The replication fork.

Open the fork: leading strand grows continuously, lagging strand in 5′→3′ pieces that ligase seals.

Copy 60 bp of both templates with every new piece built 5′→3′ and no nicks left.

Thinking the discontinuous strand grows 3′→5′.

• Watson and Crick proposed semiconservative replication: the strands separate and each acts as a template, so every new molecule has one parental and one newly made strand.
• Meselson and Stahl (1958) grew E. coli for many generations in ¹⁵NH₄Cl so their DNA became heavy, then moved them to ¹⁴NH₄Cl. DNA was separated by density in a caesium chloride (CsCl) gradient.
• E. coli divides in 20 minutes. After one generation (20 min) all DNA had hybrid density. After two generations (40 min) there were equal amounts of hybrid and light DNA, as predicted by the semiconservative model.
• Taylor and colleagues (1958) used radioactive thymidine on Vicia faba (faba beans) and showed that DNA in chromosomes also replicates semiconservatively.
• The main enzyme is DNA-dependent DNA polymerase. E. coli, with 4.6 × 10⁶ bp, finishes replication in about 18 minutes, an average of about 2000 bp per second, with high accuracy.
• Deoxyribonucleoside triphosphates act both as substrates and as the energy source, from their two terminal high-energy phosphates.
• The helix cannot open fully at once, so replication happens in a small opened region called the replication fork. It begins at a defined site, the origin of replication.
• DNA polymerase adds nucleotides only in the 5′→3′ direction. So on the template with 3′→5′ polarity synthesis is continuous, while on the template with 5′→3′ polarity it is discontinuous, and the short pieces made there are later sealed together by DNA ligase.
• In eukaryotes DNA replicates in the S phase of the cell cycle. If replication is not followed by cell division, polyploidy results.

Teaching this idea? Put the simulation on your own page or LMS with this code.