Leading vs Lagging strand (Continuous vs Discontinuous synthesis)

Why does the antiparallel arrangement of DNA strands require different synthesis patterns at a replication fork?

The two DNA template strands run in opposite directions, so DNA polymerase synthesises one new strand continuously toward the advancing replication fork, while the other is produced discontinuously away from it.

Why can DNA polymerase synthesise a new DNA strand only in the 5′ to 3′ direction?

DNA polymerase can only add an incoming nucleotide to the free 3′ hydroxyl group (3′-OH) of a growing strand, causing the new DNA strand to extend in the 5′ to 3′ direction.

Why is synthesis of the leading strand continuous?

The leading-strand template is oriented so that DNA polymerase can follow the advancing replication fork while continuously adding nucleotides in the 5′ to 3′ direction from a single RNA primer.

Why is synthesis of the lagging strand discontinuous?

Its template runs in the opposite orientation, so DNA polymerase must synthesise DNA away from the advancing replication fork in separate sections called Okazaki fragments.

What are Okazaki fragments and where are they produced?

Okazaki fragments are short sections of newly synthesised DNA produced discontinuously on the lagging strand during DNA replication.

Why does the lagging strand require many RNA primers while the leading strand generally requires only one?

The leading strand can be extended continuously from one initial RNA primer, whereas each Okazaki fragment on the lagging strand requires its own primer.

What is the main function of DNA polymerase III during DNA replication?

DNA polymerase III extends an existing primer or DNA strand by adding complementary DNA nucleotides to its free 3′ end, producing the new strand in the 5′ to 3′ direction.

How does DNA polymerase I help complete the lagging strand?

DNA polymerase I removes the RNA primers from between Okazaki fragments and replaces them with the appropriate DNA nucleotides.

What is the role of DNA ligase during lagging-strand synthesis?

DNA ligase joins adjacent DNA sections by sealing nicks in the sugar-phosphate backbone and forming the required phosphodiester bonds between neighbouring nucleotides.

What happens to the lagging strand if DNA ligase cannot function?

The Okazaki fragments remain separated by unsealed nicks in the sugar-phosphate backbone, preventing the lagging strand from becoming a continuous DNA molecule.

What is the function of single-strand binding proteins (SSBs) after helicase separates DNA strands?

SSBs attach to the exposed single-stranded DNA, stabilising the separated templates and preventing them from re-annealing before they are copied.

How does topoisomerase, including bacterial DNA gyrase, assist DNA replication?

Topoisomerase relieves torsional strain and excessive supercoiling that develops in the DNA ahead of the advancing replication fork as helicase unwinds the double helix.

How can both antiparallel DNA strands be copied simultaneously if DNA polymerase works only in the 5′ to 3′ direction?

The leading strand is synthesised continuously in the direction of fork movement, while the lagging strand is produced discontinuously as Okazaki fragments in the opposite direction.

In what direction does the leading strand grow relative to the advancing replication fork?

The leading strand is synthesised in the same overall direction as the movement of the replication fork, allowing continuous DNA synthesis.

In what direction are individual Okazaki fragments synthesised relative to replication fork movement?

Each Okazaki fragment is synthesised in the direction opposite to the movement of the replication fork, although every fragment is still built in the 5′ to 3′ direction.

Why are RNA primers replaced with DNA before replication is completed?

The temporary RNA primers must be removed and replaced with DNA nucleotides so the finished molecule contains a continuous DNA backbone rather than sections of RNA.

What is the final result of coordinated leading- and lagging-strand synthesis?

Replication produces two double-stranded DNA molecules, with each molecule containing one original parental strand and one newly synthesised strand, demonstrating semiconservative replication.

Why does the lagging strand require many more RNA primers than the leading strand?

The lagging strand is assembled from numerous Okazaki fragments, and each fragment needs a separate primer to provide the starting 3′-OH required by DNA polymerase.

If a new DNA strand is being synthesised continuously in the 5′ to 3′ direction toward an advancing replication fork, which strand is it?

It is the leading strand, because its template orientation allows DNA polymerase to synthesise the new strand continuously toward the advancing fork.

Why is DNA replication described as semi-discontinuous?

DNA replication is continuous on the leading strand but discontinuous on the lagging strand, where DNA is assembled from multiple Okazaki fragments that are later joine