Leading Strand, Lagging Strand and Okazaki Fragments
Biology · Molecular Basis of Inheritance · NEET
DNA polymerase can only add new nucleotides in the 5'→3' direction. So on the template that runs 3'→5', the new strand is made in one smooth piece (the leading strand). On the other template (5'→3'), the new strand is made in short broken pieces called Okazaki fragments (the lagging strand), which DNA ligase later joins together. Memory hook: "Leading = Long and non-stop; Lagging = Little pieces, Ligase seals."
At the replication fork, the leading strand (green) is made in one continuous 5'→3' piece toward the fork, while the lagging strand (red) is made as short Okazaki fragments (5'→3', but away from the fork) that DNA ligase joins.
Your doubts, answered
Why is one strand continuous (leading) and the other discontinuous (lagging)?
Because DNA polymerase can add nucleotides only in the 5'→3' direction. The two template strands run in opposite directions (antiparallel). On the template with 3'→5' polarity, the polymerase can move smoothly toward the replication fork, so the new leading strand is continuous. On the template with 5'→3' polarity, the polymerase must move away from the fork, so it can only work in short bursts, making the discontinuous lagging strand. NCERT states this directly.
What are Okazaki fragments and which enzyme joins them?
Okazaki fragments are the short pieces of DNA made on the lagging strand during discontinuous replication. Each fragment is made 5'→3' but in the direction away from the replication fork. NCERT says: 'The discontinuously synthesised fragments are later joined by the enzyme DNA ligase.' So the answer to 'which enzyme joins Okazaki fragments' is always DNA ligase, not DNA polymerase.
In which direction are Okazaki fragments made — towards or away from the fork?
Each individual Okazaki fragment is polymerised 5'→3', but the overall lagging strand grows AWAY from the replication fork. This exact point was the 2017 NEET answer: 'Okazaki fragments elongate the lagging strand away from the replication fork.' Do not confuse the direction of one fragment (5'→3') with the direction of overall growth (away from fork).
Does DNA polymerase work in both directions during replication?
No. DNA-dependent DNA polymerase catalyses polymerisation only in one direction, that is 5'→3'. This single-direction rule is the whole reason a lagging strand exists. NEET 2024 tested exactly this: the correct statement is 'DNA-dependent DNA polymerase catalyses polymerisation in 5'→3' direction' — options saying 3'→5' or both directions are wrong.
⚠️ The NEET trap ✗ Okazaki fragments elongate the lagging strand towards the replication fork. ✓ Okazaki fragments elongate the lagging strand AWAY from the replication fork (each fragment is still made 5'→3', but overall growth is away from the fork). 🧠 NEET 2017 tested this exact trap. Students pick 'towards the fork' by reflex. Remember: leading strand grows toward the fork, lagging strand grows away from it.
Real NEET questions
NEET 2017
During DNA replication, Okazaki fragments are used to elongate
A · The leading strand towards replication fork
B · The lagging strand towards replication fork
C · The leading strand away from replication fork
D · The lagging strand away from the replication fork ✓
Solution: Okazaki fragments make up the lagging strand. Because DNA polymerase works only 5'→3' and the template here has 5'→3' polarity, replication is discontinuous and the fragments grow in the direction away from the replication fork. These fragments are later joined by DNA ligase (NCERT Ch 5). So the correct answer is 'the lagging strand away from the replication fork'.
NEET 2024
Which of the following statements is correct regarding the process of replication in E. coli?
A · DNA dependent RNA polymerase catalyses polymerization in one direction, 5'→3'
B · DNA dependent DNA polymerase catalyses polymerization in 5'→3' as well as 3'→5' direction
C · DNA dependent DNA polymerase catalyses polymerization in 5'→3' direction ✓
D · DNA dependent DNA polymerase catalyses polymerization in one direction, 3'→5'
Solution: NCERT: 'The DNA-dependent DNA polymerases catalyse polymerisation only in one direction, that is 5'→3'.' This one-direction rule is exactly why one strand (leading) is continuous and the other (lagging) is discontinuous. Options with 3'→5' or both directions are wrong.
NEET 2016 (Phase 1)
A complex of ribosomes attached to single strand of RNA is known as:
A · Polysome ✓
B · Polymer
C · Polypeptide
D · Okazaki fragment
Solution: The correct answer is Polysome (polyribosome). 'Okazaki fragment' is a distractor placed here to catch students who link the word to replication — Okazaki fragments belong to the lagging strand of DNA replication, not to ribosomes on mRNA.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the difference between leading and lagging strand in one line?
Leading strand is synthesised continuously toward the replication fork; lagging strand is synthesised discontinuously (as Okazaki fragments) away from the fork and then sealed by DNA ligase.
Are Okazaki fragments in NCERT?
NCERT does not use the exact term 'Okazaki fragments'. It describes them as 'the discontinuously synthesised fragments' that are 'later joined by the enzyme DNA ligase.' The term itself is extension content, but NEET has directly asked it (2017).
Which enzyme joins Okazaki fragments?
DNA ligase. It seals the nicks between adjacent Okazaki fragments to make one continuous lagging strand. DNA polymerase adds the nucleotides; ligase joins the pieces.
Why does DNA polymerase add nucleotides only in the 5'→3' direction?
DNA polymerase needs a free 3'-OH end to attach the next nucleotide, and the energy for the bond comes from the incoming nucleotide's triphosphate. This chemistry only works when the chain grows 5'→3'. For NEET, just remember the rule: polymerisation is only 5'→3'.
What is a replication fork?
It is the small Y-shaped opening in the DNA helix where the two strands separate and new strands are made. NCERT says the full strands are not separated at once (too much energy); replication happens within this small opening called the replication fork.
