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?

Connection for AP® Courses

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 differences 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® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® 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.
Enduring Understanding 3.A Heritable information provides for continuity of life.
Essential Knowledge 3.A.1 DNA, and in some cases RNA, is the primary source of heritable information.
Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 3.3 The student is able to describe representations and models that illustrate how genetic information is copied for transmission between generations.

The Science Practice Challenge Questions contain 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 14.12): conservative, semi-conservative, and dispersive.

Illustration shows the conservative, semi-conservative, and dispersive models of DNA synthesis. In the conservative model, when DNA is replicated and both newly synthesized strands are paired together. In the semi-conservative model, each newly synthesized strand pairs with a parent strand. In the dispersive model, newly synthesized DNA is interspersed with parent DNA within both DNA strands.
Figure 14.12  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 several generations in a medium containing a “heavy” isotope of nitrogen (15N) that gets incorporated into nitrogenous bases, and eventually into the DNA (Figure 14.13).

Illustration shows an experiment in which E. coli was grown initially in media containing ^{15}N nucleotides. When the DNA was extracted and run in an ultracentrifuge, a band of DNA appeared low in the tube. The culture was next placed in ^{14}N medium. After one generation, all of the DNA appeared in the middle of the tube, indicating that the DNA was a mixture of half ^{14}N and half ^{15}N DNA. After two generations, half of the DNA appeared in the middle of the tube, and half appeared higher up, indicating that half the DNA contained 50% ^{15}N, and half contained ^{14}N only. In subsequent generations, more and more of the DNA appeared in the upper, ^{14}N band.
Figure 14.13 Meselson and Stahl experimented with E. coligrown first in heavy nitrogen (15N) then in 14N. DNA grown in 15N (red band) is heavier than DNA grown in 14N (orange band), and sediments to a lower level in cesium chloride solution in an ultracentrifuge. When DNA grown in 15N is switched to media containing 14N, after one round of cell division the DNA sediments halfway between the 15N and 14N levels, indicating that it now contains fifty percent 14N. In subsequent cell divisions, an increasing amount of DNA contains 14N only. This data supports the semi-conservative replication model. (credit: modification of work by Mariana Ruiz Villareal)
 

The E. coli culture was then shifted into medium containing 14N and allowed to grow for one generation. The cells were harvested and the DNA was isolated. The DNA was centrifuged at high speeds in an ultracentrifuge. Some cells were allowed to grow for one more life cycle in 14N 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 15N will band at a higher density position than that grown in 14N. Meselson and Stahl noted that after one generation of growth in 14N after they had been shifted from 15N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15N and 14N. This suggested either a semi-conservative or dispersive mode of replication. The DNA harvested from cells grown for two generations in 14N formed two bands: one DNA band was at the intermediate position between 15N and 14N, and the other corresponded to the band of 14N 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.


LINK TO LEARNING

Click through this tutorial on DNA replication.

One theory of aging is that the body’s ability to fix mistakes in its DNA decreases as we age. How can this affect DNA replication?
  1. Aging causes accumulation of DNA mutations and DNA damage of only the nuclear DNA and the mistakes will be passed down to new cells causing age related diseases.
  2. Aging results in ineffective DNA repair mechanism so that the mistakes in the DNA will be passed down to new cells. This could lead to the development of age-related diseases.
  3. Aging causes DNA polymerase to function abnormally. This is the sole reason which causes defects in DNA replication.
  4. DNA replication of only fast growing cells is affected by aging.

 SCIENCE PRACTICE CONNECTION FOR AP® COURSES

ACTIVITY

Design (but do not implement) an experiment to test the three models of DNA replication. 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.