My Campbell's Biology textbook contains the following diagram related to the semi-conservative model of DNA replication proposed by Watson and Crick. I have highlighted where my confusion arises in red:
So, I understand what goes on in the first replication--that's pretty much straightforward. However, what I don't understand is why in the second replication, the light blue strand isn't paired with a dark blue strand. After all, aren't the light blue strand and dark blue strand complementary, per the results of the first replication?
After the light blue and dark blue DNA strands separate to serve as templates in Replication #2, why don't we see two dark+light double-helices?
I believe the reason you are having trouble understanding the concept is due to a poor usage of colors in the diagram. Don't focus on the colors, but on the concept. It's the same for both replication events. Each strand of a double helix is used as a template to make a new complimentary strand, giving rise to two new DNA helices from the original. In each new double helix, one strand should match that of the double helix it came from before, as it was the template and is the same strand. While the other strand, the complimentary strand, has been newly synthesized to match the complimentary strand. In this diagram, every newly synthesized strand is shown in light blue.
Start by looking at the first double-helix of DNA (navy blue). During replication, the DNA is unwound and each navy strand is used as a template to create the newly synthesized (complimentary) strand, which is shown in light blue. In this first all navy double helix, you have two navy strands that are each used as a template strand and new complimentary strands (light blue) are synthesized to match . This produces the second two helices (navy and light blue). They each have one strand from the original helix (navy), and one new strand (light blue).
The process is exactly the same for the second replication event, only, a new color was not introduced to show a newly synthesized strand, so one cannot differentiate between the original (template) strand and the new complimentary strand. Both are pictured as light blue.
For the final replication event, only focus on the top navy and light blue helix first. Just as before, the two strands are separated from each other and are both used as templates to create a new strand. The navy blue strand is used as a template for a new light blue strand to be transcribed. This produces the top-most helix, where the original strand is shown as navy and the newly transcribed (complimentary) strand is light blue.
Just as the navy strand was a template, the light blue strand from the bicolored helix is used as a template strand as well to create the second all light blue helix. This light blue template strand is used to synthesize a new light blue strand, creating the all light blue helix. One of the strands is the original light blue one from the navy and light blue helix, while the other strand is the newly synthesized one.
It might've been easier if they had used a different color for the final round of replication, like orange. Then to show the newly synthesized strands in the final round, the 1st and 4th double helix would consist of a navy (original) strand and an orange (newly synthesized) strand. The second and third double helices would then have each had a light blue (original) strand and an orange (newly synthesized) strand. This would have made it easier to understand where each strand originally came from.
Long Answer Question: Explain the process of DNA replication. - Biology
The process by which DNA duplicates to form identical copies is known as replication.
Semi-conservative method of replication:
1. After replication, each daughter DNA molecule has one old and other new strands.
2. As parental DNA is partly conserved in each daughter's DNA, the process of replication is called semi-conservative.
3. The model of semi-conservative replication was proposed by Watson and Crick.
4. The semi-conservative model of DNA replication using the heavy isotope of nitrogen N15 and E. coli was experimentally proved by Meselson and Stahl (1958).
Mechanism of replication is as follows:
a. Activation of Nucleotides:
i. The four types of nucleotides of DNA i.e. dAMP, dGMP, dCMP and dTMP are present in the nucleoplasm.
ii. They are activated by ATP in presence of an enzyme phosphorylase.
iii. This results in the formation of deoxyribonucleotide triphosphates i.e. dATP, dGTP, dCTP and dTTP. This process is known as Phosphorylation.
b. Point of Origin or Initiation point:
i. Replication begins at a specific point &lsquoO&rsquo origin and terminates at point &lsquoT&rsquo.
ii. Origin is flanked by &lsquoT&rsquo sites. The unit of DNA in which replication occurs is called replicon.
iii. In prokaryotes, there is only one replicon however in eukaryotes, there are several replicons in tandem.
