Course: SAT 170 Introductory Biotechnology

Unit 3 Learning Module

Page 2 of 3
DNA-The Molecule of Heredity (continued)

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Figure 6

Organization of DNA

As you may recall from our last chapter, DNA is located within the nucleus of eukaryotes. As mentioned previously, this DNA functions as our genetic information as it provides directions for the synthesis of our proteins. The segments of DNA that code for proteins are known as genes. For humans, it is estimated that there are somewhere between 25, 000 to 30, 000 genes amongst approximately 3 billion nucleotides of DNA. The complete set of genes for an organism is known as that organism’s genome.

DNA primarily exists in two forms, known as chromatin and chromosomes. Chromatin represents a loose, uncondensed form of DNA that is highly accessible for the synthesis of proteins. As a cell prepares for cell division, the DNA becomes tightly coiled into the familiar chromosome structure to ensure accurate separation of the chromosomes once division is complete. Genetic testing is often done to evaluate the number and integrity of these chromosomes in order to determine risk for certain genetic disorders. Once a sample of DNA is collected for genetic testing, the chromosomes can be evaluated for their shape, number and physical appearance, which is known as the karyotype.

See Figure 4: A typical karyotype arranges the chromosomes from largest to smallest and will use different staining techniques to evaluate the composition of those chromosomes.

DNA Replication

Understanding the process of DNA replication became an important goal for scientists, as many believed this would help them understand how it was that DNA was able to carry genetic information from one generation to the next. There were several models that were proposed for the process of DNA replication, and it took an elegant experiment performed by Matthew Messelson and Franklin Stahl in 1958 to determine which of these models was accurate. Since they were evaluating three models, they essentially had three hypotheses that they were testing. When there are multiple hypotheses for one experiment we often describe them as alternative hypotheses. The first model was known as the Conservative Model, which explained that during DNA replication the strands of the parent DNA molecule would serve as templates to produce daughter strands, but once replication was complete the result would be one DNA molecule composed of the original (parent) strands and a second DNA molecule composed of two new (daughter) strands. In contrast the semi-conservative model predicted that the strands of the parent DNA molecule would each serve as a template for a new daughter strand and the resulting two DNA molecules would each be composed on of one parent and one daughter strand. The last of the models being tested was the dispersive model. This model outlined DNA replication as a seemingly more random process, where the strands of the parent DNA molecules would serve as templates for the daughter strands but the resulting two DNA molecules would be composed of DNA strands that had both parent and daughter fragments.

See Figure 5: The three different DNA models are illustrated here. The conservative model (shown as konservativ), the semi-conservative model (shown as semikonservativ) and the dispersive model (shown as dispers).

Once again, radioisotopes were utilized. This time, Meselson and Stahl decided to grow bacteria on a medium containing a particular isotope, ^15N. The result was that that bacteria’s entire DNA contained the ^15N isotope. Once the ^15N isotope was incorporated into their DNA, the bacteria were switched to a medium containing a different isotope, ^14N (which is lighter than the ^15N isotope). After tracing the isotopes present in the new DNA molecules, it was discovered that they all exhibited a mix of the ^15N (heavy) and ^14N (light) isotopes. This allowed the researchers to rule out the conservative model for DNA replication, as it would have predicted that there would be two distinct isotopes; the ^15N that would have remained in the parent DNA molecule and the ^14N that would have been present only in the daughter DNA molecule. This left Meselson and Stahl with two remaining hypotheses to test. A second round of replication on the ^14N media would help them to differentiate between the two models. If the dispersive model were accurate, they would have expected that after a second round of replication all of the DNA molecules would have been just a little lighter since those molecules would have been composed of a mixture of the original ^15N and the new ^14N isotopes. This was not what they observed. Instead, they saw that after the second round of replication, they ended up with some DNA molecules that were a mix of the original ^15N and the new ^14N, and other DNA molecules that were had only the ^14N isotope (from the second round of replication).

See Figure 6: This image summarizes the results of the Meselson Stahl experiment. The heavy N15 isotope is shown in red, while the light N14 isotope is shown in orange. Notice the change in the heaviness (density) of the bands after each round of DNA replication (or generation).