Question

During DNA replication, Okazaki fragments are used to elongate

• A. the leading strand away from replication fork
• B. the lagging strand towards replication fork
• C. the lagging strand away from the replication fork
• D. the leading strand towards replication fork

Answer 1

The correct option is C the lagging strand away from the replication fork

DNA replication is the process by which new DNA molecules are synthesised using the parental strands as templates. The main enzyme involved in the synthesis is DNA polymerase.

Replication fork is a structure formed when a small stretch of DNA is unwound during DNA replication, while the rest of DNA remains as a helix. At the replication fork, one parental strand is in the 5’ to 3’ direction away from the fork and the other parental strand is in the 3’ to 5’ direction towards the fork.

DNA polymerase adds new nucleotides on the 3'-OH position of the previous nucleotide by a nucleophilic attack. Thus, DNA polymerase can polymerise nucleotides only in one direction 5' $\to$ 3'. This feature of DNA polymerase results in a semi-discontinuous synthesis of DNA.

• The DNA strand formed using the 3’→ 5’ parental template will be synthesised continuously with the help of DNA polymerase in the 5' $\to$ 3' direction i.e towards the replication fork. This new strand is called the leading strand.
• The DNA strand formed using the 5' $\to$ 3’ parental template will have 3’ $\to$ 5’ polarity. Thus, it cannot be synthesised continuously using DNA polymerase. The synthesis of this strand is done discontinuously by forming short segments known as Okazaki fragments away from the replication fork and it is called lagging strand.
• The leading strand can be extended from one primer alone, whereas the lagging strand needs a new primer for each of the short Okazaki fragments.