This primer serves to prime and lay a foundation for the DNA synthesis of the leading strand to take place continuously. A primer is a short nucleic acid sequence that provides a starting point for DNA synthesis. And so, for maintaining the antiparallel nature of DNA the lagging strand is synthesized discontinuously. The discontinuous segments of the DNA lagging strand are known as the Okazaki fragments. These fragments are later joined together as one complete strand by the enzyme DNA ligase.
The synthesizing of the fragments of the lagging strand is always in the direction away from the replication fork. Later when the fragments are joined the primers are removed altogether.
Both are replicated strands of DNA. The leading strand grows continuously whereas the lagging strand grows discontinuously. Only one RNA primer is required for the leading strand. Many RNA primers are required for the lagging strand.
The leading strand is synthesized towards the direction of the replication fork. The lagging strand is synthesized opposite to the direction of the replication fork. The leading strand is rapidly synthesized. The lagging strand is slowly synthesized. The leading strand is a complete strand. Generally, DNA undergoes replication during the cell cycle as a step of preparing the cell for the division. During the process, each strand of the DNA double helix serves as templates for replication.
Figure 1: DNA Replication. During replication, the DNA double helix unwinds to form the replication fork. Therefore, we call this strand as the leading strand. In the leading strand, DNA polymerase can add nucleotides continuously, and the growth of the new DNA strand occurs towards the replication fork.
The lagging strand is the second strand of the DNA double helix. On that account, the replication process is not continuous, and it occurs through the formation of Okazaki fragments. Generally, Okazaki fragments are short segments of DNA, about nucleotides long. Figure 2: Continuity of the Leading and Lagging Strand. Development of a leading strand is fast as opposed to a lagging strand whose formation is slower.
Further, the formation of a leading strand starts quickly with replication. Development of lagging strand requires a little time as compared to a leading strand. It needs RNA primer and is a non-continuous process. It is synthesized in the opposite way of the movement of the replication fork and this is why it is discontinuous.
Remember that primase takes time for the fork to open before inserting the primer. There is no gap or breaks between them.
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