structure is called the _________________.
Ans: nucleolus
Difficulty: 2
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Multiple Choice
25. The enzyme that some organisms use to replicate DNA at the 5' ends of chromosomes is
called:
A) DNA polymerase.
B) telomerase.
C) DNA ligase.
D) replicase.
Ans: B
Difficulty: 1
26.
A)
B)
C)
D)
Which of the following is not usually seen when a cell loses telomerase activity?
Telomeres shorten slightly with each cell division.
After 30-50 divisions cells show signs of senescence and then die.
In the immune system certain capacity for protection is gradually lost.
Many normal somatic cells gain the ability to divide indefinitely.
Ans: D
Difficulty: 2
27. The chromosome can be considered a dynamic organelle for _______________ of
DNA.
A) expression
B) segregation
C) replication
D) packaging
E) all of the above
Ans: E
Difficulty: 1
28.
A)
B)
C)
D)
E)
The components of a chromosome include one long DNA molecule and:
phospholipids.
proteins.
carbohydrates.
steroids.
RNA.
Ans: B
Difficulty: 1
29.
A)
B)
C)
D)
By weight, chromatin consists roughly of:
1/3 DNA, 1/3 histones, & 1/3 nonhistones.
1/3 DNA, 2/3 acidic proteins.
1/3 DNA, 1/3 histones, & 1/3 basic proteins.
1/4 DNA, 1/4 RNA, 1/4 histones, 1/4 nonhistones.
Ans: A
Difficulty: 2
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30.
A)
B)
C)
D)
E)
Histones and DNA have a strong attraction for each other because:
DNA is positively charged and histones are negatively charged.
both DNA and proteins are hydrophobic.
DNA is negatively charged and histones are positively charged.
like substances share common charges.
none of the above
Ans: C
Difficulty: 2
31.
A)
B)
C)
D)
Which of the following is not a role ascribed to nonhistone proteins found in chromatin?
structural
replication
chromosome segregation
nucleosome packers
Ans: D
Difficulty: 2
32. Histones appear to have a singular role in the packaging of DNA. The nonhistone
proteins in chromatin have which of the following functions?
A) different functions for different tissues, such as tissue-specific transcription factors
B) activity at different moments in the life cycle
C) centromere function activity
D) all of the above
Ans: D
Difficulty: 1
33.
A)
B)
C)
D)
The first level of compaction of DNA consists of:
DNA winding around histones to form small nucleosomes.
tight coiling of DNA with nucleosomes into higher order structures.
high level compaction into metaphase-type chromosomes.
histone, DNA, and nonhistone covalent bonding.
Ans: A
Difficulty: 1
34. An electron micrograph of chromatin, which looks like a bead on a string, allows
measurement of structural dimensions. The beads appear to have a diameter of about
________ Å and the string a diameter of about ________ Å.
A) 20, 100
B) 20, 0.1
C) 100, 20
D) 120, 100
Ans: C
Difficulty: 2
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35. Nucleosomal chromatin with a diameter of 100Å supercoils into a 300Å superhelix. The
histone that appears to be responsible for this compaction is:
A) H4.
B) H3.
C) H2.
D) H1.
Ans: D
Difficulty: 2
36. In DNA, the 300Å fiber is formed into structural loops. Responsibility for this function
appears to lie with:
A) histone H4.
B) nucleosomase.
C) histone H1.
D) certain nonhistone proteins.
Ans: D
Difficulty: 1
37. At what stage of mitosis do we first see a chromosome that is compacted 250-fold over
the 40-fold compacted 300Å fiber?
A) prophase
B) metaphase
C) anaphase
D) telophase
E) never achieves this level of compaction
Ans: B
Difficulty: 1
38. Which of the following is not a piece of evidence that supports the radial loop-scaffold
model of DNA packaging?
A) Histone-depleted metaphase chromosomes still maintain their X-like shapes.
B) Electron micrographs of whole-mounted mitotic chromosomes show loops of chromatin
at the periphery of the chromosomes.
C) Topoisomerase II assists in 100Å packaging.
