Steps of DNA replication in order, explained simply: initiation, elongation and termination, the enzymes at each step, and the prokaryote and eukaryote differences for Class 12, AP and IB Biology

DNA replication is asked in every biology course from Class 12 and IGCSE to AP and IB, and it is asked the same way every time: the steps in order, what each enzyme does, why one strand is made in pieces, and how prokaryotes and eukaryotes differ. Students lose marks by knowing the words without the order, or the order without the reasons. This post gives the steps of DNA replication in order, with the reason for each, the enzymes in one table, a labelled diagram, the prokaryote and eukaryote differences, and the six questions examiners actually ask.

The idea in one paragraph

DNA is two strands wound together, held by base pairs (A with T, G with C), running in opposite directions (antiparallel: one strand 5' to 3', the other 3' to 5'). To copy it, the cell unwinds the two strands and uses each as a template to build a new partner strand. The result is two double helices, each with one old strand and one new one, which is why replication is called semi-conservative. The whole thing happens before a cell divides, in the S phase of the cell cycle.

The steps of DNA replication in order

1. Initiation

• The origin is found. Replication starts at a specific sequence, the origin of replication. A bacterial chromosome has one origin; a eukaryotic chromosome has many, so a very long molecule can be copied in a reasonable time.
• Helicase unwinds the helix. Helicase breaks the hydrogen bonds between the base pairs and separates the two strands, opening a replication bubble with a fork at each end. The forks move outwards as replication goes on.
• Single-strand binding proteins hold the strands apart so they cannot re-pair before they are copied.
• Topoisomerase relieves the tension. Unwinding twists the DNA ahead of the fork; topoisomerase cuts and rejoins the strand to release the supercoiling. (In bacteria the enzyme is DNA gyrase.)
• Primase lays a primer. DNA polymerase cannot start a strand from nothing; it can only extend an existing 3' end. Primase makes a short RNA primer (about ten nucleotides) on the template to give polymerase a starting point.

2. Elongation

• DNA polymerase III adds nucleotides. It reads the template strand and adds the complementary nucleotide to the 3' end of the growing strand, so the new strand always grows 5' to 3'. It also proofreads: a wrong base is removed and replaced before the next one is added.
• The leading strand is made continuously. On the template that runs 3' to 5' towards the fork, the new strand grows 5' to 3' in the same direction the fork moves, so one primer is enough and synthesis is continuous.
• The lagging strand is made in Okazaki fragments. On the other template, the new strand has to grow away from the fork. As the fork opens more template, primase lays a new primer and polymerase makes a short fragment (about 1,000 to 2,000 nucleotides in bacteria, 100 to 200 in eukaryotes) back towards the previous one. Each fragment is an Okazaki fragment.
• DNA polymerase I replaces the primers. It removes each RNA primer and fills the gap with DNA.
• Ligase joins the fragments. DNA ligase forms the phosphodiester bond between adjacent fragments, making the lagging strand continuous.

3. Termination

• In prokaryotes the two forks travelling round the circular chromosome meet at termination sequences opposite the origin, and the two circles are separated.
• In eukaryotes neighbouring bubbles merge, and the linear ends (telomeres) present a problem: the last primer on the lagging strand cannot be replaced with DNA, so the chromosome would shorten every division. Telomerase, an enzyme carrying its own RNA template, extends the end so nothing coding is lost.

The enzymes, in one table

Exam tip: examiners want the function, not just the name. "Helicase" earns nothing; "helicase breaks the hydrogen bonds between complementary bases and unwinds the double helix" earns the mark.

The diagram

Prokaryotes and eukaryotes

Class 12 (NCERT) students need the prokaryote column with polymerase III and I named; AP and IB students need both columns and the reason for telomerase.

The six questions examiners ask

1. State the steps of DNA replication in order. Initiation, elongation, termination, with two actions in each.
2. Explain why the lagging strand is synthesised discontinuously. Polymerase adds only to a 3' end, the strands are antiparallel, so one strand must be built away from the fork in fragments.
3. Describe the role of three named enzymes. Function first, then name.
4. Explain what is meant by semi-conservative, and the evidence. One old strand and one new in each daughter helix; Meselson and Stahl's nitrogen isotopes and density gradient.
5. Compare replication in prokaryotes and eukaryotes. Origins, polymerases, telomeres.
6. Explain why the primer is RNA and what happens to it. Primase can start a strand, polymerase cannot; polymerase I removes it and fills with DNA; ligase seals.

How to learn it in three sittings

Sitting one: draw the fork from memory, label the five strands and structures (two parents, leading, lagging, primer, fragment), and say the direction rule aloud. Sitting two: the enzyme table, covered, from memory, function before name. Sitting three: write the six answers above in exam sentences, timed. Students who learn it as a story of a fork moving, rather than a list, keep it to the exam and beyond.

What we do

Our biology tutoring is one tutor with one student, on the student's own board (CBSE Class 11 and 12, IGCSE, A-Level, AP, IB), and molecular biology is taught on a shared board where the fork gets drawn and redrawn until the student can do it alone. The first 30-minute class is free; bring the chapter.

Questions parents ask

See how we teach this, 1-on-1 online →