Question

The mode of DNA replication in E. coli is
a. Conservative and unidirectional
b. Semi-conservative and unidirectional
c. Conservative and bidirectional
d. Semi-conservative and bidirectional

Answer 1

Hint: DNA replication is the process by which a parent DNA strand gets copied into two daughter DNA strands, each having one parent strand that acts as a template during DNA synthesis. E. coli is a prokaryotic organism that has a circular double-stranded DNA molecule.

Complete answer:
> Option (A) is incorrect. The mode of DNA replication in E. coli is not conservative and unidirectional but it is semi-conservative and bidirectional.

> Option (B) is incorrect. The mode of DNA replication in E. coli is semi-conservative and bidirectional but not unidirectional.

> Option (C) is incorrect. DNA is not replicated conservatively in E. coli but it is replicated semi-conservatively as out of the two strands of the daughter DNA, one strand comes from the parent DNA. Thus, the mode of DNA replication in E. coli is semi-conservative and bidirectional.

> Option (D) is correct. The DNA is replicated semi-conservatively in E. coli because during the initial stage of DNA replication i.e., DNA denaturation, the double-stranded DNA gets converted into single-stranded DNA (ss-DNA) by breaking of hydrogen bonds between them and this ss-DNA will act as a template for the synthesis of another strand of daughter DNA. Thus, single parent DNA gives rise to two daughter DNA strands each having one of its strands coming from the parent DNA.

Also, the replication of DNA is bidirectional in E. coli because it takes place in both directions i.e., $5' \to 3'$ and $3' \to 5'$ directions that will form the leading or continuous strand and lagging or a discontinuous strand. Thus, the mode of DNA replication in E. coli is semi-conservative and bidirectional.

Hence, the correct answer is option (D).

Note: DNA replication in E. coli is semi-conservative and bidirectional. The enzyme playing a key role in the process of synthesizing DNA strands is the DNA polymerase enzyme. The two strands synthesized by DNA polymerase are the leading and the lagging strands. The lagging strand (when DNA synthesis occurs in $3' \to 5'$ direction) consists of short pieces of DNA known as Okazaki fragments.