D) Special, irregularly spaced repetitive base sequences associate with nonhistone proteins
to define the chromatin loops.
E) All of the above are such evidence.
Ans: C
Difficulty: 3
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39. The total compaction of DNA as seen in metaphase chromosomes is approximately
______ fold.
A) 10,000
B) 50,000
C) 100,000
D) 40
Ans: A
Difficulty: 2
40. Technique of preparing mitotic chromosomes involves a staining procedure using
Giemsa stain. One variation of this technique allows low-resolution of chromosome
bands while a second variation allows a high-resolution. At low resolution we can see
about _________ bands while at high-resolution we see about _______.
A) 300, 2,000
B) 300, 1,000
C) 100, 2,000
D) 100, 1,000
Ans: A
Difficulty: 2
41.
A)
B)
C)
D)
Chromosome banding patterns can be used for which of the following?
Analyze chromosomal differences between species.
Locate genes.
Reveal the cause of certain genetic diseases.
All of the above
Ans: D
Difficulty: 1
42. If one looks at the chromosome banding pattern of certain primates it can be seen that
humans and another primate have banding patterns nearly identical on 13 of the
chromosomes. This primate is the:
A) gorilla
B) chimpanzee
C) orangutan
D) baboon
Ans: B
Difficulty: 1
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43. In the medical literature can be found a case history of a young boy with defects in four
X-linked traits. When looking at the banding patterns in his karyotype, which of the
following can be seen that apparently explains the situation?
A) A duplication near the centromere.
B) Loss of all interband regions.
C) Deletion of a small white band between two darker bands.
D) A fusion of two small chromosomes to form the number two chromosome.
Ans: C
Difficulty: 3
44.
A)
B)
C)
D)
With regard to the typical human cell, which of the following appears to be true?
There is only one origin of replication.
There are multiple origins of replication that function consecutively.
There are multiple origins of replication that function simultaneously.
Replication is not understood well enough to postulate on replication origins.
Ans: C
Difficulty: 3
45.
A)
B)
C)
D)
In mammalian cells, replication proceeds:
unidirectionally.
bidirectionally.
unidirectionally from many origins.
bidirectionally from only one origin.
Ans: B
Difficulty: 2
46. The 10,000 origins of replication in mammalian cells suggests that concerning origin of
replication there is:
A) at least one per loop of DNA.
B) probably one every other loop of DNA.
C) a random distribution across the genome.
D) a precise pattern of distribution relative to other replication points.
Ans: A
Difficulty: 2
47. Yeast research with autonomously replicating sequences (ARS), along with digestion of
chromatin with DNase I, have led scientists to determine that:
A) DNA is not protected inside a nucleosome.
B) ARSs are really plasmids.
C) RSs bond irreversibly to replication enzymes.
D) origins of replication are accessible regions of DNA devoid of nucleosomes.
Ans: D
Difficulty: 3
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48.
A)
B)
C)
D)
Which of the following is not true about DNA polymerase?
It cannot begin the 5' end of a new strand without a primer.
It can only travel in the 5'-to-3' direction.
It can reconstruct the 3' end of each newly made DNA strand.
It can replace RNA primer with DNA at the 5' end.
Ans: D
Difficulty: 2
49.
A)
B)
C)
D)
E)
Which of the following are properties of telomeres?
Consist of special repetitive DNA sequences.
Prevent the loss of DNA through incomplete replication.
Cap the ends of each chromosome.
Number of repeats varies with cell type.
All of the above
Ans: E
Difficulty: 1
50. Telomeres appear to function in at least two ways during the replication of
chromosomes. One way is to provide a primer for DNA polymerase. The second is to:
A) allow the chromosome to shorten each generation to speed up replication.
B) attract telomerase.
C) permit the hairpin turn at the end to be cleaved.
D) all of the above
E) none of the above
Ans: B
Difficulty: 1
51. The two chromatids of each replicated chromosome must separate from one another and
segregate during:
A) mitosis.
B) meiosis I.
C) meiosis II.
D) both a and b
E) both a and c
Ans: E
Difficulty: 2
52.