iv. At the point &lsquoO&rsquo, enzyme endonuclease nicks one of the strands of DNA, temporarily.
v. The nick occurs in the sugar-phosphate backbone or the phosphodiester bond.
c. Unwinding of DNA molecule:
i. Enzyme DNA helicase breaks weak hydrogen bonds in the vicinity of &lsquoO&rsquo.
ii. The strands of DNA separate and unwind. This unwinding is bidirectional and continues as &lsquoY&rsquo shaped replication fork.
iii. Each separated strand acts as a template.
iv. The two separated strands are prevented from recoiling (rejoining) by SSBP (Single-strand binding proteins).
v. SSB proteins remain attached to both the separated strands for facilitating the synthesis of new polynucleotide strands.
d. Replicating fork:
i. The point formed due to the unwinding and separation of two strands appears like a Y-shaped fork, called replicating/ replication fork.
ii. The unwinding of strands imposes strain which is relieved by the super-helix relaxing enzyme.
e. Synthesis of new strands:
i. Each separated strand acts as a mould or template for the synthesis of a new complementary strand.
ii. It requires a small RNA molecule, called RNA primer.
iii. RNA primer attaches to the 3&rsquo end of the template strand and attracts complementary nucleotides from the surrounding nucleoplasm.
iv. These nucleotides bind to the complementary nucleotides on the template strand by forming hydrogen bonds (i.e. A=T or T=A G = C or C = G).
v. The newly bound consecutive nucleotides get interconnected by phosphodiester bonds, forming a polynucleotide strand.
vi. The synthesis of a new complementary strand is catalyzed by enzyme DNA polymerase. 7. The new complementary strand is always formed in 5&rsquo&rarr 3&rsquo direction.
f. Leading and Lagging strand:
i. The template strand with free 3&rsquo end is called a leading template and with free 5&rsquo end is called a lagging template.
ii. The process of replication always starts at the C-3 end of the template strand and proceeds towards C-5 end.
iii. As both the strands of the parental DNA are antiparallel, new strands are always formed in 5&rsquo &rarr 3&rsquo direction.
iv. One of the newly synthesized strands which develop continuously towards the replicating fork is called the leading strand.
v. Another new strand develops discontinuously away from the replicating fork and is called the lagging strand.
vi. Maturation of Okazaki fragments: DNA synthesis on the lagging template takes place in the form of small fragments called as Okazaki fragments (named after scientist Okazaki).
vii. Okazaki fragments are joined by the enzyme DNA ligase.
viii. RNA primers are removed by DNA polymerase and replaced by DNA sequence with the help of DNA polymerase-I in prokaryotes and DNA polymerase-&alpha in eukaryotes.
ix. Finally, DNA gyrase (topoisomerase) enzyme forms a double helix to form daughter DNA molecules.
g. Formation of daughter DNA molecules:
i. At the end of the replication, two daughter DNA molecules are formed.
ii. In each daughter's DNA, one strand is parental and the other one is totally newly synthesized.
iii. Thus, 50% is contributed by mother DNA. Hence, it is described as semiconservative replication.
The three different DNA replication models suggested by scientists include:
- Conservative model of DNA replication
- Semi-conservative model of DNAreplication
- Dispersive model of DNA replication
Conservative model of DNA: According to this model the parental DNA will be conserved in the next generation. For example, a cell containing a double stranded parental DNA (represented by black color) will divide by mitosis into two daughter cells. Now, one daughter cell will contain the parental double stranded DNA and the other daughter cell will contain daughter DNA double strands (represented by red color).
Semiconservative model: In this model, half of the parental DNA would be conserved in the next generation. For example, if a call contains double stranded parental DNA (represented by black color) and ready to divide by mitosis – in each of the daughter cell, half of the parental DNA would be conserved. In other words, in each new cell, one strand of DNA would be parental and the other strand would be daughter DNA. Now, you can make a difference between conservative and semiconservative model of dna replication.
