DNA Replication : DNA Replication The Process of making copies of DNA molecules to pass on to new cells is called DNA replication
Purpose of replication : Purpose of replication Replication is to make a copy of entire complement of DNA within a cell.
It takes place at the chromosome level during interphase of cell cycle.
Duplication of DNA : Duplication of DNA
Steps of replication : Steps of replication The replication of DNA begins at the sequence of nucleotides called the origin of replication. it begins with helicase unwinds a segments of DNA and breaks the hydrogen bonds between the to complementary strands.
Slide 5 : Origins initiate replication at different times .
Formation of DNA replication fork : Formation of DNA replication fork
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Experiments to show the mode of conservation of DNA : 1. Conservative replication would somehow produce an entirely new DNA strand during replication. Experiments to show the mode of conservation of DNA
Slide 9 : Semiconservative replication would produce two DNA molecules,
each of which was composed of one-half of the parental DNA along with an entirely new complementary strand. In other words the new DNA would consist of one new and one old strand of DNA. The existing strands would serve as complementary templates for the new strand.
Slide 10 : Dispersive replication involved the breaking of the parental strands during replication, and somehow, a reassembly of molecules that were a mix of old and new fragments on each strand of DNA.
Slide 11 : Experiments have shown that the two DNA strands in a double helix survive replication intact , but are seperated in to different daughter helics. As a result replication is called a semi conservative process.
Meselson-Stahl experiment : The Meselson-Stahl experiment involved the growth of E. coli bacteria on a growth medium containing heavy nitrogen (Nitrogen-15 as opposed to the more common, but lighter molecular weight isotope, Nitrogen-14). The first generation of bacteria was grown on a medium where the sole source of N was Nitrogen-15. The bacteria were then transferred to a medium with light (Nitrogen-14) medium. Watson and Crick had predicted that DNA replication was semi-conservative. If it was, then the DNA produced by bacteria grown on light medium would be intermediate between heavy and light. It was. DNA replication involves a great many building blocks, enzymes and a great deal of ATP energy (remember that after the S phase of the cell cycle cells have a G phase to regenerate energy for cell division). Only occurring in a cell once per (cell) generation, DNA replication in humans occurs at a rate of 50 nucleotides per second, 500/second in prokaryotes. Nucleotides have to be assembled and available in the nucleus, along with energy to make bonds between nucleotides. Meselson-Stahl experiment
Meselson and Stahl (1958) : Meselson and Stahl (1958)
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Replication errors : Replication errors Missense mutations are a type of replication errors .In this example,the nucleotide adenine was replaced by cytosine in the genetic code introducing an incorrect amino acid in to the protein sequence.
Slide 16 : The consequences of replication error are quite variable,some causes such as cancer while others have no health effects.errors made or copied during the formation of sex cells are heritable.
Replication errors : Replication errors The enzyme DNA polymerase is able to detect when the wrong bases have been paired during replication ,if this occurs replication is stopped, while the mistake is corrected.
Nature of the Genetic Material : Nature of the Genetic Material Property 1 - it must contain, in a stable form, information encoding the organism’s structure, function, development and reproduction
Property 2 - it must replicate accurately so progeny cells have the same genetic makeup
Property 3 - it must be capable of some variation (mutation)
to permit evolution