A)
B)
C)
D)
E)
In DNA, most satellite sequences are found in:
chromosome arms.
telomeres.
centromeres.
spaces around the dark bands.
none of the above
Ans: C
Difficulty: 1
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53. One of the functions of a centromere is to contribute to proper chromosome segregation.
The other function is to:
A) hold sister chromatids together.
B) ensure that proper chromosome arm length is maintained.
C) allow easy karyotyping.
D) develop meiotic chromosome replication.
E) all of the above
Ans: A
Difficulty: 2
54.
A)
B)
C)
D)
During mitosis, kinetochores develop during:
telophase.
anaphase.
metaphase.
prophase.
Ans: D
Difficulty: 1
55.
A)
B)
C)
D)
In yeast chromosomes, centromeres:
help distinguish one chromosome from another.
are closely related in sequence.
are only 10-15 bp long.
play various roles in chromosome segregation.
Ans: B
Difficulty: 2
56.
A)
B)
C)
D)
In higher eukaryotes, kinetochores attach to:
one spindle fiber.
one spindle fiber on each side.
multiple spindle fibers.
multiple repeating structural subunits.
Ans: C
Difficulty: 2
57. The single-celled yeast, Saccharomyces cerevisiae, was the organism of choice as raw
material for constructing artificial chromosomes because of all the following except:
A) it is easy to manipulate.
B) it is unicellular.
C) its genetic machinery is similar to the one in higher organisms.
D) its origins of replication have been defined as discrete, small segments of DNA.
Ans: B
Difficulty: 3
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58.
A)
B)
C)
D)
E)
A yeast artificial chromosome (YAC) usually consists of:
a yeast centromere.
a yeast origin of replication.
telomere sequences.
suitable selectable markers.
all of the above
Ans: E
Difficulty: 1
59. Which of the following appears to be true concerning yeast artificial chromosomes
(YACs) and segregation errors during cell division?
A) 11,000 bp YACs show 50% errors.
B) 55,000 bp YACs show 1.5% errors.
C) >100,000 bp YACs show 0.3% errors.
D) all of the above are true
Ans: D
Difficulty: 2
60. Even though not well understood, certain chromosome features, known as _________,
restrict DNA unwinding to specific regions and prevent decompaction from spreading
beyond a certain point.
A) rosettes
B) telomerases
C) nucleosomes
D) boundary elements
Ans: D
Difficulty: 2
61.
A)
B)
C)
D)
Many, but not all, DNase hypersensitive (DH) sites are found:
at the 5' ends of genes to be transcribed by the cell.
in and near nucleosomes.
near histone-rich regions.
all of the above
Ans: A
Difficulty: 2
62. Which of the following is true concerning the location of DNase hypersensitive (DH)
sites in higher organisms?
A) In red blood cells the DH site is at the 5' end of the gene for hemoglobin.
B) In muscle cells the DH site exposes myoglobin.
C) In a type of white blood cell DH sites are at the 5' ends of the genes for antibodies.
D) In nerve cells DH sites expose the genes for neurotransmitters.
E) All of the above
Ans: E
Difficulty: 2
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63. From DNA research, sites of transcription and therefore most of the genes along the
length of the chromosome appear to be found in:
A) heterochromatin.
B) euchromatin.
C) constitutive heterochromatin.
D) centromeric regions.
Ans: B
Difficulty: 1
64. When a chromosomal rearrangement such as an inversion occurs placing a known gene
into or next to a heterochromatic region, the gene's expression:
A) will turn on.
B) may be amplified.
C) may cease.
D) none of the above
Ans: C
Difficulty: 1
65. In the position effect variegation of red and white eye color in Drosophila, eyes are
produced:
A) one red and one white.
B) red or white depending on which gene is dominant.
C) with every other eye facet red or white.
D) with varying sizes and positions of red and white patches.
E) none of the above
Ans: D
Difficulty: 2
66.
A)
B)
C)
D)
E)
In which of the following cases will a Barr body be seen?
XX
XY
XO
XXY
both a and d
Ans: E
Difficulty: 2
67. During development, an XX female embryo will form one of the X chromosomes in
each cell into a Barr body. This usually occurs:
A) about two weeks after zygote formation.