Dispersive model: In this model, when parental DNA replicates, it forms a mixture of parental DNA and daughter DNA in the next generation.
Experimental Proof for the Semiconservative Model of DNA Replication:
This experiment was done in 1958 by two young scientists of that time. In order to perform the experiment, they used E. coli bacteria, Nitrogen isotopes and Caesium chloride density gradient. We will discuss this experiment in different steps.
In this step, scientists grew E. coli bacteria in a heavy nitrogen isotope solution i.e., NH4Cl ( 15 N). We know that DNA contains a large amount of nitrogen and if a bacterium is grown in a medium containing nitrogen of specific isotope (in this case, 15 N), the bacteria would use the heavy isotope of nitrogen to build DNA. So, the daughter strands of newly replicated DNA would vary by weight and could be easily separated by density gradient centrifugation.
After growing bacterium for several generations in a heavy nitrogen isotope environment, Meselson and Stahl pulled some samples of E. coli out of the growth medium for testing. They centrifuged the sample for initial separation and then they added salt to the bacteria so that the bacteria could release its DNA contents in order to further analyze the sample. When the DNAs of bacteria were transferred to Cesium chloride density gradient, it formed a band at the bottom (similar to the heavy density of Cesium chloride). The band formed by 15 N-DNA was called as Fo generation.
In step-2, the N-15 labeled DNA containing bacteria are transferred to N-14 solution (NH4Cl solution) to grow for all subsequent generations so that any new DNA strands produced would have a lower density than the original parental N-15 DNA. Now, the new strands formed after 20 minutes (generation time of E. coli) are again sent for density gradient and a new band formed which had a lower density than the Fo generation DNA. This new band formed DNA was called F1 generation.
Likewise, after 40 minutes, two bands were seen, one at the middle and one at the top (light density). However, after 60 minutes, the hybrid density (shown in the middle) got thinner and the light density band got thicker. The N-15—N-14 DNA depleted because scientists never reintroduced N-15 nitrogen in the medium.
Meselson and Stahl after the experiment concluded that DNA replicates semiconservatively, meaning that the double stranded DNA first separate and each DNA strand serves as a template for the synthesis of new complimentary strand. In other words, we can conclude that semiconservative model of dna replication exists in nature.
What is semi-conservative replication of DNA? And why is it important?
Semi-conservative DNA replication involves splitting open the parent cells DNA duplex and exposing both strands. Now these strands are accessible to replication machinery to act as a template, so that the sequence can be 'read' and a daughter strand synthesised that is complementary to each parent strand. This will produce two DNA duplexes which have one parent strand and one daughter strand. The semi-conservative mechanism minimises errors in DNA replication, because the template gives DNA polymerase something accurate to copy from. This is very important because cells want to minimise errors in replication when dividing, so that mutations (such as incorrect bases, deletions, insertions) are not brought into the genome. Mutations in genes can cause proteins to become non-functional and lead to disease, including cancer.
How does the Meselson and Stahl Experiment provide evidence for the semi conservative theory of DNA replication?
Before we start on the specifics of the experiment itself we need to understand how DNA replication occurs on a molecular level. DNA is made up of four different bases A, C, G and T. Each base is attached to a deoxyribose sugar and a phosphate group. The sugars and phosphates are covalently bonded to each other in a long line to form the backbone of the linear DNA model. This single stranded DNA is not stable and is vulnerable to damage as the bases that carry genetic information are left exposed. This is avoided by storing DNA as a double stranded molecule in which the bases pair between the backbones like the rungs of a ladder. A always pairs with T and C always pairs with G. This is convenient for DNA replication as it means the DNA strands can be separated and used as a template for the new strand.
