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Basics of DNA Replication
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Abstract
By the end of this section, you will be able to:
• Explain how the structure of DNA reveals the replication process
• Describe the Meselson and Stahl experiments
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In this section, you will explore the following questions:
•
•
How does the structure of DNA provide for the process of replication?
How did the Meselson and Stahl experiments support the semi-conservative nature of replication?
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2 Connection for AP
The Watson and Crick model suggested a way in which DNA could be replicated during cell division.
Basically, the two strands unwind and separate where the hydrogen bonds connect the nucleotides. Each
parental strand then serves as a template for a new, complementary daughter strand. Replication is said to
be semi-conservative because the original information encoded in each parental strand is conserved (kept)
in the daughter molecules. Thus, a newly replicated molecule of DNA consists of one old strand and one
new strand.
Meselson and Stahl used density dierences in nitrogen isotopes to investigate replication,
and their experiments supported the semi-conservative model. However, the process of replication is more
complex than their model's simple description.
®
Information presented and the examples highlighted in the section support concepts outlined in Big
Idea 3 and Big Idea 4 of the AP
®
Biology Curriculum Framework. The Learning Objectives listed in the
®
Curriculum Framework provide a transparent foundation for the AP
laboratory experience, instructional activities, and AP
Biology course, an inquiry-based
exam questions.
A Learning Objective merges
required content with one or more of the seven Science Practices.
Big Idea 3
Living systems store, retrieve, transmit and respond to information essential to life
processes.
continued on next page
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Portal module: m10151
Enduring Un-
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Heritable information provides for continuity of life.
derstanding
3.A
Essential
3.A.1 DNA, and in some cases RNA, is the primary source of heritable information.
Knowledge
Science
Prac-
tice
1.2
The student can describe representations and models of natural or man-made
phenomena and systems in the domain.
Learning
jective
Ob-
3.3
The student is able to describe representations and models that illustrate how
genetic information is copied for transmission between generations.
Table 1
Teacher Support:
Before the Meselson and Stahl experiment in 1958, scientists did not
know how chromosomes replicated.
Watson and Crick had suggested that replication was semi-
conservative, but other scientists favored one of two other hypotheses, shown in Figure 1.
The
Meselson and Stahl experiment can be confusing. Take time to walk students through the process.
Take some time to trace a mythical family tree, assume no recombination over time and illustrate
how a chromosome from a distant ancestor might have ended up in a modern-day person.
The Science Practices Assessment Ancillary contains additional test questions for this section that will help
you prepare for the AP exam. These questions address the following standards:
[APLO 2.34][APLO 3.3][APLO 4.1]
The elucidation of the structure of the double helix provided a hint as to how DNA divides and makes
copies of itself. This model suggests that the two strands of the double helix separate during replication,
and each strand serves as a template from which the new complementary strand is copied. What was not
clear was how the replication took place.
There were three models suggested (Figure 1):
conservative,
semi-conservative, and dispersive.
Figure 1: The three suggested models of DNA replication. Grey indicates the original DNA strands,
and blue indicates newly synthesized DNA.
In conservative replication, the parental DNA remains together, and the newly formed daughter strands
are together. The semi-conservative method suggests that each of the two parental DNA strands act as a
template for new DNA to be synthesized; after replication, each double-stranded DNA includes one parental
or old strand and one new strand. In the dispersive model, both copies of DNA have double-stranded
segments of parental DNA and newly synthesized DNA interspersed.
Meselson and Stahl were interested in understanding how DNA replicates. They grew E. coli for sev15
eral generations in a medium containing a heavy isotope of nitrogen (
nitrogenous bases, and eventually into the DNA (Figure 2).
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N) that gets incorporated into
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Figure 2: Meselson and Stahl experimented with E. coli grown rst in heavy nitrogen (15 N) then in
14
N. DNA grown in 15 N (red band) is heavier than DNA grown in 14 N (orange band), and sediments to
a lower level in cesium chloride solution in an ultracentrifuge. When DNA grown in 15 N is switched to
media containing 14 N, after one round of cell division the DNA sediments halfway between the 15 N and
14
N levels, indicating that it now contains fty percent 14 N. In subsequent cell divisions, an increasing
amount of DNA contains 14 N only. This data supports the semi-conservative replication model. (credit:
modication of work by Mariana Ruiz Villareal)
The E. coli culture was then shifted into medium containing
The cells were harvested and the DNA was isolated.
14
N and allowed to grow for one generation.
The DNA was centrifuged at high speeds in an
ultracentrifuge. Some cells were allowed to grow for one more life cycle in
14
N and spun again. During the
density gradient centrifugation, the DNA is loaded into a gradient (typically a salt such as cesium chloride
or sucrose) and spun at high speeds of 50,000 to 60,000 rpm. Under these circumstances, the DNA will form
a band according to its density in the gradient. DNA grown in
than that grown in
been shifted from
exclusively in
15
14
15
15
N will band at a higher density position
N. Meselson and Stahl noted that after one generation of growth in
14
N after they had
N, the single band observed was intermediate in position in between DNA of cells grown
N and
14
N. This suggested either a semi-conservative or dispersive mode of replication. The
DNA harvested from cells grown for two generations in
intermediate position between
15
N and
14
14
N formed two bands: one DNA band was at the
N, and the other corresponded to the band of
14
N DNA. These
results could only be explained if DNA replicates in a semi-conservative manner. Therefore, the other two
modes were ruled out.
During DNA replication, each of the two strands that make up the double helix serves as a template
from which new strands are copied. The new strand will be complementary to the parental or old strand.
When two daughter DNA copies are formed, they have the same sequence and are divided equally into the
two daughter cells.
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Click through this tutorial
Link to Learning:
on DNA replication.
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Science Practice Connection for AP
:
®
Courses:
Design (but do not implement) an experiment to test the three models of DNA repli-
cation.
Summarize the results you would expect if each of the three models of DNA
replication were correct.
Assume you have access in a laboratory to the following: an
experimental organism such as E. coli, an unlimited variety of isotopes, test tube and
centrifuge, and organic growth media.
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Portal module: m10151
Teacher Support:
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This activity is an application of Learning Objective 3.3 and Science Practice
1.2 because by designing an experiment, students are describing the three models that were proposed
to illustrate how genetic information is copied.
3 Section Summary
The model for DNA replication suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied.
In
conservative replication, the parental DNA is conserved, and the daughter DNA is newly synthesized. The
semi-conservative method suggests that each of the two parental DNA strands acts as template for new DNA
to be synthesized; after replication, each double-stranded DNA includes one parental or old strand and
one new strand. The dispersive mode suggested that the two copies of the DNA would have segments of
parental DNA and newly synthesized DNA.
4 Review Questions
Exercise 2
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Exercise 3
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Exercise 4
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Exercise 5
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Exercise 6
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5 Critical Thinking Questions
Exercise 7
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Exercise 8
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Exercise 9
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6 Test Prep for AP
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Exercise 10
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Exercise 11
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