Leading and Lagging Strand Synthesis

Why can DNA synthesis occur only in the 5′ to 3′ direction

DNA polymerases add nucleotides to the free 3′-OH end of a growing DNA strand.

What is the leading strand

The new DNA strand synthesized continuously toward the advancing replication fork.

What is the lagging strand

The new DNA strand synthesized discontinuously in short fragments away from the advancing replication fork.

What are Okazaki fragments

Short DNA segments synthesized on the lagging strand.

Why is leading-strand synthesis continuous

Its template is oriented so that DNA polymerase can synthesize toward the advancing fork.

Why is lagging-strand synthesis discontinuous

Its template orientation requires repeated synthesis away from the advancing fork.

How many RNA primers are generally needed to initiate leading-strand synthesis at one fork

One primer, assuming continuous synthesis from that initiation point.

Why does the lagging strand require multiple RNA primers

Each Okazaki fragment must be initiated with its own primer.

In which direction does DNA polymerase extend an Okazaki fragment

5′ to 3′, away from the replication fork.

What happens to RNA primers on the lagging strand

DNA polymerase I removes them and replaces them with DNA; ligase seals the remaining nicks.

What do the leading and lagging strands have in common

Both are synthesized 5′ to 3′ using parental DNA templates and require primers.

How do leading and lagging strands differ in their synthesis

The leading strand is continuous; the lagging strand is discontinuous and formed from Okazaki fragments.

What is the role of helicase at a replication fork

It unwinds the parental double helix as the fork advances.

What is the role of primase at the replication fork

It produces RNA primers for DNA synthesis.

What is the role of SSB proteins at the replication fork

They stabilize the separated parental DNA strands.

What is the role of DNA polymerase III at the replication fork

It extends the leading strand and synthesizes Okazaki fragments on the lagging strand.

What is the role of DNA polymerase I after Okazaki fragments are synthesized

It removes RNA primers and fills the gaps with DNA.

What is the role of DNA ligase after primer replacement

It seals the nicks between adjacent DNA fragments.