There are three possible ways to use the DNA template to replicate DNA conservative, semi-conservatively or dispersive replication. Meselson and Stahl's experiment sought to establish which one was correct. Conservative replication separates the original strands to use as templates for new strands. The new strands combine to form an entirely new double stranded molecule while the original strands re-join so the original molecule is completely conserved. Semi conservative replication uses the original strands as templates but there is no further separation or re-joining, instead the new molecules are a mixture with one side as the original and the other as new DNA. Dispersive replication is similar to semi-conservative replication but the original and new strand constantly swap so that each individual DNA strand is a mixture of old and new.
To work out which method is used they grew E. coli bacteria in a medium containing N 15 , a heavier isotope of nitrogen, which is an element incorporated in DNA bases. The bacteria were then removed and placed in a medium containing only N 14 , so that all new DNA would be lighter than the original DNA. By letting the bacteria divide and replicate their DNA and then extracting and separating out the DNA according to weight they would be able to distinguish between these methods.
After the first division half of the DNA will contain N 15 and half N 14 , but the weights will be distributed differently depending on the method of replication used. When the DNA was extracted and separated it made one band, meaning all the DNA molecules were the same weight. They were therefore able to disregard the conservative replication as this would have made two bands, one entirely of N 15 and one of N 14 . In the other two possible methods the strands contain equal amounts of original and new DNA.
To distinguish between semi-conservative and dispersive replication the bacteria were allowed to divide again and the extracted DNA formed two bands. This confirmed that the method of replication was semi conservative because half of the new DNA would be formed using the N 15 strand as a template and half with a N 14 strand template, so half of the new DNA molecules would be heavier than the other. In contrast dispersive replication would produce DNA that all had an equal proportion of N 15 to N 14 all of the DNA would be the same weight. This proved that the method of DNA replication is semi conservative.
If I were answering this question in a tutorial I would use a diagram with different colours to represent the different weight strands to explain this as this makes it a lot clearer and easier to follow. I would also recommend doing this in an exam as a correct diagram shows the examiner that you understand the principles of the experiment, but you would need to accompany this with an explanation of how the results they found support the conclusion they made.
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The Conservative Replication Model Biology Essay
In the dispersive replication model, the original DNA double helix breaks apart into fragments and each fragment then serves as a template for a new DNA fragment. As a result, every cell division produces two cells with varying amounts of old and new DNA.
According to the semi-conservative replication model, the two original DNA strands (i.e., the two complementary halves of the double helix) separate during replication each strand then serves as a template for a new DNA strand, which means that each newly synthesized double helix is a combination of one old (original) and one new DNA strand.
When these three models were first proposed, scientists had few clues about the process occurring at the molecular level during DNA replication. Fortunately, the models yielded different predictions about the distribution of old versus new DNA in newly divided cells. These predictions were as follows:
According to the semi-conservative model, after one round of replication, every new DNA double helix would be a hybrid that consisted of one strand of old DNA bound to one strand of newly synthesized DNA. Then, during the second round of replication, the hybrids would separate, and each strand would pair with a newly synthesized strand. Afterward, only half of the new DNA double helices would be hybrids the other half would be completely new. Every successive round of replication therefore would result in fewer hybrids and more completely new double helices.
According to the conservative model, after one round of replication, half of the new DNA double helices would be composed of completely old, (original) DNA, and the other half would be completely new. Then, during the second round of replication, each double helix would be copied in its entirety. Afterward, one-quarter of the double helices would be completely old, and three-quarters would be completely new. Thus, each successive round of replication would result in a greater proportion of completely new DNA double helices, while the number of completely original DNA double helices would remain constant.
According to the dispersive model, every round of replication would result in hybrids, or DNA double helices that are part original DNA and part new DNA. Each subsequent round of replication would then produce double helices with greater amounts of new DNA.
Matthew Meselson and Franklin Stahl were well familiar with these three predictions, and they reasoned that if there were a way to distinguish old versus new DNA, it should be possible to test each prediction. As they were aware of previous studies that had relied on isotope labels as a mode to differentiate between parental and progeny molecules, they decided to see whether the same technique could be used to differentiate between parental and progeny DNA.
