Semiconservative Replication of DNA

DNA in the chromosomes of most organisms is double-helical-, i.e., it consists of two polydeoxynucleotide chains (or strands) twisted around one another in the form of a helix. The genetic information is contained in the sequence of nucleotides along one of the chains, with the sequence in one being complementary to that in the other (Chap. 7). A replica of DNA is one that is an exact copy of itself. 

Question What feature of the DNA structure provides the basis for reproduction of the original nucleotide sequence in a replica  

Each chain of double-helical DNA is bound to the other through complementary base pairs, with adenine (A) in one being hydrogen-bonded to thymine (T) in the other, and guanine (G) to cytosine (C). Watson and Crick proposed that, to achieve precise copying of a nucleotide (base) sequence, the two chains of the DNA must unwind from one another to allow each single chain to act as a template for the synthesis of a new one. Thus, the assembly of the sequence in the newly synthesized 

Question What is meant by the term semiconservative replication of DNA  

Semiconservative replication refers to the conservation of just one half of the parental DNA structure when it undergoes replication to give two daughter molecules. Thus, in Fig. 16-1, each chain of the parental DNA acts as a template and remains intact through the doubling process. 

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The semiconservative replication of DNA at the chromosomal level was shown by J. H. Taylor and coworkers. Using autoradiography and bean seedling root cells in tissue culture, they showed that, after a part of a cycle of duplication with [3H]thymidine (a selective label for DNA), the two chromosomes, descended from an original unlabeled chromosome, were both labeled. Following an additional duplication in the absence of labeled thymidine, the labeled chromosome yielded one labeled and one unlabeled descendant, as predicted by the semiconservative mechanism. 

Semiconservative replication of DNA. The two replicas consist of one parental strand plus one daughter strand. Each base in a daughter strand is selected by the requirement that it form a base pair with the parental base. 

The semiconservative replication of DNA means that a newly formed DNA molecule has one new strand and one strand from the original DNA. The experimental evidence for semiconservative replication comes from density-gradient centrifugation (Figure 10.3). If replication were a conservative process, the original DNA would have two heavy strands and all newly formed DNA would have light strands. 

Fig. 32. Scheme showing results of experiment by Meselson and Stahl (1958) demonstrating semiconservative replication of DNA (explanation in text). 

Replication of DNA is an enzymatic process that starts with the partial unwinding of the double helix. Just before the cell division, the double strand begins to unwind. As the strands separate and bases are exposed, new nucleotides line up on each strand in a complementary fashion, A to T, and C to G. Two new strands now begin to grow, which are complementary to their old template strands. Two new identical DNA double helices are produced in this way, and these two new molecules can then be passed on, one to each daughter cell. As each of the new DNA molecules contains one strand of old DNA, and one new, the process is called semiconservative replication. 

Replication of DNA occurs with very high fidelity and at a designated time in the cell cycle. Replication is semiconservative, each strand acting as template for a new daughter strand. It is carried out in three identifiable phases initiation, elongation, and termination. The reaction starts at the origin and usually proceeds bidirectionally. 

Subsequent studies initiated by others have shown that the DNA in all chromosomes regardless of their size replicate in this semiconservative fashion. Thus, the semiconservative mode of DNA replication is well established and essentially universal. The genetic information implanted in... 

Figure 5.15. Detection of Semiconservative Replication of E. coli DNA by density-gradient centrifugation The...

Figure 5.16. Diagram of Semiconservative Replication. Parental DNA is shown in blue and newly synthesized DNA in red. [After M. Meselson and F. W. Stahl. Proc. Natl. Acad. Sci. U.S.A. 44(1958) 671.]...

In the replication of DNA, the two strands of a double helix unwind and separate as new chains are synthesized. Each parent strand acts as a template for the formation of a new complementary strand. Thus, the replication of DNA is semiconservative—each daughter molecule receives one strand from the parent DNA molecule. The replication of DNA is a complex process carried out by many proteins, including several DNA polymerases. The activated precursors in the... 

Explain how the Meselson-Stahl experiment supports the semiconservative model of DNA replication. 

Semiconservative Nature of DNA Replication Sequential Nature of Replication (Figure 24.9)... 

FIGURE 10.3 The experimental evidence for semiconservative replication. Heavy DNA labeled with forms a band at the bottom of the tube, and light DNA with N forms a band at the top. DNA that forms a band at an intermediate position has one heavy strand and one light strand. 

When a molecule of DNA is replicated, each of the two strands is used as a template to create a complementary strand. When a cell divides into two, each of the two cells has one of the original template strands and one of the new strands. This process is called semiconservative replication. When DNA molecules are rephcated, the strands are separated at origins of replications. Synthesis occurs in both directions from the origin along replication forks. 

Recall Why is the replication of DNA referred to as a semiconservative process What is the experimental evidence for the semiconservative nature of the process What experimental results would you expect if replication of DNA were a conservative process ... 

Reflect and Apply In the Meselson-Stahl experiment that established the semiconservative nature of DNA replication, the extraction method produced short fragments of DNA. What sort of results might have been obtained with longer pieces of DNA ... 

Fig. 21-8 Replication of DNA double helix, (a) DNA synthesis at a pair of replication forks with expansion of the replication bubble. DNA chains are not shown in helical form, (ft) Complementary chains are synthesized via base pairing. (After J. McMurry and M. E. Castellion, Fundamentals of General, Organic, and Biological Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 1992.) (c) Semiconservative replication. (After J. D. DeLeo, Fundamentals of Chemistry General, Organic, and Biological, Scott, Foresman, Glenview, IL, 1988.)...

The single-strand DNA penetrates the cell, where it has converted by enzymes to a duplex replicative form through Watson-Crick base pairing. The replicative form is then reproduced by a mechanism similar to that used for the semiconservative replication of the duplex chromosome of the host cell. Finally, after this stage of replication, the mechanism shifts to one in which the replicative form serves as a template to produce copies of the single-strand DNA found in the mature virus. 

You have a double-strand linear DNA molecule, the appropriate primers, all the enzymes required for DNA replication, four P-labeled deoxyribonucleoside triphosphates, Mg " ion, and the means to detect newly synthesized radioactive DNA. Why is this system not sufficient to distinguish between conservative and semiconservative replication of the DNA molecule ... 

DNA synthesis is a replication process involving the copying of the deoxyribonucleo-tide sequence of an original template that is not destroyed in the process. The semiconservative mode of DNA replication in viruses and bacteria is supported by a wide variety of experiments, and the direct evidence for semiconservative replication of eukaryote DNA has been summarized by Taylor (1963) and Prescott (1970). 

Interview between Pauling and then prospective graduate student Matthew Meselson, who during his subsequent tenure with Pauling did the Meselson-Stahl experiment, which proved the semiconservative nature of DNA replication. 

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