In prokaryotes, three main types of polymerases are known: DNA pol I, DNA pol II, and DNA pol III. When the bond between the phosphates is broken, the energy released is used to form the phosphodiester bond between the incoming nucleotide and the existing chain. Each of these is made up of the corresponding nucleotide with three phosphates attached. In addition to ATP, there are also TTP, CTP, and GTP. ATP structurally is an adenine nucleotide which has three phosphate groups attached breaking off the third phosphate releases energy. The addition of nucleotides requires energy this energy is obtained from the nucleotides that have three phosphates attached to them. The next important enzyme is DNA polymerase III, also known as DNA pol III, which adds nucleotides one by one to the growing DNA chain (Figure 2). Figure 1: DNA replication in prokaryotes, which have one circular chromosome. Single-strand binding proteins (Figure 2) coat the single strands of DNA near the replication fork to prevent the single-stranded DNA from winding back into a double helix. Two replication forks are formed at the origin of replication and these get extended bi-directionally as replication proceeds. As the DNA opens up, Y-shaped structures called replication forks are formed ( Figure 1). ATP hydrolysis is required for this process because it requires energy. An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs. This sequence of base pairs is recognized by certain proteins that bind to this site. The origin of replication is approximately 245 base pairs long and is rich in AT sequences. coli has a single origin of replication on its one chromosome, as do most prokaryotes ( Figure 1). How does the replication machinery know where to start? It turns out that there are specific nucleotide sequences called origins of replication where replication begins. DNA replication in prokaryotes has been extensively studied, so we will learn the basic process of prokaryotic DNA replication, then focus on the differences between prokaryotes and eukaryotes. While there are many similarities in the DNA replication process, these structural differences necessitate some differences in the DNA replication process in these two life forms. The eukaryotic chromosome is linear and highly coiled around proteins. The prokaryotic chromosome is a circular molecule with a less extensive coiling structure than eukaryotic chromosomes.
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