12–2 Chromosomes and DNA Replication
Section 12–2
1 FOCUS
D
NA is present in such large amounts in many tissues that
it’s easy to extract and analyze. But where is DNA found in
the cell? How is it organized? Where are the genes that Mendel
first described a century and a half ago?
DNA and Chromosomes
Objectives
Key Concept
• What happens during DNA
replication?
Vocabulary
Prokaryotic cells lack nuclei and many of the organelles found
in eukaryotes. Their DNA molecules are located in the cytoplasm. Most prokaryotes have a single circular DNA molecule
that contains nearly all of the cell’s genetic information. This
large DNA molecule is usually referred to as the cell’s chromosome, as shown in Figure 12–8.
Eukaryotic DNA is a bit more complicated. Many eukaryotes
have as much as 1000 times the amount of DNA as prokaryotes.
This DNA is not found free in the cytoplasm. Eukaryotic DNA is
generally located in the cell nucleus in the form of a number of
chromosomes. The number of chromosomes varies widely from
one species to the next. For example, diploid human cells have
46 chromosomes, Drosophila cells have 8, and giant sequoia tree
cells have 22.
chromatin
histone
replication
DNA polymerase
Reading Strategy:
Asking Questions Before
you read, study the diagram in
Figure 12–11. Make a list of
questions about the diagram.
As you read, write down the
answers to your questions.
DNA Length DNA molecules are surprisingly long. The
E. coli Bacterium
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DNA and
Chromosomes
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Demonstration
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To give students an idea of how large
a DNA molecule is compared to the
cell into which it is packed, show students a ball formed from a piece of
string that is about 1 meter long.
Then, draw a circle on the board that
has a diameter of about 1 mm. If
needed, review the metric equivalents so that students can see the
relationship between micrometers
(diameter of a bacterium) and
millimeters (10-3 to 1) and millimeters to meters (also 10-3 to 1). Explain
that the string represents the DNA
molecule that must fit inside the circle drawn on the board. Encourage
students to speculate about how the
DNA molecule is able to fit into the
cell and why it is so long.
Bases on the
Chromosome
SECTION RESOURCES
Technology:
• Teaching Resources, Section Review 12–2,
Chapter 12 Exploration
Save 12–2
• Reading and Study Workbook A, Section
e
• Adapted Reading and Study Workbook B,
Section 12–2
• Lesson Plans, Section 12–2
• iText, Section 12–1
• Animated Biological Concepts Videotape
Library, 21
• Transparencies Plus, Section 12–2
• Lab Simulations CD-ROM, DNA Structure
and Replication
r
Print:
Tim
Explain that the suffix -ase is used to
denote molecules that are enzymes,
proteins that catalyze biochemical
reactions. Challenge students to look
carefully at the term DNA polymerase.
Ask: Based on the root of the word,
what do you think DNA polymerase does? (It synthesizes the DNA
molecule by adding individual
nucleotides to the DNA polymer.)
Encourage students to preview all
diagrams in this section before reading. For Figure 12–11, have students
list questions for the diagram and
write answers to those questions as
they read.
왔
Chromosome
Vocabulary Preview
Reading Strategy
chromosome of the prokaryote E. coli, which can live in the
human colon (large intestine), contains 4,639,221 base pairs. The
length of such a DNA molecule is roughly 1.6 mm, which doesn’t
sound like much until you think about the small size of a bacterium. To fit inside a typical bacterium, the DNA molecule must
be folded into a space only one one-thousandth of its length.
Figure 12–8 Most prokaryotes, such as
this E. coli bacterium, have only a single
circular chromosome. This chromosome
holds most of the organism’s DNA.
12.2.1 Summarize the events of
DNA replication.
12.2.2 Relate the DNA molecule
to chromosome structure.
2 INSTRUCT
DNA and RNA
295
To get a rough idea of what this means, think of a large
school backpack. Then, imagine trying to pack a 300-meter
length of rope into the backpack! Figure 12–9, which shows DNA
spilling out from a ruptured bacterium, indicates how dramatically the DNA must be folded to fit within the cell.
12–2 (continued)
Build Science Skills
Using Models Give students a very
long piece of yarn (30–50 cm), some
beads, short wooden dowels, and a
film canister. Challenge students to
use the materials to help them fit the
yarn inside the container, making
sure that the yarn can be unwound
fairly easily. Encourage students to
make comparisons between their
models and chromatin.
