Back To CourseBiology 101: Intro to Biology
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April teaches high school science and holds a master's degree in education.
The model that Watson and Crick proposed in 1953 to describe the molecular structure of DNA was a landmark discovery. But at the time, many scientists weren't convinced that the model was right. Along with their structural model of DNA, Watson and Crick also proposed a model to describe how DNA is copied inside a cell. Many scientists thought their model of DNA production didn't make sense, and it led some people to doubt whether they were even right about the double helix. Let's learn more about the science behind DNA discoveries to find out how this problem was solved.
Scientists had known for a very long time that organisms make copies of their DNA. Making extra copies of the instructions in DNA allows an organism to grow and reproduce. The scientific word for 'copy' is 'replication.' So when we talk about DNA making copies of itself, we call it DNA replication.
Let's quickly review the things we've already learned about DNA. A DNA chain is composed of smaller components called nucleotides. Each nucleotide is composed of a sugar, a phosphate and a nitrogenous base. The nucleotides are arranged into two strands that link together like rungs on a ladder, and the ladder is twisted into a shape we call a double helix. Watson and Crick first proposed this structural model, and further scientific study has shown that they were basically correct. So, why were they challenged by the scientific community?
Watson and Crick had proposed that in order to copy itself, DNA would have to open down the center, sort of like a zipper coming apart, so that a new DNA strand could be built on top of the exposed strands. Following the rules of complimentary base pairing, adenine would pair with thymine, and cytosine would pair with guanine. This idea was called a template model, since one DNA strand serves as the template for a new one.
Watson and Crick figured that this model would result in two new double strands of DNA, each one with one strand of parent (or template) DNA and one strand of daughter (or newly-synthesized) DNA. They called this the semi-conservative model, because half of the parent DNA was conserved in each new DNA molecule.
Scientists looked at the double helix of DNA and wondered how in the world it could possibly open itself up without getting tangled or torn apart. So they thought up some other ideas about how DNA replication works. One hypothesis, called the dispersive model, suggested that DNA only copied itself for short chunks at a time, producing new strands that alternated parent and daughter DNA. Another idea, called the conservative model, argued that DNA didn't split open at all, but somehow kept the parent strands intact while creating an entirely new and separate copy.
Nobody knew for sure how DNA replication really worked until two scientists named Matthew Meselson and Franklin Stahl devised an ingenious experiment in 1958. They realized that they could test all three models at once by keeping track of what happens to one parental DNA strand as it generates a series of copies.
Each model predicts a different distribution of parent DNA following a round of DNA replication. If Meselson and Stahl were able to keep track of parent versus new DNA, they could either support or refute the predictions of the three different models.
Meselson and Stahl decided the best way to tag the parent DNA would be to change one of the atoms in the parent DNA molecule. Remember that nitrogen is found in the nitrogenous bases of each nucleotide. So they decided to use an isotope of nitrogen to distinguish between parent and newly-copied DNA. The isotope of nitrogen had an extra neutron in the nucleus, which made it heavier.
You can see from any periodic table that most nitrogen atoms have an atomic weight of 14. We call these atoms N-14. But an isotope with an extra neutron has a weight of 15, so we call it N-15. The scientists decided to start with parent DNA molecules that only contained N-15. If only N-14 nucleotides were available during DNA replication, they would be able to tell which parts had come from the original double strand and which parts had been created during the replication process.
In order to make DNA go through many rounds of replication, Meselson and Stahl harnessed the reproductive powers of the common bacteria E. coli. They made sure that the first batch of bacteria contained only N-15 DNA. Then, they put the bacteria into a medium that only contained N-14 atoms. That way, whenever the bacteria reproduced, they would be forced to incorporate the N-14 into their new DNA. The scientists sat back and let the bacteria go to work.
With each new generation of bacteria, Meselson and Stahl took a sample so that they could see how the N-15 DNA was being distributed in the daughter molecules. Now, you may be wondering, how could they tell the difference between N-15 and N-14 DNA? It's not like you can actually see an isotope of nitrogen. How did the scientists know how much N-15 was inside each molecule?
The answer is the atomic weight. Because N-15 has one extra neutron, it's slightly heavier than N-14 and therefore makes the DNA molecule more dense. We can separate DNA molecules based on the differences in their densities. To do this, we use a centrifuge, a device that spins a test tube at very high speeds. When a test tube is spun inside a centrifuge, all the contents are pushed toward the bottom. The substances that are heaviest sink farther down the tube, and the lighter substances float. So if you apply a centrifugal force to a mixture of two types of DNA, the heavier N-15 DNA sinks to a lower level than the N-14 molecules.
