AP Biology

## Learning Outcomes

- Describe why DNA replication is semiconservative.
- Explain the roles of helicase, DNA polymerase, and DNA ligase during replication.
- Predict the effect of inhibiting helicase, DNA polymerase, primase, topoisomerase, or ligase.
- Compare leading strand and lagging strand synthesis.
- Explain why DNA polymerase synthesizes DNA only in the 5′ to 3′ direction.
- Apply DNA replication concepts to AP-style multiple-choice and free-response questions.

## AP Biology Syllabus

In AP Biology Unit 6, DNA replication is tested as part of gene expression and regulation because cells must accurately copy genetic information before division. You are expected to understand the logic of the process rather than memorize every protein in a replication complex. Exam questions often ask you to connect enzyme function, DNA strand directionality, complementary base pairing, and semiconservative replication.

- DNA replication produces two DNA molecules, each with one parental strand and one newly made strand.
- DNA strands are antiparallel, so replication differs on the two template strands.
- Helicase unwinds and separates the DNA strands at the origin of replication.
- Topoisomerase relieves supercoiling caused by DNA unwinding.
- Primase makes RNA primers because DNA polymerase cannot start a new strand by itself.
- DNA polymerase adds DNA nucleotides to the 3′ end of a growing strand, so new DNA is synthesized 5′ to 3′.
- DNA ligase joins Okazaki fragments on the lagging strand.
- Free-response questions may ask you to predict what happens when one enzyme is missing or inhibited.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why is DNA replication described as semiconservative?
2. Which enzyme unwinds the DNA double helix, and which enzyme joins Okazaki fragments?
3. Why does DNA polymerase need an RNA primer?
4. Why is one new DNA strand made continuously while the other is made in fragments?
5. If DNA ligase is inhibited, which strand is most affected and why?

## Introduction

DNA replication is the process by which a cell copies its DNA before cell division. The purpose is simple: each daughter cell must receive a complete set of genetic instructions. The process is accurate because each original DNA strand acts as a template for a new complementary strand. Adenine pairs with thymine, and cytosine pairs with guanine, so the sequence of one strand determines the sequence of the other.

For AP Biology, the highest-yield part of DNA replication is enzyme function. Helicase opens the double helix, DNA polymerase adds new DNA nucleotides, and DNA ligase seals gaps between DNA fragments. These roles are closely tied to DNA directionality: DNA polymerase can add nucleotides only to a free 3′ hydroxyl group, meaning new DNA is always built in the 5′ to 3′ direction.

> **Key Term: DNA replication**
> The process by which DNA is copied before cell division, producing two DNA molecules from one original DNA molecule.

### Semiconservative Replication and the Replication Fork

DNA replication is called semiconservative because each new DNA molecule contains one old strand and one new strand. The original strands are not destroyed or kept together as one fully old molecule. Instead, the two original strands separate, and each serves as a template for a new complementary strand.

For example, imagine a short DNA molecule with two original strands. After one round of replication, there are two DNA molecules. Each has one parental strand and one newly synthesized strand. After a second round, half of the DNA molecules contain one original strand and one new strand, while the other half contain two strands made after the original starting point. This pattern is the logic behind classic isotope-labeling experiments such as the Meselson-Stahl experiment.

> **Key Term: semiconservative replication**
> A model of DNA replication in which each daughter DNA molecule contains one parental strand and one newly synthesized strand.

Replication begins at specific DNA sequences called origins of replication. At an origin, the DNA strands start to separate, forming a Y-shaped region called a replication fork. In bacteria, chromosomes are usually circular and may have a single main origin of replication. In eukaryotes, chromosomes are linear and much longer, so they use many origins of replication to copy DNA in a reasonable time.

> **Key Term: replication fork**
> The Y-shaped region where parental DNA strands are separated and new DNA strands are synthesized.

As the double helix opens, the DNA ahead of the fork becomes more tightly twisted. This extra twisting is called supercoiling. Topoisomerase helps solve this problem by making temporary breaks in the DNA backbone, relieving strain, and resealing the DNA. AP questions often include topoisomerase in answer choices, so do not confuse it with helicase.

> **Key Term: topoisomerase**
> An enzyme that relieves supercoiling during DNA replication by temporarily cutting and rejoining the DNA backbone.
>
> **Test Tip:**
> If a question asks which enzyme unwinds or separates the two DNA strands, choose helicase. If it asks which enzyme relieves supercoiling or twisting ahead of the fork, choose topoisomerase.

### Helicase Opens the Double Helix

Helicase acts early in DNA replication. Its job is to unwind the DNA double helix and separate the two strands by disrupting hydrogen bonds between complementary bases. A-T base pairs have two hydrogen bonds, while G-C base pairs have three. Because G-C pairs have more hydrogen bonds, DNA with a higher G-C content generally requires more energy to separate than DNA with a lower G-C content.

