AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/6/6-2)
Unit 6 · Topic 6.2
6.2 DNA Replication
Before a cell divides, it copies its DNA so each daughter cell gets a full set. Replication is semiconservative: each new double helix keeps one original strand and gets one new strand, built by DNA polymerase in the 5′ → 3′ direction with help from helicase, topoisomerase, RNA primers and ligase.
Key terms
- semiconservative replication
- helicase
- DNA polymerase
- leading strand
- lagging strand
- ligase
Semiconservative replication
Because base pairing is specific, each strand of DNA carries the information to rebuild its partner. During replication the two strands separate, and each acts as a template for a new complementary strand. The two resulting DNA molecules are each half old and half new. That's what semiconservative means.
Meselson and Stahl showed this in 1958. They grew bacteria on heavy nitrogen (¹⁵N) so all their DNA was heavy, then moved them to normal nitrogen (¹⁴N) and measured DNA density after each round of replication. After one round, all the DNA was intermediate (hybrid) in density. After two rounds, half was intermediate and half was light. Only the semiconservative model predicts both results.
The enzymes and what they do
Replication starts at specific spots called origins of replication. Bacteria usually have one origin on their circular chromosome; eukaryotes have many origins on each linear chromosome so the job finishes in time. At each origin, the DNA opens into a bubble with a Y-shaped replication fork at each end.
| Enzyme or molecule | Job |
|---|---|
| Helicase | Unwinds the double helix and separates the two strands at the replication fork |
| Topoisomerase | Relieves the over-twisting (supercoiling) that builds up ahead of the fork as the helix unwinds |
| RNA primers | Short RNA starter pieces that give DNA polymerase a 3′ end to build from |
| DNA polymerase | Adds DNA nucleotides that pair with the template, only onto a 3′ end, so new DNA grows 5′ → 3′; also proofreads |
| Ligase | Seals the gaps between DNA pieces on the lagging strand, joining them into one continuous strand |
Leading and lagging strands
DNA polymerase can only add nucleotides to the 3′ end of a growing strand, so new DNA is always made 5′ → 3′, reading the template 3′ → 5′. The two template strands are antiparallel, which creates a problem at the fork.
On one template, the polymerase can move in the same direction the fork is opening and build continuously. That new strand is the leading strand, and it needs only one primer. On the other template, the polymerase has to work away from the fork. It builds short pieces (called Okazaki fragments) one at a time, each starting from its own RNA primer, as more template is exposed. This discontinuous strand is the lagging strand. Later the RNA primers are replaced with DNA and ligase seals the pieces together.
You only need to know the names helicase, topoisomerase, DNA polymerase, ligase and RNA polymerase. Other enzymes in the process (such as the one that makes primers) are beyond the scope of the exam.
Why accuracy matters
Accurate copying keeps the hereditary instructions the same from one generation to the next: daughter cells and offspring get the same instructions as the parent. DNA polymerase proofreads as it goes, and other repair systems fix many remaining errors, so mistakes are rare. The few errors that slip through become mutations (6.7), a source of the variation that evolution acts on.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Predicting a Meselson–Stahl result
Bacteria with only ¹⁵N (heavy) DNA are moved to a medium with only ¹⁴N and allowed to replicate. What fractions of DNA molecules will be hybrid (¹⁵N/¹⁴N) and light (¹⁴N/¹⁴N) after (a) one generation, (b) two generations, (c) three generations?
Show the solutionHide the solution
- Step 1: Start: one heavy double helix (two heavy strands).
- Step 2: (a) Each heavy strand pairs with a new light strand: 2 hybrid molecules. Fraction hybrid = 1, light = 0.
- Step 3: (b) The 2 hybrid molecules have 2 heavy and 2 light strands. Each strand gets a light partner: 2 hybrid + 2 light = 4 molecules. Hybrid = ½, light = ½.
- Step 4: (c) There are still only 2 heavy strands, so there are still 2 hybrid molecules, now out of 8. Hybrid = 2/8 = ¼, light = ¾.
- Step 5: Pattern: the number of hybrid molecules stays at 2, while the total doubles each generation.
Answer: (a) all hybrid; (b) ½ hybrid, ½ light; (c) ¼ hybrid, ¾ light.
- Example 2
Leading or lagging?
A replication fork is moving to the right. The top template strand runs 3′ → 5′ from left to right, and the bottom template strand runs 5′ → 3′ from left to right. Which new strand is made continuously?
Show the solutionHide the solution
- Step 1: New DNA is made 5′ → 3′, so it must be built antiparallel to its template, reading the template 3′ → 5′.
- Step 2: The top template runs 3′ → 5′ from left to right, so polymerase reading it moves left to right, the same way the fork moves. The new top strand is the leading strand, made continuously.
- Step 3: The bottom template runs 5′ → 3′ from left to right, so the polymerase must move right to left, away from the fork. The new bottom strand is the lagging strand, made in fragments.
Answer: The new strand built on the top template (the one read 3′ → 5′ in the fork's direction) is the continuous leading strand; the one on the bottom template is the lagging strand.
Common mistakes
- Saying DNA polymerase reads the template 5′ → 3′. It reads the template 3′ → 5′ and builds the new strand 5′ → 3′.
- Saying helicase relieves supercoiling. Helicase separates the strands; topoisomerase relieves the twisting ahead of the fork.
- Thinking only the lagging strand needs a primer. Both strands need RNA primers; the leading strand just needs one at the start, while the lagging strand needs one for each fragment.
- Describing replication as conservative (one all-old and one all-new helix). Each new molecule has one old and one new strand.
On the exam
- Model-based questions often show a replication fork and ask you to identify the leading and lagging strands, label the 5′ and 3′ ends, or predict what happens if an enzyme like ligase doesn't work (the lagging strand stays in fragments).
- Be ready to use the Meselson–Stahl results to argue for semiconservative replication and against the other models.
Connected topics
Videos
Check yourself
4 questions on 6.2 DNA Replication. Pick an answer to see if you got it, and why.
| Generation in ¹⁴N medium | Heavy DNA (%) | Intermediate DNA (%) | Light DNA (%) |
|---|---|---|---|
| 0 | 100 | 0 | 0 |
| 1 | 0 | 100 | 0 |
| 2 | 0 | 50 | 50 |
Experimental data: bacteria were grown for many generations in a medium with only heavy nitrogen (¹⁵N), then moved to a medium with only light nitrogen (¹⁴N). DNA was extracted after each round of replication and separated by density.
Which model of DNA replication do the results support, and why?
If the bacteria went through one more round of replication in ¹⁴N (generation 3), what would be expected?
Which of the following best explains why one strand of DNA is synthesized in short fragments during replication?
A mutant cell produces a nonfunctional DNA ligase. Which of the following would most likely be found in this cell after DNA replication?
0 of 4 answered