Thus they began their experiment by choosing two isotopes of nitrogen as their labels:
The common and lighter 14N
The rare and heavier 15N ("heavy" nitrogen)
As their sedimentation method they used a technique known as cesium chloride (CsCl) equilibrium density gradient centrifugation.
Meselson and Stahl opted for nitrogen because it is an essential component of DNA therefore, whenever a cell divides and its DNA replicates, it incorporates new N atoms into the DNA of its daughter cells, depending on which model was correct.
If several different density species of DNA are present," they predicted, "each will form a band at the position where the density of the CsCl solution is equal to the density of that species. In this way, DNA labeled with heavy nitrogen (15N) may be resolved from unlabeled DNA"
They then continued their experiment by growing a culture of E. coli bacteria in a medium that had the heavier 15N (in the form of 15N-labeled ammonium chloride) as its only source of nitrogen. The bacteria divide will contain 15N.
Then they changed the medium to one containing only 14N-labeled ammonium salts as the sole nitrogen source. So, from that point onward, every new strand of DNA would be built with 14N rather than 15N.
Just before the addition of 14N and periodically thereafter, as the bacterial cells grew and replicated Meselson and Stahl sampled DNA for use in equilibrium density gradient centrifugation to determine how much 15N (from the original or old DNA) versus 14N (from the new DNA) was present.
For the centrifugation procedure, they mixed the DNA samples with a solution of cesium chloride and then centrifuged the samples for enough time to allow the heavier 15N and lighter 14N DNA to move to different positions in the centrifuge tube.
By centrifugation, the scientists found that DNA composed entirely of 15N-labeled DNA (i.e., DNA collected just prior to changing the culture from one containing only 15N to one containing only 14N) formed a single distinct band, because both of its strands were made entirely in the "heavy" nitrogen medium. Following a single round of replication, the DNA again formed a single distinct band, but the band was located in a different position along the centrifugation gradient. Specifically, it was found midway between where all the 15N and the entire 14N DNA would have migrated, in other words, halfway between "heavy" and "light"
Based on these findings, the scientists were immediately able to exclude the conservative model of replication as a possibility. After all, if DNA replicated conservatively, there should have been two distinct bands after a single round of replication half of the new DNA would have migrated to the same position as it did before the culture was transferred to the 14N-containing medium (i.e., to the "heavy" position), and only the other half would have migrated to the new position (i.e., to the "light" position). That left the scientists with only two options: either DNA replicated semi-conservatively, as Watson and Crick had predicted, or it replicated dispersively.
To differentiate between the two, Meselson and Stahl had to let the cells divide again and then sample the DNA after a second round of replication.
After that second round of replication, the scientists found that the DNA separated into two distinct bands: one in a position where DNA containing only 14N would be expected to migrate, and the other in a position where hybrid DNA (containing half 14N and half 15N) would be expected to migrate. The scientists continued to observe the same two bands after several successive rounds of replication. These results were consistent with the semi-conservative model of replication and the reality that, when DNA replicated, each new double helix was built with one old strand and one new strand. If the dispersive model were the correct model, the scientists would have continued to observe only a single band after every round of replication.
Semi-conservative replication made sense in light of the double helix structural model of DNA, in particular its complementary nature and the fact that adenine always pairs with thymine and cytosine always pairs with guanine. Looking at this model, it is easy to imagine that during replication, each strand serves as a template for the synthesis of a new strand, with complementary bases being added in the order indicated.
|Date||May 2017||Marks available||1||Reference code||17M.1.SL.TZ1.21|
|Level||Standard level||Paper||Paper 1||Time zone||Time zone 1|
|Command term||Question number||21||Adapted from||N/A|
Cladograms can be created by comparing DNA or protein sequences. The cladogram on the left is based on DNA sequences and the cladogram on the right is based on comparing protein sequences.
