Replication conservative

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. 

Figure 11-2. The prokaryotic DNA replication fork. A schematic representation of semi-conservative replication of DNA by different mechanisms on the leading and lagging strands by DNA polymerase III (DNA pol III) is shown. Other enzymes and accessory proteins that participate in initiation, elongation, and ligation phases of the process are indicated, with DNA pol I depicted as having just dissociated from a completed Okasaki fragment. SSBs, single-stranded DNA binding proteins.

This result argued against conservative replication, an alternative hypothesis in which one progeny DNA... 

Are there any other possible uses for the construction of complex topological species One possible application is in the mass production of DNA polyhedral catenanes by biological means, such as the polymerase chain reaction (PCR) (Saiki et al. 1986) or by production in vivo. Figure 21 illustrates that semi-conservative replication (the mechanism used by DNA polymerases) cannot reproduce a stable branch. The DNA with different sequences in the two arms of the branch (cartooned as dashed and solid lines) leads to two heterologous duplex DNA molecules, rather than a second branched molecule. 

Multiple replicons In eukaryotes, replication of chromosomal DNA occurs only in the S phase of the cell cycle. As for bacterial DNA (see Topic F3), eukaryotic DNA is replicated semi-conservatively. Replication of each linear DNA molecule in a chromosome starts at many origins, one every 3-300 kb of DNA depending on the species and tissue, and proceeds bi-directionally from each origin. The use of multiple... 

An unseen pattern. What result would Meselson and Stahl have obtained if the replication of DNA were conservative (i.e., the parental double helix stayed together) Give the expected distribution of DNA molecules after 1.0 and 2.0 generations for conservative replication. 

In conservative replication, after 1.0 generation, half of the molecules would be the other half 4n-14n. 

Meselson and Stahl Confirmed the Watson-Ciick model of semi conservative replication of DNA. 

Figure 4.15 Detection of semi conservative replication of f. call DNA by density-gradient centrifugation. The position of a band of DNA depends on its content of and N, After 1.0 generation, all of the DNA molecules were hybrids containing equal amounts of and N. [From M. Meselson and R W. Stahl. Proc. Noti Acad Sci. U. S. A. 44(195S) 671-682.]... 

One historically important application of density gradient methods is the classic Meselson and Stahl (1958) experiment in which DNA strands of different densities (incorporating natural N or enriched with N isotopes) were used to demonstrate the semi-conservative replication of the DNA double helix that is central to molecular biology. 

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. 

The experiments done in Kornberg s laboratory illustrate the complexity of the reactions involved in extending the DNA chain, but there are many other aspects to semi-conservative replication of DNA, among them the unwinding of the intertwined strands, the initiation of DNA synthesis, the formation of deox-yribonucleotide intermediates, and the connection of the intermediates. We can only consider these various aspects of DNA synthesis briefly. For further details, the reader is referred to the book of Wells and Inman [235],... 

Before semi-conservative replication of DNA can take place, the two intertwined parental DNA strands must be untwisted. The mechanism of this unwinding is not clear. Two have been proposed (1) the formation of a single-strand break which would allow a portion of the strand to unwind the unwound strand could then be rejoined by a ligase after semi-conservative replication, or (2) the existence of a specific enzyme which would temporarily unwind the DNA has been suggested. A protein capable of unwinding DNA in vitro has been isolated from E. coli [238]. 

Because a mutant of E. coli referred to as pol A , defective in polymerase I, replicates DNA normally, it has been suspected that at least in some cases DNA polymerase I might not be the enzyme responsible for DNA semi-conservative replication. This observation led to a search for other DNA polymerases. In E. coli, two other DNA polymerases were discovered polymerase II and III. 

The Increase in DNA synthesis is not due to stimulatory influences on (semi-conservative) DNA replication but represents DNA repair. This was verified by keeping the cells under conditions that prevent normal (semi-conservative) replication but permit repair ("unscheduled DNA synthesis"). Two major routes are suggested by which NO-Fru-Trp could impart DNA damage and, thus, assume mutagenic properties. 

Conservative Replication— two original template DNA strands together in a double helix, producing a copy composed of two new strands containing all of the new DNA base pairs. The mechanism of replication was one of three models originally proposed for DNA replication. One of the three original models proposed for DNA replication that has now been withdrawn. 

Semi-conservative Replication— the mechanism by which DNA is replicated in all known cells, producing two copies that each contain one of the original strands and one new strand... 

The process by which new DNA molecules are S5mthesized is called DNA replication. In this process, each DNA strand serves as a template for the synthesis of a new strand, producing two new DNA molecules, each containing a parent strand and a daughter strand (Figure 2.7). This is called semi-conservative replication. If replication were conservative, then the parent DNA strand would remain intact in the double helix form at the end of the process. This was one of the original hypotheses for DNA replication, but it has been proven that the semi-conservative process is what actually occurs. 

The two double helices formed are identical. Each double helix contains one strand from the old DNA and one newly synthesised strand. This is called semi-conservative replication (Figure 28.19). 

Elucidation of the double-helical structure of DNA lends itself to an elegant but simple mechanism of perpetuation of the DNA information during duplication, called semi-conservative replication. In this model (Fig. 2), the two strands of DNA separate, and each then acts as the template for synthesis of a new daughter strand based on base pair complementarity and strand polarity. Thus, the two strands of the DNA double helix, though not identical in sequence, are equivalent in information content. 

Although excision repair requires DNA synthesis, it is distinct from normal semi-conservative replication because it occurs throughout the cell cycle and may utilize nonreplicative DNA polymerases in both prokaryotes and eukaryotes. Pol II and Pol I in E.coli and DNA polymerase... 

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