Chromosome Structure The DNA in eukaryotic cells is
왖
DNA Synthesis
Mitosis
Synthesis of New DNA Molecules
How can you investigate when and where cells
synthesize DNA? Scientists have done this by briefly
adding radioactively labeled thymine to the medium
in which a cell grows. A cell that is synthesizing DNA
will take the labeled thymine nucleotide into its DNA.
The graph shows the total amount of radioactive label
taken into DNA from thymine during an eight-hour
period between two cell divisions.
1. Interpreting Graphics Contrast the amounts of
radioactivity incorporated during the following
times: (a) the first four hours of the experiment,
(b) the next two hours, and (c) the final two hours.
2. Drawing Conclusions Is DNA synthesized
continually during the cell cycle (the period
Inclusion/Special Needs
Show students a picture of a bacterial cell and
an animal cell. Talk about the location of DNA in
each cell and where DNA replication occurs.
Then, discuss situations in which the cell would
need copies of its DNA molecule. Elicit from students that replication occurs when new cells are
needed, either during healing and growth
(mitosis) or during the production of sex cells
(meiosis).
296
Chapter 12
Mitosis
Radioactive Label
Review the cell cycle with students.
Remind them that interphase is the
phase that occurs before mitosis, or
cell division. After the cell has divided, its daughter cells are in
interphase until they divide. Discuss
the three phases of interphase:
G1, S, and G2. Remind students that
cell growth occurs during G1, DNA
replication occurs during S, and the
cell prepares for mitosis in G2.
Answers
1. (a) No radioactivity is incorporated.
(b) Very large amounts of radioactivity are incorporated. (c) No additional
radioactivity is incorporated.
2. No, DNA is synthesized at one
point in the cell cycle about halfway
between the two cell divisions. DNA
synthesis lasts about an hour.
3. The chromosomes in the cell
nucleus would contain the most
radioactivity because thymine is part
of the DNA molecule and chromosomes are composed of DNA.
Figure 12–9 The DNA in a
bacterium is about 1000 times as
long as the bacterium itself. It must
therefore be very tightly folded.
Using Analogies Compare DNA in
a bacterium to a rope jammed into a
backpack.
packed even more tightly. A human cell contains almost 1000
times as many base pairs of DNA as a bacterium. The nucleus of
a human cell contains more than 1 meter of DNA. How is so
much DNA folded into tiny chromosomes? The answer can be
found in the composition of eukaryotic chromosomes.
Eukaryotic chromosomes contain both DNA and protein,
tightly packed together to form a substance called chromatin.
Chromatin consists of DNA that is tightly coiled around proteins called histones, as shown in Figure 12–10. Together, the
DNA and histone molecules form a beadlike structure called a
nucleosome. Nucleosomes pack with one another to form a thick
fiber, which is shortened by a system of loops and coils.
During most of the cell cycle, these fibers are dispersed in the
nucleus so that individual chromosomes are not visible. During
mitosis, however, the fibers of each individual chromosome are
drawn together, forming the tightly packed chromosomes you can
see through a light microscope in dividing cells. The tight packing of nucleosomes may help separate chromosomes during
mitosis. There is also some evidence that changes in chromatin
structure and histone-DNA binding are associated with changes
in gene activity and expression.
0
2
4
6
Time (hours)
8
between cell divisions)? If not, during what phase is
it synthesized? How long is that phase?
3. Predicting Which organelle or cell structure
would you expect to contain the most radioactivity
after this experiment? Explain.
Less Proficient Readers
Encourage students to outline this section as
they read it. Students should include at least
two main points for each green head in the section. Students can also use illustrations in their
outlines to help describe the main points. Check
the outlines to make sure that students have
included all Vocabulary terms and Key
Concepts.
Use Visuals
Nucleosome
Chromosome
DNA
double
helix
Coils
Supercoils
Histones
왖
What do nucleosomes do? Nucleosomes seem to be able to
fold enormous lengths of DNA into the tiny space available in
the cell nucleus. This is such an important function that the
histone proteins themselves have changed very little during
evolution—probably because mistakes in DNA folding could
harm a cell’s ability to reproduce.
Figure 12–10 Eukaryotic
chromosomes contain DNA
wrapped around proteins called
histones. The strands of nucleosomes are tightly coiled and
supercoiled to form chromosomes.
Interpreting Graphics What is
each DNA-histone complex called?
What is chromatin?
