![]() Topoisomerase breaks and reforms DNA’s phosphate backbone ahead of the replication fork, thereby relieving the pressure that results from this supercoiling. The DNA tends to become more highly coiled ahead of the replication fork. DNA polymerase can now extend this RNA primer, adding nucleotides one by one that are complementary to the template strand ( ).Ī replication fork is formed when helicase separates the DNA strands at the origin of replication. Because this sequence primes the DNA synthesis, it is appropriately called the primer. Another enzyme, RNA primase, synthesizes an RNA primer that is about five to ten nucleotides long and complementary to the DNA. Then how does it add the first nucleotide? The problem is solved with the help of a primer that provides the free 3′-OH end. This essentially means that it cannot add nucleotides if a free 3′-OH group is not available. It also requires a free 3′-OH group to which it can add nucleotides by forming a phosphodiester bond between the 3′-OH end and the 5′ phosphate of the next nucleotide. DNA polymerase is able to add nucleotides only in the 5′ to 3′ direction (a new DNA strand can be only extended in this direction). Single-strand binding proteins 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. ATP hydrolysis is required for this process. An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs. The origin of replication is recognized by certain proteins that bind to this site. coli, which has a single origin of replication on its one chromosome (as do most prokaryotes), it is approximately 245 base pairs long and is rich in AT sequences. How does the replication machinery know where to begin? It turns out that there are specific nucleotide sequences called origins of replication where replication begins. It is now known that DNA pol III is the enzyme required for DNA synthesis DNA pol I and DNA pol II are primarily required for repair. 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 growing chain. The addition of nucleotides requires energy this energy is obtained from the nucleotides that have three phosphates attached to them, similar to ATP which has three phosphate groups attached. One of the key players is the enzyme DNA polymerase, also known as DNA pol, which adds nucleotides one by one to the growing DNA chain that are complementary to the template strand. ![]() The process is quite rapid and occurs without many mistakes.ĭNA replication employs a large number of proteins and enzymes, each of which plays a critical role during the process. This means that approximately 1000 nucleotides are added per second. coli has 4.6 million base pairs in a single circular chromosome and all of it gets replicated in approximately 42 minutes, starting from a single origin of replication and proceeding around the circle in both directions. Discuss the role of different enzymes and proteins in supporting this processĭNA replication has been extremely well studied in prokaryotes primarily because of the small size of the genome and the mutants that are available.Explain the process of DNA replication in prokaryotes.By the end of this section, you will be able to: ![]()
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