partWatson and Crick observed that the two strands of DNA are anti-parallel and complementary to each other with respect to their base sequences. This type of arrangement in DNA molecule led to the hypothesis that DNA replication is semi-conservative. It means that the double stranded DNA molecule separates and then, each of the separated strand acts as a template for the synthesis of a new complementary strand. As a result, each daughter DNA molecule would have one parental strand and a newly synthesized daughter strand.
Since only one parental strand is conserved in each daughter molecule, it is known as semi-conservative mode of replicatio
Semiconservative replication would produce two copies that each contained one of the original strands and one new strand.Conservative replication would leave the two original template DNA strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.Dispersive replication would produce two copies of the DNA, both containing distinct regions of DNA composed of either both original strands or both new strands.
The deciphering of the structure of DNA by Watson and Crick in 1953 suggested that each strand of the double helix would serve as a template for synthesis of a new stran Explanation
Watson and Crick observed that the two strands of DNA are anti-parallel and complementary to each other with respect to their base sequences. This type of arrangement in DNA molecule led to the hypothesis that DNA replication is semi-conservative. It means that the double stranded DNA molecule separates and then, each of the separated strand acts as a template for the synthesis of a new complementary strand. As a result, each daughter DNA molecule would have one parental strand and a newly synthesized daughter strand.
Since only one parental strand is conserved in each daughter molecule, it is known as semi-conservative mode of replication.
Semiconservative replication would produce two copies that each contained one of the original strands and one new strand. Conservative replication would leave the two original template DNA strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.
it is relating to or being genetic replication in which a double stranded molecule of nucleic acid separates into two single strand each of which serves as a template for the formation of a complementary strand that together with the template forms a complete molecule.
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Semiconservative replication describes the mechanism of DNA replication in all known cells. It derives its name from the fact that it produces two copies of the original DNA molecule, each of which contains one of original strand, and one newly-synthesized strand.
The Meselson and Stahl Experiment Described
Meselson and Stahl tested the hypothesis of DNA replication. They cultured bacteria in a 15N medium. 15N is a heavy isotope of nitrogen so the DNA synthesized is of heavy density. They then shifted the bacteria to a 14N medium, DNA was isolated at different times corresponding to replication cycles 0, 1, and 2. After one replication cycle, the DNA was all of intermediate density. This rules out the conservative replication model, which predicts that both heavy density DNA and light density DNA will be present, but none of intermediate density will be present. This result is consistent with the semiconservative replication model, which predicts that all DNA molecules will consist of one 15N-labeled DNA strand and one 14N-labeled DNA strand. The result does not rule out the dispersive replication model, which also predicts that all DNA will be of intermediate density, consisting of interspersed double-stranded 15N-labeled and 14N-labeled segments.
After two replication cycles, two bands of DNA were seen, one of intermediate density and one of light density. This result is exactly what the semiconservative model predicts: half should be 15N-14N intermediate density DNA and half should be 14N-14N light density DNA. This result rules out the dispersive replication model, which predicts that after replication cycle 1, the DNA density of all DNA molecules will gradually become lower, so no intermediate density DNA should remain at replication cycle 2. The semiconservative model is correct.
The semiconservative mode of DNA replication was suggested (1953) by Watson and Crick along with the double-helix model of DNA. Here the replication of DNA involves the progressive separation of the two strands of DNA molecules by breaking up of hydrogen bonds between base pairs.
Each strand, acting as template, synthesizes their complementary new strand on itself taking raw materials from the nuclear sap. Thus two daughter DNA helices are formed. Each daughter DNA helix has one old or parental and one new strand.
It indicates that in each daughter DNA helix, one parental strand is retained and conserved while its complementary strand is new. Hence, according to this mode of DNA replication, the parental DNA is partially conserved in each new daughter DNA molecule. So this mode of DNA replication is called semiconservative
Mechanism of DNA Replication:
DNA replication is a multistep complex process. It is catalysed by the multi-enzyme complex, often called the replication apparatus, or the replisome, and needs the involvement of several other proteins. DNA replication always begins at certain unique and fixed points of DNA called origin.
Each prokaryotic chromosome has a single origin but every eukaryotic chromosome has several origins (e.g., the giant salivary chromosome of Drosophila contains 7,000 origins), phage T2 has one primary and one secondary origin. In presence of primary origin, the secondary origin remains nonfunctional. But when the primary origin is deleted, then the secondary origin takes over as the functional origin.
Studies with various organisms show that the replication of DNA molecules in both prokaryotes and eukaryotes is bidirectional. However, bidirectional replication is not universal. The chromosome of coli-phage P2which, like the lambda chromosome, is circular during replication, replicates unidirectional from a unique origin.
The double-stranded DNA does not replicate. Hence, before replication, two strands of DNA must be separated gradually from the point of replication origin in the form of replication eye or Y-shaped replication fork.
During separation, DNA synthesis also occurs usually bi-directionally or rarely unidirectional. DNA is replicated by enzymes known as DNA directed DNA polymerase or simply DNA polymerases. This enzyme utilizes free single- stranded DNA as a template on which the synthesis of daughter complementary strand takes place.
To fulfil the requirement of DNA polymerase activity, two enzymes and DNA helicase are required. DNA gyrase reduces the linking of DNA strands and DNA helicase first binds to the origin points and induces the unwinding of complementary strands of DNA double helix to make it single-stranded.
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the correct answer is (b) apical meristems are sometimes changed into floral meristems.