B) by the end of the first trimester.
C) at the 16-cell stage.
D) in the X that is the most recessive.
Ans: A
Difficulty: 2
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68. Drosophila polytene chromosomes go through 10 rounds of replication without
undergoing mitosis. The resulting sister chromatids will consist of ________ double
helixes.
A) 2,048
B) 1,024
C) twice as many
D) 10 times as many
Ans: B
Difficulty: 2
69. Drosophila chromosomes contain bands that range in size from 3 kb to more than 150
kb. These bands total about ________.
A) 5,000
B) 2,048
C) 2,500
D) 4,000
Ans: A
Difficulty: 2
70. At certain times, a region of the polytene chromosome may swell to form a large,
diffuse structure called a:
A) lampbrush.
B) centromere.
C) chromocenter.
D) chromosomal puff.
Ans: D
Difficulty: 1
71.
A)
B)
C)
D)
E)
The nucleolus consists of:
long loops of DNA from several chromosomes.
ribosomal RNA (rRNA) genes.
growing rRNA transcripts.
completed transcripts from arrays of rRNA genes.
all of the above
Ans: E
Difficulty: 2
72. Certain reversible changes in chromatin structure allow a variety of chromosome
functions. These include:
A) coordinated compaction for segregation and transport.
B) universal unwinding for replication.
C) selective unwinding for gene expression.
D) all of the above
Ans: D
Difficulty: 2
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Matching
Match the following terms with the definition that best fits.
a) telomer
b) nucleosome
c) replicon
d) kinetochore
e) constitutive heterochromatin
f) facultive heterochromatin
g) Barr bodies
h) chromocenter
i) chromosomal puff
j) nucleolar organizer
73. ______ centromere
Ans: d
Difficulty: 1
74. ______ a short tandomly repeated G-C rich DNA sequence near the end of
chromosomes.
Ans: a
Difficulty: 2
75. ______ a genetic element that behaves as an autonomous unit during DNA replication.
Ans: c
Difficulty: 2
76. ______ the chromosomal region that contains the ribosomal RNA genes.
Ans: j
Difficulty: 2
77. ______ made of repetitious DNA that is late to replicate and is transcriptionally
inactive.
Ans: e
Difficulty: 2
78. ______ inactivated chromosomal DNA.
Ans: f
Difficulty: 2
79. ______ a localized swelling of a specific region of a polytene chromosome due to
localized synthesis of DNA or RNA.
Ans: i
Difficulty: 2
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80. ______ a beadlike structure of eukaryotic chromosomes, consisting of a core of eight
histone molecules wrapped by a DNA segment about 150 base pairs in length and
separated by “linker” DNA.
Ans: b
Difficulty: 2
81. ______ a condensed single X chromosome seen in the nuclei of somatic cells of female
mammals.
Ans: g
Difficulty: 2
82. ______ a formation in polytene chromosomes where heterochromatic segments near the
centromere adhere to one another.
Ans: h
Difficulty: 2
True or False
83. The linear chromosomes of eukaryotic cells terminate at both ends in protective caps
called telomeres.
Ans: True
Difficulty: 1
84. During metaphase of mitosis the chromosomes look like a tangled mass of grainy string
scattered throughout the nucleus.
Ans: False
Difficulty: 1
85. Chemical and physical analysis of chromosomes within a cell has shown us that a
chromosome contains several long linear molecules of DNA.
Ans: False
Difficulty: 1
86. That histones have changed so little throughout evolution underscores the importance of
their contribution to chromatin structure.
Ans: True
Difficulty: 2
87. Nucleosomes are evenly spaced along a chromosome, but they do not have a welldefined arrangement along the chromatin.
Ans: False
Difficulty: 2
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88. At this time the loops and scaffold concept of higher level chromatin packaging remains
a hypothesis.
Ans: True
Difficulty: 2
89. It appears that chromosome bands produced by staining embody discrete differences in
base composition over long distances.
Ans: False
Difficulty: 2
90. It is possible to use chromosome bands to assist in location of genes.
Ans: True
Difficulty: 1
91. In all placental mammals, the diploid genome carries roughly 6 billion base pairs, but is
packaged into different numbers of chromosomes with different banding patterns.
