One strand (the leading strand), complementary to the 3′ to 5′ parental DNA strand, is synthesized continuously towards the replication fork because the polymerase can add nucleotides in this direction. As we know, the DNA double helix is anti-parallel that is, one strand is in the 5′ to 3′ direction and the other is oriented in the 3′ to 5′ direction. DNA polymerase can only extend in the 5′ to 3′ direction, which poses a slight problem at the replication fork. The replication fork moves at the rate of 1000 nucleotides per second. DNA ligase seals the gaps between the Okazaki fragments, joining the fragments into a single DNA molecule. DNA polymerase I replaces the RNA primer with DNA. On the leading strand, DNA is synthesized continuously, whereas on the lagging strand, DNA is synthesized in short stretches called Okazaki fragments. DNA polymerase III uses this primer to synthesize the daughter DNA strand. Single-strand binding proteins bind to the single-stranded DNA to prevent the helix from re-forming. 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. Figure: DNA Replication in Prokaryotes: A replication fork is formed when helicase separates the DNA strands at the origin of replication. DNA polymerase then extends this RNA primer, adding nucleotides one by one that are complementary to the template strand. A primer provides the free 3′-OH end to start replication. Another enzyme, RNA primase, synthesizes an RNA primer that is about five to ten nucleotides long and complementary to the DNA, priming DNA synthesis. This 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 extended only in this direction). Single-strand binding proteins coat the strands of DNA near the replication fork to prevent the single-stranded DNA from winding back into a double helix. Two replication forks at the origin of replication are 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. There are specific nucleotide sequences called origins of replication where replication begins. 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, which adds nucleotides one by one to the growing DNA chain that are complementary to the template strand. \)ĭNA replication employs a large number of proteins and enzymes, each of which plays a critical role during the process.
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