what go you mean by semiconservative mode of replication? Explain the mechanism of the same with suitable diagram ?
Answers
Answer:
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.
Answer:
Semiconservative replication describes the mechanism of DNA replication in all known cells. This process is known as semi-conservative replication because two copies of the original DNA molecule are produced. Each copy contains one original strand and one newly-synthesized strand.