Ans: True
Difficulty: 2
92. It appears that all origins of replication are active during all mitotic divisions that create
an organism.
Ans: False
Difficulty: 2
93. While duplicating an entire chromosome, DNA polymerase cannot begin the 5' end of a
new strand without a primer.
Ans: True
Difficulty: 2
94. Telomeres prevent the transcription of genes brought into their vicinity.
Ans: True
Difficulty: 1
95. Telomerase is an enzyme that consists of a protein portion in association with RNA.
Ans: True
Difficulty: 1
96. If removal of a DNA sequence disrupts chromosome segregation and reinsertion of the
same sequence restores stable transmission, the sequence must lie outside the
centromere.
Ans: False
Difficulty: 2
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97. To build a yeast artificial chromosome (YAC), scientists must insert parts of the yeast
into a plasmid.
Ans: True
Difficulty: 2
98. Yeast artificial chromosomes (YACs) that are more than 100,000 bp long show a
frequency of segregation errors nearly identical to that of natural yeast chromosomes.
Ans: False
Difficulty: 1
99. When looking for the relationship between chromosome banding and number of genes
per band in polytene chromosomes, one finds a direct one-to-one relationship.
Ans: False
Difficulty: 2
Short Answer
100. Describe how researchers learned from physical and chemical studies that each
chromosome within a cell nucleus contains one long linear molecule of DNA.
Ans: In one study researchers used the base analog 5-bromodeoxyuridine to label
newly synthesized DNA in metaphase chromosomes consisting of two sister
chromatids. The two chromatids stained in a fashion consistent with the idea that
each chromatid contains one long double helix of DNA produced by a
semiconservative mode of replication. In a second study, they placed
chromosomal DNA between two cylinders, stretched the DNA by rotating one of
the cylinders, and measured the DNA's rate of recoil. Shorter molecules recoil
faster than longer ones. When they applied this measure to the DNA in a
Drosophila chromosome, the length of the DNA molecule was sufficient to
account for the entire DNA in a single chromosome. It is also possible to use
pulsed gel electrophoresis to separate the DNA molecules of a relatively small
genome by size; with the yeast genome, the number and sizes of the separated
DNA molecules correspond to the number and sizes expected if each chromosome
contains a single DNA molecule.
Difficulty: 4
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101. Discuss how a nucleosome is constructed from DNA and histone proteins.
Ans: On an electron micrograph of chromatin, the chromatin fibers resemble beads on a
string, with the beads having a diameter of about 100Å, and the string a diameter
of about 20Å, which is just DNA. Student may sketch a figure similar to a text
figure and explain how DNA wraps around histone cores to form the chromatin
fiber's observed beads-on-a-string structure. Each bead is a nucleosome containing
about 160 bp of DNA wrapped around a core of 8 histones – 2 each of H2A, H2B,
H3, and H4 (student sketch may show the arrangement). The 160 bp wrap twice
around the core octamer and 40 bp form linker DNA to connect nucleosomes.
Histone H1 lies outside the core and appears to associate with DNA where it
enters and leaves the nucleosome.
Difficulty: 4
102. Describe the ways in which telomeres are critical to chromosome function.
Ans: Telomeres prevent chromosomal shortening during replication that could
dismantle vital genes nucleotide by nucleotide. They also maintain the integrity of
the chromosomal ends. Broken chromosomes without telomeres are recognized as
defective by the cellular DNA repair machinery. The repair machinery will
frequently put the broken ends back together. Sometimes the unprotected broken
ends are repaired incorrectly and may result in chromosome fusion or the
chromosome may be degraded entirely. Both fusion and degradation upset proper
chromosome number and function.
Difficulty: 4
103. Ignoring the detailed description of how DNA polymerase functions, discuss the roles
that proteins play in reproducing chromatin structure.
