Replication, semiconservative

Comparison of Parameters of DNA Replication in E. coli and Human Cells 

In both bacterial and plasmid DNA, replication is initiated at a unique origin and proceeds in both directions along the DNA molecule to a termination site that is located approximately 180 degrees from the origin. Thus, replication of DNA is, in most cases, bidirectional replication. The ability of DNA to replicate from many origins and in both directions means that cells can replicate all of their DNA in a fairly short period of time. The shortest generation time for the bacterium E. coli is approximately 30 minutes in rich medium in the laboratory whereas the shortest division time for a human cell is approximately 24 hours (Table 24-1). 

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. 

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Semiconservative replication (Section 28.3) The process by which DNA molecules are made containing one strand of old DNA and one strand of new DNA. 

Implicit in the functioning of the Watson-Crick DNA model is the idea that the strands of a DNA molecule must separate and new daughter strands must be synthesized in response to the sequence of bases in the mother strand. This is called semiconservative replication. Still, conservative replication, in which both strands of a daughter molecule are newly synthesized, could not be ruled out by consideration of the structure of DNA alone. 

Genetic information is transmitted from parent to progeny by replication of parental DNA, a process in which two daughter DNA molecules are produced that are each identical to the parental DNA molecule. During DNA replication, the two complementary strands of parental DNA are pulled apart. Each of these parental strands is then used as a template for the synthesis of a new complementary strand (semiconservative replication). During cell division, each daughter cell receives one of the two identical DNA molecules. 

The two major types of nucleic acids are DNA and RNA. Nucleic acids are polyphosphate esters containing the phosphate, sugar, and base moieties. Nucleic acids contain one of five purine or pyrimidine bases that are coupled within double-stranded helices. DNA, which is an essential part of the cell s chromosome, contains the information for the synthesis of protein molecules. For double-stranded nucleic acids, as the two strands separate, they act as a template for the construction of a complementary chain. The reproduction or duplication of the DNA chains is called replication. The DNA undergoes semiconservative replication where each of the two new strands contains one of the original strands. 

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. 

DNA Replication Is Semiconservative Each DNA strand serves as a template for the synthesis of a new strand, producing two new DNA molecules, each with one new strand and one old strand. This is semiconservative replication. 

Watson and Crick proposed the hypothesis of semiconservative replication soon after publication of their 1953 paper on the structure of DNA, and the hypothesis was proved by ingeniously designed experiments carried out by Matthew Meselson and Franklin Stahl in... 

Terms in bold are defined template 950 semiconservative replication 950 replication fork 951 origin 952 Okazaki fragments 952 leading strand 952 lagging strand 952 nucleases 952 exonuclease 952 endonuclease 952 DNA polymerase I 952 primer 954 primer terminus 954... 

The Meselson-Stahl experiment (see Fig. 25-2) proved that DNA undergoes semiconservative replication ini , coli. In the dispersive model of DNA replication, the parent DNA strands are cleaved into pieces of random size, then joined with pieces of newly replicated DNA to yield daughter duplexes. In the Meselson-Stahl experiment, each strand would contain random segments of heavy and light DNA. Explain how the results of Meselson and Stahl s experiment ruled out such a model. 

When the two strands of the DNA double helix are separated, each can serve as a template for the replication of a new complementary strand. This produces two daughter molecules, each of which contains two DISA strands with an antiparallel orientation (see Figure 29.3). This process is called semiconservative replication because, although the parental duplex is separated into two halves (and, therefore, is not "conserved" as an entity), each of the individual parental strands remains intact in one of the two new duplexes (Figure 29.8). The enzymes involved in the DlsA replication process are template-directed polymerases that can synthesize the complementary sequence of each strand with extraordinary fidelity. The reactions described in this section were first known fiom... 

That the DNA content doubles prior to cell division was established by microspectrophotometry. It was clear that both daughter cells must receive one or more identical molecules of DNA. However, it was not known whether the original double-stranded DNA molecule was copied in such a way that an entirely new double-stranded DNA was formed or whether, as we now know to be the case, the two chains of the original molecule separated. The latter is called semiconservative replication, each of the separated strands having a new complementary strand synthesized along it to form the two identical double-stranded molecules. 

The Universality of Semiconservative Replication Overview of DNA Replication in Bacteria... 

Initiation of a second round of replication leads to a replication eye at the initiation site of replication (fig. 26.3). As synthesis proceeds the size of the replication eye becomes larger at this stage the replicating chromosome is referred to as a theta structure because it has the appearance of the Greek letter d. Semiconservative replication is consistent with the density of the autoradiographic tracks made by different parts of the chromosome after one and two rounds of replication in [3H]thymidine (see fig. 26.3). 

Simulated autoradiographs of the E. coli chromosome after one or more replications in the presence of [3H]thymidine. After one round of replication, the autoradiograph shows a circular structure that is uniformly labeled. The second round of replication begins with the formation of a replication eye. One branch in the replication eye is twice as strongly labeled as the remainder of the chromosome, indicating that this branch contains two labeled strands. This structure is consistent with semiconservative replication for the E. coli chromosome. 

Semiconservative replication. Duplication of DNA in which the daughter duplex carries one old strand and one new strand. 

UNSCHEDULED DNA SYNTHESIS (UDS) DNA synthesis that occurs at a stage in the cell cycle other than S incorporation of precursors (e.g., tritiated thymidine) into ENA in the absence of semiconservative replication a manifestation of genetic repair, whose occurrence has been used as an indicator of induced DNA damage. 

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. 

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. 

The double-helical model of DNA and the presence of specific base pairs immediately suggested how the genetic material might replicate. The sequence of bases of one strand of the double helix precisely determines the sequence of the other strand a guanine base on one strand is always paired with a cytosine base on the other strand, and so on. Thus, separation of a double helix into its two component chains would yield two single-stranded templates onto which new double helices could be constructed, each of which would have the same sequence of bases as the parent double helix. Consequently, as DNA is replicated, one of the chains of each daughter DNA molecule would be newly synthesized, whereas the other would be passed unchanged from the parent DNA molecule. This distribution of parental atoms is achieved by semiconservative replication.. 

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.]...

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