What is the reason that cladograms based on DNA sequences are more reliable predictors of the phylogenetic relationship of species than cladograms based on protein sequences?
The Embryo Project Encyclopedia
In 1954 Max Delbrück published "On the Replication of Desoxyribonucleic Acid (DNA)" to question the semi-conservative DNA replication mechanism proposed that James Watson and Francis Crick had proposed in 1953. In his article published in the Proceedings of the National Academy of Sciences, Delbrück offers an alternative DNA replication mechanism, later called dispersive replication. Unlike other articles before it, "On the Replication" presents ways to experimentally test different DNA replication theories. The article sparked a debate in the 1950s over how DNA replicated, which culminated in 1957 and 1958 with the Meselson-Stahl experiment supporting semi-conservative DNA replication as suggested by Watson and Crick. "On the Replication" played a major role in the study of DNA in the 1950s, a period of time during which scientists gained a better understanding of DNA as a whole and its role in genetic inheritance.
"On the Replication" was Delbrück's response to two 1953 articles by Watson and Crick concerning DNA. Prior to the Watson-Crick publications, scientists had determined that genes, which are the biological factors that control heritable traits, are comprised of DNA. However, the physical structure of DNA remained unknown. In addition, scientists did not know how the properties of DNA translated into the passing of genetic information from one cell to another. Watson and Crick addressed those questions in their article.
In their first 1953 article, Watson and Crick described their proposed structure of DNA. They described DNA as a double helix that contains two long, helical molecular strands. The DNA strands consist of a backbone with individual molecular units called bases attached to the backbone. Like the rungs of a ladder, in Watson-Crick DNA the DNA bases point inward so that the base of one strand joins with the base of the other, thereby providing the connecting points between the two DNA strands. Each DNA base has a specific partner meaning that each base only pairs with one other type of base. Therefore, according to Watson and Crick, by knowing the identity of the bases on one DNA strand one could determine the bases of the other strand. Joined at the bases, the DNA strands coil around each other along a vertical axis like two pieces of rope. The strands also have a beginning and an end and each run in opposite directions, so that the beginning of one DNA strand lines up with the end of the other. As of 2017, Watson and Crick's DNA structure remains the accepted structure for DNA.
In "On the Replication," Delbrück did not contest the structure of DNA that Watson and Crick had proposed, but instead he questioned the replication mechanism Watson and Crick proposed in a second article. In their second article, Watson and Crick proposed what later became known as semi-conservative DNA replication. According to Watson and Crick, because each strand of DNA contained bases that corresponded to the other strand, the strands themselves could serve as individual templates for self-replication. Watson and Crick claimed that during replication, the two strands of DNA untwisted completely and separated, each strand serving as a parent template on which new, daughter DNA strands are constructed. In their article, Watson and Crick acknowledged the issue of the strands getting tangled when uncoiled, but they dismissed the problem as something that could be overcome.
Delbrück, a researcher at the California Institute of Technology in Pasadena, California, addressed the problem of DNA strands tangling during replication with his 1954 article, "On the Replication of Desoxyribonucleic Acid (DNA)." Delbrück was a scientist educated in physics. However, when he published his article in 1954, Delbrück studied biology in search of fundamental laws that governed gene replication within basic organisms. Following the publication of Watson and Crick's articles in 1953, Delbrück wrote to Watson about how he thought the untwisting of DNA strands posed a problem for replication. He hypothesized different alternate replication mechanisms before settling on the one he wrote about in 1954 called dispersive replication.
In "On the Replication," Delbrück details his dispersive replication model and experimental designs to test that model, along with the logic and reasoning behind both. The six-page article begins with a description of the Watson-Crick model of DNA and DNA replication. Delbrück subsequently calls the model into question and proposes multiple alternative theories before settling on his favored mechanism, dispersive replication. Using diagrams and illustrations as aids, Delbrück details his model and concludes how different aspects of his mechanism could yield experimental results distinguishing the model from others. "On the Replication" ends with a brief summary and list of references.
