The replication fork opens because helicase travels along the double helix breaking the hydrogen bonds between base pairs, splitting one parent duplex into two single-stranded templates. DNA polymerase can only add nucleotides in the 5'→3' direction, and it can only extend an existing strand — it cannot start one from scratch. That single rule produces the whole asymmetry you see here: on the leading strand, the exposed template already runs 3'→5' toward the fork, so polymerase rides the fork continuously, synthesizing new DNA in one unbroken piece. On the lagging strand, the template runs the wrong way, so primase must lay down a short RNA primer behind the fork first, and polymerase then synthesizes a short stretch back toward the previous fragment before detaching and waiting for the next primer — producing the short, discontinuous Okazaki fragments that give the lagging strand its stop-start rhythm. Once a fragment reaches the 5' end of the one before it, DNA ligase seals the nick between them into one continuous backbone.
Real example: this exact fork geometry — one continuous strand, one fragmented strand stitched together by ligase — copies the entire genome of every dividing cell in your body, and the same base-pairing and polymerase chemistry is what a PCR test amplifies to detect viral DNA.
leading strand: continuous synthesis, follows fork directly (5'->3' matches fork direction)
lagging strand: primase -> short primer -> polymerase extends -> Okazaki fragment -> ligase seals nick
fork speed (bp/s) set by slider; both strands always move at the same fork rate