DNA replication must occur before any type of cell division, whether the cell is prokaryotic or eukaryotic. DNA replication assures that any daughter cells produced will have a complete copy of the DNA necessary for the cell to survive.
As organisms have from thousands to millions or even billions of base-pairs of DNA, this process could seem overwhelmingly complicated. However, cells utilize a relatively simple mechanism to copy their DNA rapidly and accurately. Several different enzymes are involved in this process - some unwind the DNA from its double helix, some separate the two strands of DNA, and some build new strands of DNA complementary to each of the original strands. After the DNA is replicated, cells employ several other enzymes to ‘proofread’ their work and correct mistakes from the replication process. The end result of DNA replication is two complete and accurate copies of a cell’s DNA.
The hypothesis of DNA replication
DNA was proven as the hereditary material and Watson et al. had deciphered its structure. What remained was to determine how DNA copied its information and how that was expressed in the phenotype. Based on the Watson-Crick model of DNA, three methods of DNA replication were suggested. They are called conservative, semiconservative, and dispersive replication.
1. In conservative replication the double helix remains completely intact during the replication process, and an entirely new double helix is formed without destroying the original copy.
Conservative model of DNA replication.
2. In semiconservative replication the bonds between the bases
are broken and the DNAmolecule "unzips" into two strands. Alongside each of the two strands forms a new strand with the appropriate base pairs. Thus the final copies are half original DNA and half new DNA, in contrast to conservative replication in which the copies are either completely original or completely new.
The semiconservative model of DNA structure.
3. Dispersive replication the DNA molecule is broken up into many small segments. Alongside each segment forms an appropriate complementary segment, and then all of the segments are joined back together into two molecules of DNA. The final product is two strands of DNA with small pieces from the original DNA and small pieces from the new DNA.
The dispersive replication model of DNA replication.
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 process
Promoter proteins are produced and bind to DNA at several sites.
Helicases attach to promoter proteins and break the hydrogen bonds linking the bases together to open the helix up.
RNA polymerase reads the exposed nucleotides and produces an RNA primer (approximately 10 nucleotides in length).
DNA polymerase replicates the DNA (base-pairing) forming the new strand in the 5' to 3' direction.
Because DNA polymerase only reads in the 3' to 5' direction, and forms the new strand in the opposite direction, there is a directional problem. Therefore one molecule of DNA produces a continuous leading strand in one direction. On the lagging strand, new primers have to form at many sites and the DNA is broken up into many small fragments called Okazaki fragments.
The process of DNA replication occurs at many sites, called replication bubbles, along the entire DNA strand.
When DNA polymerase reaches the 5' end of the RNA primer, it is released and other enzymes remove the RNA primers and replace them with the proper nucleotides.
DNA ligase joins together all of the large leading fragments and the many small Okazaki fragments.
DNA polymerase also checks and corrects any mistakes in base pairing.
Topoisomerases prevent kinks as the parent DNA is unzipped.
Growth of replication forks as DNA is replicated base by base.
DNA replication must occur before any type of cell division, whether the cell is prokaryotic or eukaryotic. DNA replication assures that any daughter cells produced will have a complete copy of the DNA necessary for the cell to survive.