Every time Meselson and Stahl wanted to take a sample of the bacterial DNA, they had to chop up the tiny organisms and empty all the contents into a test tube. They mixed in a salt solution and then spun the test tube for many hours to make the substances separate out. Then they used special techniques to see how far the DNA molecules sank inside the tube.
When they sampled their first group of bacteria, Meselson and Stahl saw a darkened band in the test tube where the N-15 DNA had sunk and gathered in one spot. But after they let the bacteria reproduce, they got much different results in their samples. The DNA still sank down in the tube, but not nearly as far as the first generation. It was a lighter form of DNA, meaning that it wasn't completely made with the N-15 isotope. After one replication, all of the DNA had been converted to a hybrid of N-15 and N-14 DNA.
Right away, Meselson and Stahl knew that they could rule out one of the three models. The conservative model, which suggested that the original DNA molecule remains intact, had to be false. The conservative model predicted that the centrifugation experiment would produce two distinct bands - one band representing DNA with only N-15 and one band representing DNA with only N-14. Because they observed only one band with DNA of intermediate density, they knew that each and every one of the new DNA molecules contained a mixture of both nitrogen forms.
But Meselson and Stahl still had to figure out whether DNA replication followed the dispersive or semi-conservative model. Since both models would result in a hybrid of parent and daughter DNA, the intermediate band was still consistent with both models. In order to figure out what was really going on, Meselson and Stahl had to let the bacteria keep replicating and study the DNA samples after every generation.
Before we find out what actually happened, let's think about the remaining possibilities for this experiment. How would we know whether DNA replication is dispersive or semi-conservative? What would we expect to see if either of the possible models were correct?
First, we'll assume the dispersive model correctly describes DNA replication. In dispersive replication, DNA is copied in short chunks, and the result is a molecule that alternates pieces of parent DNA with daughter DNA. After one replication, the new molecule would be 50% parent and 50% daughter DNA. After another replication, the result would be 25% of the original parent and 75% newly-copied DNA.
So in each successive generation, the amount of parent DNA would be cut in half. In the case of our experiment, this second generation would have 25% N-15 and 75% N-14 DNA. In the third generation, only 12.5% of the DNA would be N-15. So we would always expect to see one continuous band of DNA in the test tube. This band would move slightly higher up the tube with each successive generation as the DNA molecules became progressively lighter and lighter.
On the other hand, what data would we expect to see if the semi-conservative model is correct? In this model, each new DNA molecule would contain one full strand of parental DNA linked down the center with one full strand of daughter DNA. After one replication, all the new DNA would be the same density. But, after the second round of replication, two different kinds of DNA would emerge: some that were a hybrid of N-15 and N-14, like the previous round, and some that were fully composed of N-14 DNA.
This is because the original parent strands, while split apart from each other in the beginning, are conserved and kept as continuous strands of N-15 DNA. Those parent strands may partner up with new N-14 nucleotides, but they will always be connected along the length of the chain. Therefore, while the amount of N-14 will grow and grow over each generation, there will always be two DNA molecules that contain one strand each of the parent DNA. In our experiment, we would expect to see two separate bands emerge inside the test tubes: one with a growing population of N-14 DNA and one with the initial N-15 hybrids.
As it turned out, Meselson and Stahl observed a splitting of bands that became more pronounced with each new generation. They only had to observe four rounds of replication before they knew for sure that the semi-conservative model was correct.
This was a major breakthrough in the field of biology because so many scientists had been arguing over the issue of DNA replication. Meselson and Stahl were able to disprove two hypotheses, and strongly support the third one, just by performing a simple ingenious experiment.
In summary, DNA replication is the process of making copies of DNA. DNA replicates by semi-conservative replication, which means that one strand of the parent double helix is conserved in each new DNA molecule.
Meselson and Stahl were the scientists who showed that DNA follows the semi-conservative model. They were able to disprove conservative replication, whereby all parent DNA is conserved in the original molecule, after only one round of DNA replication. After four more replications, they also disproved dispersive replication, which suggests that new DNA consists of alternating parent and daughter DNA.
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Back To CourseBiology 101: Intro to Biology
22 chapters | 151 lessons | 12 flashcard sets