Question

Describe the way in which DNA is replicated with reference to the following: (a) The formation of a replication bubble with 2 replication forks in opposite directions,the involvement of Okazaki fragments in replication, the synthesis of leading and lagging strand and the direction in which chain elongation occurs.

Answer 1

Replication is a process in which DNA copies
itself to produce identical daughter molecules of DNA.

The parent DNA has two strands complemen-
tary to each other. Both the strands undergo
simultaneous replication to produce two
daughter molecules.

Each one of the newly synthesized DNA has one-half of the parental DNA (one strand from original) and one-half of new DNA . This type of replication is known as semiconservative.

The two complementary strands of DNA
separate at the site of replication to form a
bubble.
Note-Multiple replication bubbles are formed
in eukaryotic DNA molecules, which is essential
for a rapid replication process.

The replication of DNA occurs in 5’ to 3’
direction, simultaneously, on both the strands of
DNA. On one strand, the leading (continuous or
forward) strand—the DNA synthesis is
continuous.

On one other strand, the lagging
(discontinuous ) strand—the synthesis of DNA is discontinuous. Short pieces of DNA are produced on the lagging strand.

In the replication bubble, the DNA synthesis
occurs in both the directions (i.e.. bidirectional) from the point of origin.

The separation of the two strands of parent
DNA results in the formation of a replication
fork.

The active synthesis of DNA occurs in this
region. The replication fork moves along the
parent DNA as the daughter DNA molecules are synthesized.

DNA Helicases : These enzymes bind to both
the DNA strands at the replication fork.
Helicases move along the DNA helix and
separate(unwind).

The two strands separate and provide the template for new DNA synthesis.

The synthesis of a new DNA strand, catalysed by DNA polymerase III, occurs in 5’ to 3’ direction. This is antiparallel to the parent template DNA strand.

The synthesis of two new DNA strands,
simultaneously, takes place in the opposite
direction—one is in a direction (5’ to 3’) towards the replication fork which is continuous (leading strand) , the other in a direction (5’ to 3’) away from the replication fork which is discontinuous(lagging strand).

The DNA strand (leading strand) with its
3'end oriented towards the fork that can be
elongated by sequential addition of new
nucleotides. The other DNA strand (lagging
strand) with 5'-end presents some problem i.e.. There is no DNA polymerase enzyme (in any organism) that can catalyse the addition of nucleotides to the 5' end (i.e.. 3'-5' direction) of the growing chain. This problem is
solved by synthesizing this strand as a series of small fragments also known Okazaki fragments.

Okazaki fragments are produced on the lagging strand of the parent DNA.And later joined to form a continuous strand of DNA. DNA polymerase I and DNA ligase (enzyme) responsible for this process .