why is synthesis discontinuous on the lagging strand
Synthesis is discontinuous on the lagging strand because DNA synthesis occurs in a 5' to 3' direction, while the lagging strand is oriented in a 3' to 5' direction. This means that DNA polymerase has to move in the opposite direction to replication fork movement, causing short, discontinuous fragments called Okazaki fragments to be formed. These fragments are later joined together by the enzyme DNA ligase to form a continuous strand. This process of discontinuous replication occurs on the lagging strand and not the leading strand because of differences in their orientation and processing requirements.
Synthesis is discontinuous on the lagging strand during DNA replication because DNA polymerase can only synthesize DNA in the 5' to 3' direction. The lagging strand is oriented in the opposite direction, known as the 3' to 5' direction. As a result, synthesis of the lagging strand occurs in short fragments called Okazaki fragments.
Here's a step-by-step explanation of the discontinuous synthesis on the lagging strand:
1. The double-stranded DNA molecule unwinds during replication, and the leading and lagging strands are exposed.
2. DNA polymerase begins synthesizing a new strand in the leading strand in the 5' to 3' direction continuously as the DNA helix unwinds. The leading strand is oriented in the same direction as the replication fork movement, allowing continuous synthesis.
3. However, on the lagging strand, DNA polymerase encounters a problem since it can only synthesize DNA in the 5' to 3' direction. Because the lagging strand is oriented in the opposite direction, synthesis cannot occur continuously.
4. To overcome this issue, primase synthesizes short RNA primers, about 10-12 nucleotides in length, at intervals along the lagging strand. These primers provide a starting point for DNA polymerase to initiate synthesis.
5. DNA polymerase then synthesizes DNA in short stretches, about 100-200 nucleotides long, away from the replication fork, starting from each RNA primer. These short DNA fragments are called Okazaki fragments.
6. After synthesizing an Okazaki fragment, DNA polymerase dissociates from the template strand and moves further along the lagging strand to find the next RNA primer.
7. DNA ligase comes into play once all the Okazaki fragments have been synthesized. It joins the Okazaki fragments together by forming phosphodiester bonds between adjacent nucleotides, creating a continuous lagging strand.
In summary, the discontinuous synthesis on the lagging strand occurs because DNA polymerase can only synthesize DNA in the 5' to 3' direction. This requires the lagging strand to be synthesized in short fragments called Okazaki fragments, which are joined together by DNA ligase after synthesis.