[an error occurred while processing this directive]
|Chapter Six: DNA, RNA, and Protein Synthesis|
Scientists knew that DNA had to replicate (copy) itself prior to cell division so that each daughter cell would obtain all of the genetic information. Based on the Watson-Crick model of DNA, three methods of DNA replication were suggested. They are called conservative, semiconservative, and dispersive replication.
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.
A second hypothesis is known as semiconservative replication, whereby the bonds between the bases are broken and the DNA molecule "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 least likely candidate for DNA replication is dispersive replication. In this hypothesis, 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.
Through a series of experiments in 1958, two scientists, Matthew Meselson and Franklin Stahl, gathered evidence which disproved conservative and dispersive replication as possibilities for the method of DNA replication. Thus they provided strong evidence in support of semiconservative replication, which has since been accepted as the true method.
Of course, DNA replication is a bit more complex than a mere unzipping of the molecule and then copying each strand. A number of enzymes are involved in the process, and there are a few intricacies which one might not have originally expected. We will discuss the process in general first, and then we will discuss the difference between replication in prokaryotes and eukaryotes.
To begin the process of replication, an enzyme called DNA helicase moves down the DNA molecule and separates the two strands by breaking the bonds between the nitrogenous bases. This process begins at a point called the replication origin. Soon after the DNA helicase passes over a part of the DNA, an enzyme known as DNA polymerase bonds the appropriate bases with the now exposed bases on the single strand.
A complication arises in that the DNA polymerase can only work moving from the 5' end of the new DNA molecule to the new 3' end. As the DNA helicase separates the two strands of DNA, one strand ends with a 5' end and the other with a 3' end. For the one which ends with a 5' end, the DNA polymerase does not encounter a difficulty. The complementary strand which it constructs will end with a 3' end, and since DNA polymerase works from 5' to 3' on the new strand, it can follow right behind the DNA helicase.
On the other side, however, the old strand ends with a 3', meaning that the new strand will end with a 5'. In this case, the DNA polymerase actually has to work backwards, periodically beginning at the point where the DNA helicase is (which will be a 5' end on the new strand) and moving away from it until it reaches a new piece of DNA already formed. In this manner, the DNA polymerase creates small fragments of DNA called Okazaki fragments (named after Reiji Okazaki, who discovered them), which are then joined together by another enzyme called DNA ligase.
Prokaryotic DNA is arranged in a circular shape, and there is only one replication origin when replication starts. By contrast, eukaryotic DNA is linear; it does not connect end to end to form a circle. When it is replicated, there are as many as 1000 replication origins. Despite these differences, however, the underlying process of replication is the same for both prokaryotic and eukaryotic DNA.