> **Key Term: helicase**
> An enzyme that unwinds the DNA double helix and separates the two parental strands during replication.

Once helicase has opened the DNA, each separated strand can act as a template. The exposed bases guide the placement of new complementary nucleotides. If the template sequence is 3′-TACGGA-5′, the new DNA strand made from it will be 5′-ATGCCT-3′. The strands are complementary and antiparallel.

The action of helicase creates the replication fork. At this fork, several processes occur together: helicase keeps opening the DNA, topoisomerase reduces twisting ahead of the fork, primase adds primers, DNA polymerase extends new DNA, and ligase seals fragments on one strand.

If helicase were missing or inhibited, the two DNA strands would not separate properly. DNA polymerase would have little or no single-stranded template to read, so DNA synthesis would stop or be greatly reduced. In an AP-style investigation question, this could appear as a decrease in newly synthesized DNA, failure to form replication forks, or accumulation of unreplicated DNA.

> **Exam Warning:**
> Do not say helicase “adds nucleotides.” Helicase separates DNA strands. DNA polymerase adds DNA nucleotides.

### DNA Polymerase Builds New Strands

DNA polymerase is the enzyme that performs the main DNA-building step. It reads a template strand and adds complementary DNA nucleotides to a growing new strand. However, DNA polymerase has a strict rule: it can add nucleotides only to the 3′ end of an existing strand. This means the new DNA strand grows in the 5′ to 3′ direction.

> **Key Term: DNA polymerase**
> An enzyme that adds complementary DNA nucleotides to the 3′ end of a growing DNA strand during replication.

This directionality is one of the most tested ideas in DNA replication. DNA has a sugar-phosphate backbone, and each strand has a 5′ end and a 3′ end. The 5′ end has a phosphate group attached to the 5′ carbon of deoxyribose. The 3′ end has a hydroxyl group attached to the 3′ carbon. DNA polymerase links the 5′ phosphate of an incoming nucleotide to the 3′ hydroxyl of the growing strand.

DNA polymerase also cannot begin a new strand from nothing. It needs a short existing strand with a free 3′ end. This starting piece is an RNA primer, made by primase. In many AP materials, primase may be described as RNA primase or RNA polymerase acting in primer synthesis. The key point is that a short RNA sequence is laid down first, and DNA polymerase extends from it.

> **Key Term: RNA primer**
> A short RNA sequence that provides a free 3′ end for DNA polymerase to begin adding DNA nucleotides.

After DNA synthesis, RNA primers are removed and replaced with DNA. The final replicated chromosome contains DNA, not RNA primers. The primer is temporary, but it is required because DNA polymerase cannot start synthesis on a bare template strand.

DNA polymerase is also the enzyme used in polymerase chain reaction, or PCR, a biotechnology method that amplifies DNA. PCR uses repeated cycles of DNA strand separation, primer binding, and DNA polymerase extension. Because PCR uses high temperatures to separate DNA strands, it requires a heat-stable DNA polymerase that will not denature during the heating steps. If one DNA molecule is copied perfectly each cycle, the number of copies after n cycles follows:

$$
\text{number of copies} = 2^n
$$

For example, after 10 cycles, there can be $$2^{10} = 1024$$ copies of the target DNA sequence. This is a common AP application of DNA replication.

> **Test Tip:**
> When explaining why a primer is needed, write that DNA polymerase can only add nucleotides to an existing strand with a free 3′ end. Avoid vague answers such as “the primer tells it where to go” unless you also explain the 3′ end.

### Leading Strand, Lagging Strand, and Ligase

The two DNA strands in the original double helix are antiparallel. One runs 5′ to 3′ in one direction, and the other runs 3′ to 5′ in the opposite direction. Because DNA polymerase can synthesize only 5′ to 3′, the two new strands are made differently.

On one template strand, DNA polymerase can follow the replication fork as helicase opens the DNA. This new strand is synthesized continuously and is called the leading strand. DNA polymerase reads the template in the 3′ to 5′ direction and builds the new complementary strand in the 5′ to 3′ direction.

> **Key Term: leading strand**
> The new DNA strand synthesized continuously in the 5′ to 3′ direction toward the replication fork.

The other new strand is more complicated. Its template is oriented so that DNA polymerase must work away from the replication fork. Since helicase keeps opening more DNA at the fork, this strand cannot be made in one continuous stretch. Instead, it is made in short segments called Okazaki fragments. Each fragment begins with an RNA primer and is extended by DNA polymerase.