Delbrück opens his article with a description of the Watson-Crick model of DNA and the replication mechanism that Watson and Crick suggested. In summarizing Watson-Crick DNA, Delbrück highlights how DNA bases function like a genetic code for heritable traits and how those bases are added sequentially during DNA replication. Delbrück then states that his main issue with Watson and Crick's replication mechanism is that their mechanism requires DNA strands to separate.
Delbrück describes three ways DNA strands could separate before replication. In the first way, one of the DNA strands is pulled up while the other is pulled down, meaning that the two strands slide past each other and apart. The DNA strands are pulled apart from end to end so that the strands slide past each other vertically. In the second way, the DNA strands untwist as suggested by Watson and Crick. Delbrück writes that he rejects both of those hypotheses for how DNA strands separate because the mechanisms are inelegant and therefore inefficient.
Instead, Delbrück discusses in more detail a third way by which DNA strands could separate. According to Delbrück, DNA strands could also separate through a complex interaction of certain breaks and reunions at each interlock of the coil formed by the DNA strands. One strand could break along its backbone before each twist of the double helix, where the strands would normally get caught. The break in one strand provides a gap for the other strand to pass through, thereby avoiding tangling. The gap would then be resealed. Or, both strands break along their backbones at each twist. The segments below the breaks would then cross over each other in such a way that the twist is unwound. Lastly, each strand would join with the part of the opposing strand above the break, leaving the strands uncoiled. Delbrück argues that both of those processes are unfavorable. If only one strand breaks, the symmetry of DNA that is essential to the molecule's stability would be disrupted. If both strands broke at the same point and crossed over, they would join opposing strands in the wrong direction, because the strands run in opposite directions to begin with. Therefore, Delbrück concludes that DNA strands cannot separate prior to replication. Instead, Delbrück suggests that the separation of DNA strands and DNA replication occur simultaneously through a method he describes in the next part of his article.
In the next portion of the article, Delbrück provides a description of his replication mechanism, later called dispersive replication, in which DNA strands replicate and separate at the same time. The mechanism Delbrück proposes is a modification of the breaks and reunions method he suggests earlier in his article. Delbrück considers a case in which replication begins while the two DNA strands are still twisted together. Between each interlock of the DNA double helix, the DNA strands can pull apart slightly without fully separating, forming a bubble. Though not mentioned by Delbrück, the same can occur between two wound pieces of rope. In the gap formed between the DNA strands, Delbrück states that the replication of daughter strands can begin at each parent DNA strand. Delbrück describes that the replication continues until the assembling daughter strands meet an interlock of the parent helix. At that point, the parent strands each break along their backbones. The parent strands cross over each other, but instead of rejoining with the opposing parent strands, they rejoin with the opposing daughter strands. Each daughter strand is a replica of the opposing parent strand, so those strands travel in the same direction. Therefore, Delbrück states, when the parent strands attach to the opposite daughter strands, the parent strands attach in the right direction. The successful rejoining of DNA strands in the method that Delbrück describes can only occur if the daughter DNA strands started replicating first. The process continues throughout the entire DNA molecule, with each break occurring at each twist of the two parent DNA strands.
Following the description of his suggested alternative replication mechanism, Delbrück discusses why his mechanism is energetically favorable. Though not explicitly stated by Delbrück, chemicals are more likely to undergo a chemical change if less energy is required to achieve that change. When DNA strands separate and replicate, they undergo a chemical change that requires energy. In his article, Delbrück argues that his DNA replication mechanism requires the least amount of energy, so his mechanism is most likely to occur. Delbrück postulates that breaking and rejoining DNA strands results in a net zero energy consumption. He also argues that no work needs to be done to correctly coil the daughter DNA strands, because those strands are automatically coiled as they are produced. Delbrück continues his discussion by stating that at any given time during replication, only one small part of the entire DNA molecule is disrupted. The rest of the molecule retains its stable, helical configuration. In contrast, if both strand of DNA were separated before replication as Watson and Crick suggested, the entire molecule would be unstable. According to Delbrück, the benefits of minimal disruption in the DNA molecule are twofold. First, the total energy use during the process is minimized. Second, only a small part of the DNA molecule needs to be uncoiled, allowing the process to occur in the tight constraints of a cell nucleus.
