Why lagging strand is fragmented?

Answer 1

Think of DNA as a zipper. The left one labelled 5' on the top and 3' on the bottom. The right one labelled 3' on the top and 5' on the bottom.

ex.

left 5'-------------------------3'

right 3'-------------------------5'

As you 'unzip' (with helicase), the teeth of the zipper are exposed. The copying mechanism moves along the original strand in a 5' to 3' direction

ex

left 5'------

.....................\

.......................--------------------3'

.......................--------------------5'

....................../

right 3'------

(periods are merely space holders ignore them)

On the 'left' strand, the DNA Polymerase attaches to the free 5' end and moves toward the helicase

On the 'right' strand, the DNA Polymerase must attach near the helicase (zipper-handle) and move towards the top. It must start several times therefore lagging strands are fragmented.

Answer 2

Because the strands must be bonded together in a 5' carbon to 3' carbon bonding. The leading strand, after primer is laid down, can run in this direction unimpeded, but the lagging strand, due to the antiparallel direction of DNA, must be short primed and run short sections of DNA in the 5' to 3' direction.

Answer 3

The lagging strand is fragmented because DNA replication occurs in a continuous manner on the leading strand, while on the lagging strand, synthesis occurs in short, discontinuous fragments called Okazaki fragments. This is due to the antiparallel nature of the DNA double helix, which requires synthesis to proceed in opposite directions.