Figure 12–10 Use Figure 12–10 to
reinforce the structure of chromosomes. Ask: What are chromosomes
composed of? (Proteins and DNA)
Discuss the role of histones in compacting the DNA molecule. Ask:
What is a nucleosome? (A beadlike
structure composed of DNA wrapped
around a histone molecule) Why
would it be advantageous to the
cell to have its DNA molecules
tightly condensed into nucleosomes during mitosis? (To prevent
the chromatin from tangling and to
make separation and division of the
chromatin more efficient during mitosis) Then, challenge students to infer
why changes in chromatin structure
are associated with gene expression.
Help students realize that the DNA
molecule must be unwound from the
histone molecule so that the individual nucleotide bases can be exposed
if the DNA molecule is to be copied
or translated into protein.
DNA Replication
DNA Replication
When Watson and Crick discovered the double helix structure of
DNA, there was one more remarkable aspect that they recognized immediately. The structure explained how DNA could be
copied, or replicated. Each strand of the DNA double helix has
all the information needed to reconstruct the other half by the
mechanism of base pairing. Because each strand can be used to
make the other strand, the strands are said to be complementary. If you could separate the two strands, the rules of base
pairing would allow you to reconstruct the base sequence of the
other strand.
In most prokaryotes, DNA replication begins at a single
point in the chromosome and proceeds, often in two directions,
until the entire chromosome is replicated. In the larger eukaryotic chromosomes, DNA replication occurs at hundreds of places.
Replication proceeds in both directions until each chromosome
is completely copied. The sites where separation and replication
occur are called replication forks.
N S TA
Download a worksheet
on DNA replication for students to
complete, and find additional teacher
support from NSTA SciLinks.
N S TA
For: Links on DNA
replication
Visit: www.SciLinks.org
Web Code: cbn-4122
FACTS AND FIGURES
Straight talk about nomenclature
Chromatin is a general term that describes a
substance that is made up of DNA, histone and
nonhistone proteins, and some RNA that is
identified by its special staining properties.
Chromosomes are composed of chromatin.
Chromosomes have specific structures. Bacteria
usually have a single circular chromosome.
Multicellular plants and animals have
rod-shaped chromosomes. A chromatid is
found only in replicated chromosomes after
DNA replication has occurred in the S phase of
the cell cycle. Chromatids are the two highly
visible chromosome replicas held together at
their centromeres during prophase and
metaphase of mitosis and meiosis.
Answers to . . .
Chromatin is DNA and
protein tightly packed together.
Figure 12–9 Both situations involve
packing a very long, narrow material
(the DNA or rope) into a compact
space (the bacterium or backpack).
Figure 12–10 A nucleosome
DNA and RNA
297
DNA REPLICATION
12–2 (continued)
Use Visuals
Figure 12–11 As students examine
the diagram of DNA replication, have
volunteers explain what a complementary strand of DNA is and what
happens when the DNA molecule
unzips. (Hydrogen bonds break.)
Discuss how the strand being copied
acts as a template for the new strand
being synthesized. Ask: Why is the
new strand complementary to the
original strand? (It is made up of a
base sequence of purines and pyrimidines that match, or base pair, with
the sequence of purines and pyrimidines of the original strand.) Remind
students of the tightly wound chromosome structure. Have students
infer how chromosome structure
might change during replication.
(The nucleosomes unwind.)
Figure 12–11
During DNA
replication, the DNA molecule
produces two new complementary strands. Each strand of the
double helix of DNA serves as a
template for the new strand. The
electron micrograph shows a double
strand of human DNA.
Nitrogenous
bases
For: DNA Replication activity
Visit: PHSchool.com
Web Code: cbp-4122
Replication fork
DNA polymerase
Original
strand
Original
strand
Growth
New strand
Growth
New strand
Demonstration
Demonstrate that each new DNA
molecule is composed of one original
strand and one new strand. Use two
strands of red yarn to represent the
DNA molecule of a chromosome. As
you separate the strands of yarn for
replication, lay down a new strand of
green yarn to represent the replicated strand of DNA. When replication
is complete, the result will be two
sets of yarn molecules composed of
one red strand and one green strand.
Explain that these two sets of yarn
molecules are connected by centromeres (represent this with a paper
clip) until they are separated during
mitosis.
For: DNA Replication activity
Visit: PHSchool.com
Web Code: cbe-4122
Students learn about DNA
replication by producing
complementary strands of DNA.
298
Chapter 12
DNA
polymerase
KEY
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Replication
fork
FACTS AND FIGURES
Where other genes are located
All the genes in a cell are not located in the nucleus, although for many years scientists thought
they were. However, researchers began to notice
that some inherited traits—such as Leber disorder,
a type of blindness—seem to follow an unusual
inheritance pattern. Could the genes for such
traits be located somewhere else in the cell? In
the 1970s, researchers found that mitochondria
and chloroplasts have their own genes that code
for proteins.