Ans: Student may comment on the role of DNA polymerase and DNA ligase. Before
DNA synthesis can occur, the chromatin fiber must unwind. Next, as DNA
replication proceeds, newly formed DNA must associate with histones, either
preexisting histones or recently synthesized ones that have just made their way to
the nucleus. The synthesis and transport of histones must be coordinated with
DNA synthesis since the nascent DNA becomes incorporated into nucleosomes
within minutes of its formation. Finally, the nucleosomal DNA must interact in
specific ways with a variety of proteins to compact in the same pattern as before.
Student may also write about the different pattern seen in differentiating cells that
allows expression of different genes.
Difficulty: 4
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104. Describe the process of nucleosome decompaction that allows the transcription of RNA.
Ans: Because RNA polymerase is much larger than the nucleosomes in the 300Å fiber
some loosening of individual nucleosomes must occur at the beginning of genes.
Some of the nonhistone proteins known as transcription factors carry out this task
by unwinding nucleosomes and dislodging histones from DNA at the 5' end of
genes where transcription will begin. The nucleosome-free regions are now
decompacted enough for RNA polymerase and other transcription factors to have
access to the DNA and thus allow recognition of promoter regions and initiate
gene expression.
Difficulty: 4
105. Discuss how research on position effect variegation has been used to help identify the
molecules involved in heterochromatin formation.
Ans: Geneticists have used various procedures involving position effect variegation to
identify the molecules involved in heterochromatin formation. In one procedure,
they obtained mutations that either enhance the amount of variegation produced
by genes positioned near heterochromatin, or diminish the amount of variegation.
Enhancement reflects gene inactivation in more cells; diminishment reflects gene
inactivation in fewer cells. Using molecular cloning, researchers later isolated
several of the genes that had mutated, and raised antibodies against the mutant
protein products of these genes. In this way, they discovered that at least some of
the genes influencing heterochromatin formation encode DNA-binding proteins
that localize selectively to the heterochromatin.
Difficulty: 4
106. Describe the relationship between chromatin structure and gene function in eukaryotic
chromosomes.
Ans: Student may mention and describe position effect variegation and Barr body
formation as specialized cases of the relationship between chromatin structure and
gene function. The compaction of chromosomes makes DNA inaccessible to the
proteins that initiate gene expression. This compaction is greatest at metaphase
and anaphase, which curtails most gene transcription at these times. On the other
hand decompaction precedes gene expression. The opening up and closing off of
chromosomal areas depends on a dynamic network of DNA-protein interactions,
which modifies the density of chromatin packaging to appropriate levels for
genetic function and passage through the cell cycle. Once established, the
chromatin structure that determines a gene's expression is stably transmitted to
daughter cells.
Difficulty: 4
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Experimental Design and Interpretation of Data
107. You examine cells with a microscope and detect that there are two Barr bodies present
in each cell. What is most likely the genotype of the cells?
Ans: XXX
Difficulty: 4
108. You are on vacation in a remote tropical region and discover what appears to be an
unusual bacterial colony but it might also be a “clump” of small eukaryotic organisms.
You think you may have discovered a new specie and therefore bring the “animal” back
in a plastic baggie placed on ice from your cooler. Of course, your new life form is
dead by the time you arrive back in the lab but the debris can tell you a lot. Based on
DNA structure, how can you tell whether or not your “find” is eukaryotic or
prokaryotic?
Ans: Several answers are possible and include digesting the DNA with DNase and
determining if regular multiples of about 200 bp result (from protection by
eukaryotic nucleosome formation). DNA associated proteins could be isolated
and differences could indicate if the new organism was prokaryotic or eukaryotic.
Difficulty: 4
109. Manipulation of Yeast Artificial Chromosomes (YAC) can be used to test hypotheses
about eukaryotic chromosomal structure. What is a YAC and how would you use it to
test the hypothesis that there is a minimum length requirement for proper chromosomal
function?
Ans: A YAC is a bacterial plasmid with a yeast centromere, origin of replication,
telemetric sequences and usually a selectable marker gene inserted. Varying
lengths of DNA could be inserted into a plasmid and tested for normal
chromosomal function. In this manner, it had been determined that a minimum of
about 100,000 bp is necessary for normal chromosomal function.
Difficulty: 4
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