Question

Why is DNA replication bidirectional?

Answer 1

Hint: Taking biological reference we can define DNA replication as the biological process of generating two identical copies of DNA from the original parent DNA molecule. DNA replication takes place in all living organisms as the most important part for inheritance.

Complete answer:
At the time of DNA replication, each strand acts as a template for making the other strand via complementary base pairing. Complementary base pairing is a protocol of binding for DNA nucleotides like, with adenine hydrogen bonds formed by the base thymine. Similarly, the base guanine can make hydrogen bonds only with the base cytosine. Each strand has two ends, 5' end with the terminal phosphate group on the 5' carbon of the sugar and the 3' end with a terminal hydroxyl group on the 3' carbon of the sugar.
To synthesize the two strands of DNA during replication, the DNA polymerase enzyme is involved which is able to join the phosphate group (5' carbon) to the hydroxyl (OH) group (3' carbon). The two strands of DNA are complementary to each other i.e. they are oriented in opposite directions. One strand is 5' to 3' while the other DNA strand is 3' to 5' (antiparallel).
Initially, the unwinding is done by DNA helicases to form a "Y"-shaped replication forks. As they unwind in both directions around the entire molecule, new strands are produced by the formation of hydrogen bonds between nucleotides. As nucleotides attach opposite to each parent strand by, the enzyme known as DNA polymerases join the nucleotides by phosphodiester bonds. There are two antiparallel strands. However, the other parent strand - the one which is running 5' to 3' and known as the lagging strand with discontinuous fragments called Okazaki because polymerase has its activity in only one direction.
DNA polymerase enzymes can only bring new nucleotides on the 3' OH group. Therefore, to initiate the synthesis, an RNA polymerase complex also known as primase is needed which is capable of joining nucleotides without any preexisting strand. The enzyme, DNA polymerase III, has the capability to replace the primase and then to add DNA nucleotides to the primer.

Yes DNA replication is bidirectional as it runs from 5' to 3' as well as 3' to 5'.

Note:
The enzyme which is named DNA polymerase II can digest the primer and fill the gap with proper DNA nucleotides. The okazaki fragments are joined together by DNA ligase and make the segment continuous.