> **Key Term: lagging strand**
> The new DNA strand synthesized discontinuously in short fragments because DNA polymerase can only add nucleotides 5′ to 3′.
>
> **Key Term: Okazaki fragments**
> Short DNA segments made during lagging strand synthesis that are later joined into one continuous strand.

DNA ligase then joins the Okazaki fragments together. Ligase forms bonds in the sugar-phosphate backbone, producing a continuous DNA strand. Without ligase, the lagging strand would remain as many separate fragments rather than one complete strand.

> **Key Term: DNA ligase**
> An enzyme that joins DNA fragments by sealing gaps in the sugar-phosphate backbone.

A common AP free-response setup involves adding a ligase inhibitor and asking which strand is most affected. The lagging strand is most affected because it depends on ligase to join Okazaki fragments. The leading strand is made continuously, so it has much less need for fragment joining.

> **Exam Warning:**
> Do not say the lagging strand is made “backward” in the sense that DNA polymerase synthesizes 3′ to 5′. DNA polymerase still synthesizes every new DNA strand 5′ to 3′. The lagging strand is discontinuous because it is built in short 5′ to 3′ fragments away from the fork.

The correct basic order of major enzyme action is: helicase opens DNA, primase places RNA primers, DNA polymerase extends new DNA, and ligase seals fragments. Topoisomerase acts as DNA is unwound to reduce supercoiling, so it is often described as acting near the start and ahead of the replication fork.

### Applying Enzyme Roles to AP-Style Questions

AP Biology questions often test DNA replication by asking what would happen if a specific enzyme were removed, inhibited, or changed. The easiest strategy is to connect each enzyme to its direct role.

If helicase is missing, DNA does not unwind properly. There will be little exposed template DNA, so new strand synthesis cannot proceed normally. If topoisomerase is missing, helicase may still separate strands, but supercoiling and tangling ahead of the fork will interfere with replication. If primase is missing, DNA polymerase cannot begin new strands because it has no primer with a free 3′ end.

If DNA polymerase is missing, complementary DNA nucleotides are not added to the new strands. Primers may exist, and the DNA may be unwound, but full DNA synthesis will not occur. If DNA ligase is missing, leading strand synthesis can still produce long stretches, and lagging strand synthesis can still produce Okazaki fragments, but those fragments will not be joined into a continuous strand.

Consider this AP-style prompt:

> **Question:**
> A researcher removes one enzyme from a DNA replication system. Helicase, primase, and DNA polymerase are still active. The researcher observes many short DNA/RNA fragments on one strand. Which enzyme is most likely missing?
>
> **Answer:**
> DNA ligase is most likely missing. Primase is present because RNA primers exist. DNA polymerase is present because DNA fragments have been made. Helicase is present because the strands have separated. The short fragments remain unjoined because ligase is absent.

Another common style asks why a fictional organism with a DNA polymerase that can synthesize both 5′ to 3′ and 3′ to 5′ might not need Okazaki fragments. The reason is that discontinuous lagging strand synthesis exists because normal DNA polymerase cannot synthesize in the 3′ to 5′ direction. If an enzyme could truly synthesize both ways, the strand that normally lags could potentially be copied continuously.

DNA replication can also be tested alongside DNA and RNA structure. DNA contains thymine, while RNA contains uracil. DNA usually has deoxyribose and is double-stranded; RNA usually has ribose and is single-stranded. In replication, RNA appears briefly as primers, but the final product is DNA. Do not confuse replication with transcription: replication copies DNA into DNA, while transcription uses RNA polymerase to make RNA from a DNA template.

## Key Point Checklist

This article has covered the following key knowledge points:

- DNA replication copies DNA before cell division.
- Replication is semiconservative: each daughter DNA molecule has one parental strand and one new strand.
- Helicase unwinds and separates DNA strands by disrupting hydrogen bonds between bases.
- Topoisomerase relieves supercoiling ahead of the replication fork.
- DNA polymerase adds complementary DNA nucleotides to the 3′ end of a growing strand.
- New DNA is always synthesized in the 5′ to 3′ direction.
- DNA polymerase needs an RNA primer because it cannot start a strand from nothing.
- The leading strand is synthesized continuously toward the replication fork.
- The lagging strand is synthesized discontinuously as Okazaki fragments.
- DNA ligase joins Okazaki fragments by sealing the sugar-phosphate backbone.
- A ligase inhibitor mainly affects the lagging strand.
- PCR applies DNA replication principles and uses heat-stable DNA polymerase to amplify DNA.

## Key Terms and Concepts

- DNA replication
- semiconservative replication
- replication fork
- topoisomerase
- helicase
- DNA polymerase
- RNA primer
- leading strand
- lagging strand
- Okazaki fragments
- DNA ligase


DNA replication - DNA polymerase, helicase, ligase