Semiconservative mechanism

The DNA region to be replicated is copied by what is referred to as a semiconservative mechanism. 

Replication Begins at an Origin and Usually Proceeds Bidirectionally Following the confirmation of a semiconservative mechanism of replication, a host of questions arose. Are the parent DNA strands completely unwound before each is replicated Does replication begin at random places or at a unique point After initiation at any point in the DNA, does replication proceed in one direction or both ... 

Meselson and Stahl demonstrated that DNA is replicated by a semiconservative mechanism. 

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. 

Through experimentation it was determined that DNA replicates via a semiconservative mechanism. There are three possible mechanisms that can explain DNA s semiconservative replication. 

DNA is replicated by a semiconservative mechanism involving several enzymes. The leading strand is synthesized continuously in the 5 —> 3 direction. The lagging strand is synthesized in pieces in the 5 —> 3 direction the pieces are then covalently linked. 

DNA structure and function are so important for living organisms that they must possess efficient mechanisms for the rapid and accurate synthesis of DNA. DNA synthesis, referred to as replication, occurs by a semiconservative mechanism, that is, each of the two parental strands serves as a template to synthesize a new strand. 

This base-pairing template model theoretically could proceed either by a conservative or a semiconservative mechanism. In a conservative mechanism, the two daughter strands would form a new double-stranded (duplex) DNA molecule and the parental duplex would remain Intact. In a semiconservative mechanism, the parental strands are permanently separated and each forms a duplex molecule with the daughter strand base-paired to It. Definitive evidence that duplex DNA is replicated by a semiconservative mechanism came from a now classic experiment conducted by M. Meselson and W. F. Stahl, outlined In Figure 4-32. 

In the replication of natural DNA with DNA-polymerase I (Kornberg enzyme), the natural DNA is used as the template for the polyreaction of all four nucleoside-5-triphosphates (d-ATP, d-TTP, d-GTP, and d-CTP). The original double strand separates into two single strands which then act as templates for the new strands (Figure 29-4). After replication, each double strand consists of one old and one new strand. For this reason, the mechanism is also known as the semiconservative mechanism. 

During DNA replication, the two strands presumably unwind, and each strand serves as a template for the biosynthesis of a complementary polynucleotide chain. Such a semiconservative mechanism of replication finds support in two types of experimental evidence studies involving the separation of the two DNA strands during replication, and the mechanism of DNA synthesis in vitro. 

Earlier, we did a thonght experiment abont one way that DNA replication could take place. In this mechanism, termed semiconservative, each new strand of DNAis paired with one of the old strands that is, the two strands of the original DNA molecule are now divided between the two molecnles, each of which has one old strand and one new strand. However, there is another possibility, termed conservative. In this case, the two new strands are paired with each other and the two old strands remain paired with each other. Matthew Meselson and W. F. Stahl provided compelling evidence that DNA replication is semiconservative. Here is how the experiment was done. ... 

From the complementary duplex structure of DNA described in chapter 25, it is a short intuitive hop to a model for replication that satisfies the requirement for one round of DNA duplication for every cell division. In chapter 26, DNA Replication, Repair, and Recombination, key experiments demonstrating the semiconservative mode of replication in vivo are presented. This is followed by a detailed examination of the enzymology of replication, first for how it occurs in bacteria and then for how it occurs in animal cells. Also included in this chapter are select aspects of the metabolism of DNA repair and recombination. The novel process of DNA synthesis using RNA-directed DNA polymerases is also considered. First discovered as part of the mechanisms for the replication of nucleic acids in certain RNA viruses, this mode of DNA synthesis is now recognized as occurring in the cell for certain movable genetic segments and as the means whereby the ends of linear chromosomes in eukaryotes are synthesized. 

Mechanisms - The basic mechanisms of DNA replication are quite similar in eukaryotes and prokaryotes. DNA replication is semiconservative and is continuous on one strand and discontinuous on the other. As in prokaryotes, eukaryotic replication entails the assembly of short RNA primer molecules, elongation from the primers by a DNA polymerase, and (on the discontinuous strand) ligation of Okazaki fragments. A significant difference in eukaryotic and prokaryotic DNA replication is in the smaller size of the Okazaki fragments in eukaryotic cells - about 135 bases long, or about the size of the DNA on a nucleosome. 

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. 

There are three possible mechanisms that describe the fates of the DNA strands that are copied and where they will ultimately end up in the two daughter cells after mitosis. Conservative replication posits that one daughter cell will contain both parent strands and the other daughter cell will contain the two newly synthesized strands of DNA. Semiconservative replication posits that the DNA of both daughter cells contains one strand from the parent and one newly synthesized strand. The dispersive model posits that sections of parental and newly synthesized DNA are scattered throughout both strands of the daughter genomes. Experimental evidence shows that DNA replication is semiconservative. 

There are a number of different reasons why non-DNA linkers have been considered necessary to hold DNA molecules in tandem arrangement in chromosomes. The most relevant of these to the regulation of DNA synthesis are as follows the presumption that replication units of DNA within a sin e chromosome cannot be directly connected to one another without interruption of the DNA double helix the need for a mechanism to relieve torsion of the DNA molecule, torsion developed in connection with semiconservative replication. The latter proposal that non-DNA linkers mi t serve to allow rotation of the DNA double helix during semiconservative replication is no longer considered valid since it has been repeatedly demonstrated that a single-stranded break in a DNA double helix allows the intact chain to serve as a swivel and release torsion within the molecule (Vlnograd and Lebowitz, 1966). 

The second mechanism for increasing the total length of the DNA in the progenitor replicon would be by a specific breakage and reunion event between two DNA molecules. The DNA synthesizing apparatus of any replicon would by definition contain all the enzymes for a specific breakage and reunion-event. Since replication is semiconservative, there would be an endonuclease capable of cleaving a strand of DNA at the initiation site... 

The Chemistry of the DNA Molecule 97 The Watson-Crick Model of DNA 99 DNA Replication 100 Semiconservative Replication Enzymic Mechanism of Replication Problems of the Replication Model... 

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