Why does the lagging strand have to be looped out during DNA

Solution

The process of DNA replication:

During DNA replication, the double helix unwinds and the parental strands act as templates for the synthesis of new daughter strands. During this process. One strand is oriented in 5\' to 3\' direction (leading strand) and another is oriented in 3\' to 5\' direction (lagging strand) but the DNA polymerase enzyme (the prime enzyme of DNA replication) works only in 5\' to 3\' direction. So, the first strand oriented in 5\' to 3\' direction adds the nucleotides continuously one by one in the direction of replication fork, which is called as leading strand.

DNA replication in the lagging strand:

The lagging strand receives several RNA primers. Replication process in the lagging strand is more complicated and occurs in multiple fragments due to its orientation (its direction of replication is opposite to that of growing replication fork).

The DNA template of lagging strand is read by a primase, which synthesise complementary RNA primer. The primed segments are extended by DNA polymerase forming the Okazaki fragments, now the RNA primers are removed. The synthesis of lagging strand occurs using short DNA segments called Okazaki fragments each consisting of 100 to 200 nucleotides in humans (1000 to 2000 nucleotides in bacteria). The replication occurs in 5 to 3 direction, but away from the replication fork.

The lagging strand must be looped out because the DNA polymerase to move in an appropriate direction. The DNA ligase connects the Okazaki fragments by catalysing the formation of the phosphodiester bond between the Okazaki fragments of the complementary strand.