This means that approximately 1000 nucleotides are added per second. coli has 4.6 million base pairs (Mbp) in a single circular chromosome and all of it is replicated in approximately 42 minutes, starting from a single origin of replication and proceeding around the circle bidirectionally (i.e., in both directions). What would have been the conclusion of Meselson and Stahl’s experiment if, after the first generation, they had found two bands of DNA?ĭNA replication has been well studied in bacteria primarily because of the small size of the genome and the mutants that are available.The resulting DNA molecules have the same sequence and are divided equally into the two daughter cells. The new strand will be complementary to the parental or “old” strand. As a result of this experiment, we now know that during DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. Therefore, the other two models were ruled out. If DNA replication was dispersive, a single purple band positioned closer to the red 14 14 would have been observed, as more 14 was added in a dispersive manner to replace 15. These results could only be explained if DNA replicates in a semiconservative manner. The DNA harvested from cells grown for two generations in 14N formed two bands: one DNA band was at the intermediate position between 15N and 14N, and the other corresponded to the band of 14N DNA. Some cells were allowed to grow for one more generation in 14N and spun again. This suggested either a semiconservative or dispersive mode of replication. Meselson and Stahl noted that after one generation of growth in 14N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15N or 14N. DNA grown in 15N would be expected to form a band at a higher density position than that grown in 14N.
The DNA was separated by ultracentrifugation, during which the DNA formed bands according to its density. The cells were harvested and the DNA was isolated. coli culture was then shifted into a medium containing 14N and allowed to grow for one generation. coli for several generations in a medium containing a “heavy” isotope of nitrogen ( 15N) that was incorporated into nitrogenous bases and, eventually, into the DNA. Matthew Meselson (1930–) and Franklin Stahl (1929–) devised an experiment in 1958 to test which of these models correctly represents DNA replication ( Figure 11.5). In the dispersive model, all resulting DNA strands have regions of double-stranded parental DNA and regions of double-stranded daughter DNA. In the semiconservative model, parental strands separated and directed the synthesis of a daughter strand, with each resulting DNA molecule being a hybrid of a parental strand and a daughter strand. In the conservative model, parental DNA strands (blue) remained associated in one DNA molecule while new daughter strands (red) remained associated in newly formed DNA molecules. There were two competing models also suggested: conservative and dispersive, which are shown in Figure 11.4.įigure 11.4 There were three models suggested for DNA replication. In one model, semiconservative replication, the two strands of the double helix separate during DNA replication, and each strand serves as a template from which the new complementary strand is copied after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand. Separating the strands of the double helix would provide two templates for the synthesis of new complementary strands, but exactly how new DNA molecules were constructed was still unclear. The elucidation of the structure of the double helix by James Watson and Francis Crick in 1953 provided a hint as to how DNA is copied during the process of replication. Explain the process of rolling circle replication.Identify the differences between DNA replication in bacteria and eukaryotes.Describe the process of DNA replication and the functions of the enzymes involved.Explain why Okazaki fragments are formed.
Explain why DNA replication is bidirectional and includes both a leading and lagging strand.
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