Semi-conservative DNA

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. 

Alex Rosenberg Why isn t there a transfer of material in photocopying If we think about the process of photocopying, it doesn t look as though the, say, semi-conservative character of DNA copying, is a physically different process in kind for the electrostatic planting involved in say, Xerox machines. 

DNA Replication by a Semi-Conservative, Bidirectional Mechanism Figure 1-2-1... 

Althongh this description snmmarises, albeit briefly, the processes involved in DNA dnplication, it conceals a considerable amonnt of biochemistry. Nonetheless, the resnlt is two identical donble-stranded DNA molecnles. This means of replication is known as semi-conservative since, in each new DNA dnplex, one strand is identical to that from the parent molecnle, i.e. it is conserved. 

Telomeres are seqnences of six-nucleotide repeats found at the ends of the chromosomal DNA strands. Many thon-sands of repeat nnits (TTAGGG) may be present at the end of the 3 strand and (AATCCC) at the end of the 5 strand. These are present at the ends of the strands to overcome a problem posed by the semi-conservative mechanism of DNA replication, known as the end replication problem . Replication of the ends of the chromosomes presents par-ticnlar difficnlties, since DNA polymerase can only elon-... 

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.

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. 

Most of the genetic information of bacteria is contained in a single structure of fixed DNA content, a giant circular DNA molecule that replicates semi-conservatively. The enzymatic reactions involved in the biologically fundamental processes of DNA biosynthesis and genetic recombination are being elucidated in studies with bacterial systems. 

The Watson and Crick hypothesis for DNA replication proposed that each strand of DNA is used as a template for the production of one of the daughter DNA molecules. Thus the result of replication would he that one strand of DNA is present in each daughter molecule of DNA. This is a semi-conservative mechanism of replication. A simplified diagram of replication is shown in Fig. S.AIO however, the replication patterns are different in bacteria and in eukaryotes. 

DNA replication occurs only in the S phase. It occurs at many chromosomal origins, is bi-directional and semi-conservative. Sets of 20-80 replicons act as replication units that are activated in sequence. 

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... 

DNA synthesis is catalysed by DNA polymerases and requires the precursor dNTPs (dATP, dGTP, dCTP and dTTP, each of these existing as Mg2+ complexes), a template (i.e. the dsDNA being copied) and a primer (an initial deoxyribose 3 -OH to enable the reaction to insert the first new nucleotide). The reaction proceeds in a 5 to 3 direction, that is, at the end of the synthesis there is a vacant deoxyribose 3 -OH. The fidelity of the replication process is based on the incoming nucleotides base pairing with the correct base on the antiparallel template. DNA synthesis is semi-conservative (i.e. the newly synthesized strand partners its antiparallel complementary strand) and is bidirectional (because both original strands are replicated). 

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.]... 

Each strand in a parental duplex DNA acts as a template for synthesis of a daughter strand and remains base-paired to the new strand, forming a daughter duplex (semi-conservative mechanism). New strands are formed in the 5 3 direction. 

Matthew Meselson and Fanklin Stahl demonstrated in 1958 that DNA replication (see here) occurred by a semi-conservative mechanism (see here) in which a parental duplex yields two daughter duplexes. Each daughter duplex contains one strand from the original parental duplex and one newly synthesized strand, made by copying the parental strand. 

Semi-conservative-From one parental double-stranded DNA, two daughter double-stranded DNAs are made. Each daughter DNA contain one parental DNA strand and one newly made strand. (Figure 4.12)... 

These observations are all consistent with the semi-conservative model of DNA replication. 

This model/ called the "semi-conservative" model for DNA replication, is the most widely accepted model for DNA replication all data at this time seems to support this model. 

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. 

Moustacchi, E. and DiatlofF-Zito, C., DNA semi-conservative synthesis in normal and Fanconi anemia fibroblasts following treatment with 8-methoxypsoralen and near ultraviolet light or with X-rays, Genet., 70,236,1985. 

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. 

DNA replication in eukaryotic cells is fundamentally the same as that in bacteria. Replication is semi-conservative, occurs in a 5 3 direction, and requires a template, a primer and deoxy-... 

Brown, N. C. (1971) Inhibition of bacterial DNA replication by 6-(/ hydroxyphenylazo)-aracil differential effect on repair and semi-conservative synthesis in B. subtilis. J. Moiec. Biol. 59,1-16. 

The genetic material is transmitted after the duplication of the chromosomal DNA and the potentially existing plasmids. DNA replication, according to the semi-conservative mechanism, leads to the formation of two molecules that are identical to the parental chromosome or plasmid. The replication occurs almost during the entire cellular cycle at the mesosomes. When it is finished, the scission of the cytoplasm begins. 

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