What DNA replication is
DNA replication is the process of copying a DNA molecule before a cell divides, so each new cell receives a complete set of genetic instructions. It happens during the S phase of the cell cycle. In human cells, about 6 billion base pairs are copied, starting at thousands of points called origins of replication so the job finishes in hours rather than weeks.
DNA structure in brief
DNA is a double helix: two strands, each a chain of nucleotides with a sugar-phosphate backbone and abase (A, T, G or C). The bases pair across the middle by hydrogen bonds: A with T andG with C. The strands are antiparallel: one runs 5′→3′ and its partner runs 3′→5′, which is why the labels at the ends of the strands in the simulator alternate.
Step by step
- Unwinding: helicase breaks the hydrogen bonds between bases, opening a Y-shaped replication fork.
- Priming: primase makes a short RNA primer (3 bases in the model; about 10 in real cells). DNA polymerase cannot start a strand from nothing.
- Elongation: DNA polymerase III adds nucleotides that pair with the template, always to the 3′ end of the growing strand.
- Primer removal: DNA polymerase I removes the RNA primers and fills the gaps with DNA.
- Joining: DNA ligase seals the remaining nicks in the backbone, producing continuous strands.
Other proteins help too: single-strand binding proteins stop the separated strands from re-pairing, and topoisomerase relieves the twisting that builds up ahead of the fork. They are left out of the model to keep it clear.
Leading and lagging strands
DNA polymerase can only build in the 5′→3′ direction. Because the two templates are antiparallel, on one of them that direction follows the fork: the leading strand is made continuously from a single primer. On the other, 5′→3′ points away from the fork, so the lagging strand is built in short pieces calledOkazaki fragments, each with its own primer. In bacteria they are 1,000–2,000 bases long; in eukaryotes, about 100–200.
Semi-conservative replication
When copying finishes, each new molecule has one original strand (blue in the simulator) and onenew strand (orange). This is called semi-conservative replication. Matthew Meselson and Franklin Stahl proved it in 1958 by growing bacteria in heavy nitrogen, moving them to normal nitrogen, and measuring the density of the DNA after each generation.
Speed and accuracy
Bacterial replication forks move at around 1,000 bases per second; human forks at about 50. DNA polymerase alsoproofreads: it can remove a wrongly paired base immediately. Together with later repair systems, the final error rate is roughly one mistake per billion bases copied. The rare errors that slip through aremutations, which are the raw material for natural selection.
Key terms
- Template strand
- An existing strand used as the pattern for building a new one.
- Replication fork
- The Y-shaped region where the helix is being unzipped and copied.
- Okazaki fragment
- A short stretch of new DNA on the lagging strand.
- Primer
- A short RNA strand that gives DNA polymerase a 3′ end to build onto.
- 5′ and 3′ ends
- The two chemically different ends of a strand, named after carbon atoms in the sugar.
Common misconceptions
- “The new DNA molecule is completely new.” Each daughter molecule keeps one original strand.
- “The lagging strand is made more slowly or later.” Both strands are made at the same overall rate; the lagging one is just made in pieces.
- “DNA polymerase can start a strand anywhere.” It needs a primer with a free 3′ end.
Key takeaways
- Helicase unzips; primase primes; DNA polymerase builds; ligase joins.
- Bases pair A–T and G–C, so each strand is a template for its partner.
- Polymerase only works 5′→3′, giving a continuous leading strand and a fragmented lagging strand.
- Replication is semi-conservative: each copy is half old, half new.
Model note: one fork copying 40 base pairs, greatly slowed down. The two primers left at the ends are removed by enzymes from the neighbouring fork in a real chromosome, and at chromosome tips by telomerase.