The final section of "On the Replication" consists of a discussion of possible experimental results that would indicate that Delbrück's replication mechanism occurs in DNA. When Delbrück published the article, there was no experimental evidence about how DNA replicated. To craft an experimental method that would work, Delbrück first establishes that because of how parental DNA strands cross over and join with daughter DNA strands during his suggested replication mechanism, the final daughter DNA molecules would consist of alternating parental and daughter segments, thereby distinguishing the mechanism from other DNA replication mechanisms. Delbrück hypothesizes that in his mechanism if the parent DNA double helix could be labeled in some way and differentiated from newly replicated DNA, the amount of labeled and unlabeled DNA would remain equal in each new DNA double helix over many replications. Delbrück notes that other scientists had tried to label DNA by incorporating radioactive phosphorus into DNA, but were unsuccessful. Delbrück concludes his article with a brief summary.
Delbrück's dispersive replication, presented in "On the Replication," provided a plausible counterexample to the semi-conservative model of replication. While many accepted the Watson-Crick replication mechanism initially, Delbrück's article showed that questions regarding DNA replication were far from answered. In a book by historian of science Frederic Lawrence Holmes about the study of DNA replication in the 1950s, Meselson, Stahl, and the Replication of DNA: A History of "The Most Beautiful Experiment in Biology," Holmes credits Delbrück with being the first to formally outline the issues surrounding DNA replication. Like Watson and Crick's semi-conservative model, Delbrück's dispersive replication model served only as theoretical speculation. However, as Holmes emphasizes, Delbrück provided a way to find concrete, experimental evidence supporting one model or the other.
Through his publication of "On the Replication" and afterwards, Delbrück participated in the DNA replication debate during the years leading up to the Meselson-Stahl experiment. After Delbrück suggested his alternative model, other scientists provided their own theories for how DNA replicated that challenged the semi-conservative model. Some of those scientists even communicated with Delbrück directly regarding their ideas. Watson also communicated with Delbrück privately about DNA replication after the article's publication. According to Holmes, the large response to Delbrück's article prompted Watson to question his own model of DNA and suggest counter theories to his own replication mechanism.
Between 1957 and 1958, Meselson and Stahl applied the DNA labeling method suggested by Delbrück in "On the Replication" to their own experiment. Meselson and Stahl labeled parent DNA strands and then traced the distribution of parental and daughter DNA over many replication cycles. They found that DNA replicated semi-conservatively as suggested by Watson and Crick.
Scientists used the understanding of how DNA replicates to explain how heritable traits are carried and passed down from generation to generation. In "On the Replication of Desoxyribonucleic Acid (DNA)," Delbrück initiated a discussion aimed at explaining the intricacies of DNA replication and experimentally supporting a particular theory about how DNA stores and carries genetic information. Through that debate, scientists learned how DNA replicates to preserve and pass along the information contained within it.
14.3 Basics of DNA Replication
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.|
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 14.12. 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 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.
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 ( 15 N) that gets incorporated into nitrogenous bases, and eventually into the DNA (Figure 14.13).
The E. coli culture was then shifted into medium containing 14 N 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 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 15 N will band at a higher density position than that grown in 14 N. Meselson and Stahl noted that after one generation of growth in 14 N after they had been shifted from 15 N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15 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 14 N formed two bands: one DNA band was at the intermediate position between 15 N and 14 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.