In the 1980s, Douglas Wallace of Emory
University first demonstrated that human mitochondrial DNA is inherited from the mother and
later showed that most cases of Leber disorder are
due to a mutation in mitochondrial DNA. Other
mitochondrial genetic disorders have since been
discovered.
Build Science Skills
Duplicating DNA Before a cell divides, it duplicates its DNA
Applying Concepts Divide the
class into pairs. Instruct each student
to write a base pair sequence for one
strand of DNA. Partners should
exchange their DNA sequences with
each other and write the sequence of
the complementary strand. Monitor
students to be sure they understand
the process.
in a copying process called replication. This process ensures
that each resulting cell will have a complete set of DNA molecules.
During DNA replication, the DNA molecule separates
into two strands, then produces two new complementary
strands following the rules of base pairing. Each strand of
the double helix of DNA serves as a template, or model, for
the new strand.
Figure 12–11 shows the process of DNA replication. The two
strands of the double helix have separated, allowing two replication forks to form. As each new strand forms, new bases are
added following the rules of base pairing. In other words, if the
base on the old strand is adenine, thymine is added to the newly
forming strand. Likewise, guanine is always paired to cytosine.
For example, a strand that has the bases TACGTT produces
a strand with the complementary bases ATGCAA. The result is
two DNA molecules identical to each other and to the original
molecule. Note that each DNA molecule resulting from replication has one original strand and one new strand.
3 ASSESS
Evaluate Understanding
Ask students to describe the steps
that occur in the process of DNA
replication. Ask other students to
explain how DNA is packaged to fit
inside cells.
How Replication Occurs DNA replication is carried out by
a series of enzymes. These enzymes “unzip” a molecule of DNA.
The unzipping occurs when the hydrogen bonds between the
base pairs are broken and the two strands of the molecule
unwind. Each strand serves as a template for the attachment of
complementary bases.
DNA replication involves a host of enzymes and regulatory
molecules. You may recall that enzymes are highly specific. For
this reason, they are often named for the reactions they catalyze.
The principal enzyme involved in DNA replication is called
DNA polymerase (PAHL-ih-mur-ayz) because it joins individual nucleotides to produce a DNA molecule, which is, of course, a
polymer. DNA polymerase also “proofreads” each new DNA
strand, helping to maximize the odds that each molecule is a
perfect copy of the original DNA.
Reteach
Have students review Figure 12–10
and Figure 12–11. Encourage them
to draw a diagram that combines the
ideas described in each figure.
Remind students to clearly label their
diagrams.
12–2 Section Assessment
1.
Key Concept Explain how
DNA is replicated.
2. Where and in what form is
eukaryotic DNA found?
3. How are the long DNA molecules
found in eukaryotes packed into
short chromosomes?
4. How are histones related to
nucleosomes?
5. What is the role of DNA polymerase in DNA replication?
6. Critical Thinking Comparing
and Contrasting How is the
structure of chromosomes in
eukaryotes different from the
structure of chromosomes in
prokaryotes?
Creating a Venn Diagram
Make a Venn diagram that
compares the process of DNA
replication in prokaryotes and
eukaryotes. Compare the
location, steps, and end
products of the process in
each kind of cell. (For more
on Venn diagrams, see
Appendix A.)
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 12–2.
12–2 Section Assessment
1. The DNA molecule separates into two
strands, which serve as templates against
which the new strands are made, following
the rules of base pairing.
2. In the cell nucleus as chromosomes
3. DNA is tightly wound around histones,
forming nucleosomes. Nucleosomes are
tightly coiled and supercoiled to form
chromosomes.
In the Venn diagrams, students
should describe prokaryotic
DNA replication as beginning at a
single point and occurring in the
cytoplasm. Eukaryotic DNA replication begins in hundreds of
places at once and occurs in the
nucleus. DNA replication in both
eukaryotes and prokaryotes proceeds in both directions, results in
two identical strands of DNA, and
so on. You might use the diagrams
to emphasize that the process of
DNA replication is very similar in
the two types of cells.
4. Nucleosomes are composed of DNA wound
around histones.
5. Polymerizes individual nucleotides to produce DNA
6. Prokaryotes: single, circular DNA molecule;
eukaryotes: many chromosomes composed
of tightly coiled DNA and proteins called
histones
DNA and RNA
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