Introduction: What is DNA Replication?

Our bodies all began as a single fertilized egg. Through repeated rounds of cell division, that single cell develops into roughly 37 trillion cells. Amazingly, almost every single one of these cells carries the exact same genetic information (DNA).
Before a cell divides, it must create a complete, identical copy of its DNA beforehand. This process is called DNA replication. You might think, "There are so many enzyme and molecule names—this looks tough..." But don't worry! Once you master the fundamental rules, it all fits together neatly like a puzzle.

*Note: The process of using this DNA genetic information to produce proteins (transcription and translation) will be covered in detail in a separate chapter: "Transcription, Translation, and Splicing."

1. Reviewing the Basics: DNA Structure and Essential Rules

Before we dive into the mechanism of DNA replication, let's review the essential foundational rules of DNA structure.

① Complementary Base Pairing

DNA consists of long chains of nucleotides, forming a double helix where two strands twist around each other face-to-face. The bases facing each other across the strands bond only with specific partners.
・Adenine (A) pairs with Thymine (T)
・Guanine (G) pairs with Cytosine (C)
This rule is known as base complementarity. If one strand has the sequence "A-G-C-T", the other strand is automatically determined as "T-C-G-A". It is precisely this property that allows DNA to make an exact duplicate of itself.

② Directionality of the Strands (\(5'\) and \(3'\) Ends)

The carbon atoms in the deoxyribose (sugar) of a nucleotide are numbered (\(1'\) to \(5'\)). The end attached to a phosphate group is called the \(5'\) end (five-prime end), and the end with the free hydroxyl group (\(-OH\)) on the sugar is called the \(3'\) end (three-prime end).
The two strands of DNA run in opposite directions to each other (they are antiparallel). If one strand runs "\(5' \to 3'\) from top to bottom," its partner runs "\(3' \to 5'\) from top to bottom."

[Key Takeaway]
DNA strands have directionality, and the two strands face each other in opposite directions (antiparallel). Furthermore, bases always pair up specifically: A with T, and G with C!

2. Semiconservative Replication

When DNA replicates, the original double helix unwinds and separates into two single strands. Then, using each single strand as a template (a pattern or guide), a complementary new strand is synthesized.
As a result, each of the two newly formed DNA double helices consists of one original template strand and one newly synthesized strand. Because half of the original DNA molecule is conserved in each new double helix, this mode of replication is termed semiconservative replication.

[Trivia / Exam Tip]
The proof for semiconservative replication came from the famous Meselson–Stahl experiment, in which E. coli was cultured over multiple generations in isotopes of nitrogen (heavy nitrogen \(^{15}\mathrm{N}\) and normal nitrogen \(^{14}\mathrm{N}\)) followed by density gradient centrifugation. This is a very frequent topic in data-interpretation questions on the Common Test!

3. Detailed Mechanism of DNA Replication

What enzymes are hard at work at the replication site? The star of the show is the enzyme DNA polymerase.

The Star: Two Major Rules of DNA Polymerase

DNA polymerase has two extremely important characteristics (or "quirks"). These are among the most frequently tested points on the Common Test!

Rule 1: It can only synthesize a new strand in the \(5' \to 3'\) direction
DNA polymerase can only attach incoming nucleotides to the \(3'\) end of a growing strand. This means the direction of synthesis for the new strand is always strictly \(5' \to 3'\) (which means it reads the template strand in the \(3' \to 5'\) direction).

Rule 2: It cannot start synthesis from scratch (out of nowhere)
DNA polymerase cannot begin synthesis without an existing foothold of nucleotides. Therefore, a short starter strand of RNA is synthesized first. This foothold is called a primer. DNA polymerase recognizes the \(3'\) end of this primer and starts elongating the DNA strand from there.

[Common Mistake]
Many test-takers mix up whether the direction of synthesis is "\(5' \to 3'\)" or "\(3' \to 5'\)" on the actual exam!
A great memory tip is: "Synthesis says GO! (start at 5, go to 3!)" The newly formed strand always grows in the \(5' \to 3'\) direction.

Continuous and Discontinuous Synthesis

Because the two DNA strands are antiparallel, an interesting drama unfolds as the double helix unwinds.

① Leading Strand (Continuous Synthesis)
This strand is synthesized continuously in the exact same direction as the unwinding replication fork (which is \(5' \to 3'\) for the new strand). Once a single primer is laid down at the start, DNA polymerase can ride the unwinding wave and synthesize smoothly without stopping.

② Lagging Strand (Discontinuous Synthesis)
On the other strand, the direction the fork is unwinding and the required direction of DNA polymerase synthesis (\(5' \to 3'\)) are in opposite directions. As a result, every time the fork opens up a bit, a new primer must be laid down, and DNA is synthesized in short bursts moving away from the fork. It is a backstitching, piece-by-piece process.
These short, newly synthesized DNA pieces are called Okazaki fragments (named after the Japanese researchers Reiji and Tsuneko Okazaki who discovered them—a term used globally!).

Finishing Up: Forming One Continuous Strand

Eventually, the RNA primers between the Okazaki fragments on the lagging strand are degraded, removed, and replaced with DNA. Then, these separate DNA fragments are joined together seamlessly by the enzyme DNA ligase, completing a continuous strand.

[Summary: Steps of DNA Replication]
1. The double-stranded DNA unwinds.
2. A short primer (RNA) is synthesized to serve as a starting block.
3. DNA polymerase synthesizes the new strand strictly in the \(5' \to 3'\) direction.
　・Synthesized continuously toward the replication fork = Leading strand
　・Synthesized discontinuously in fragments away from the fork = Lagging strand (forms Okazaki fragments)
4. Primers are replaced with DNA, and fragments are sealed together by DNA ligase to produce two identical double helices.

4. The Cell Cycle and the Timing of DNA Replication

The sequence of events from one cell division to the next is known as the cell cycle. DNA replication does not just occur at any random time during this cycle.

The cell cycle is broadly divided into the mitotic phase (M phase) and interphase. Interphase is further subdivided into three stages:
・\(G_1\) phase (First gap phase): Preparation period for DNA synthesis (building necessary materials)
・S phase (Synthesis phase): This is when DNA replication occurs!
・\(G_2\) phase (Second gap phase): Preparation period for cell division
Consequently, after passing through the S phase, the amount of DNA inside the cell is doubled.

[Common Mistake]
Students often mistakenly assume that "DNA replication occurs during division (M phase)." However, the M phase is when already-duplicated and condensed chromosomes are distributed to daughter cells. Make sure to remember that DNA replication takes place during the "S phase" of interphase, well before division begins!

Quick Check! Q&A

Q1. In which direction does DNA polymerase synthesize a new nucleotide strand: \(5' \to 3'\) or \(3' \to 5'\)?
A1. In the \(5' \to 3'\) direction.

Q2. What is the name of the short DNA fragments synthesized on the lagging strand?
A2. Okazaki fragments.

Q3. What do we call the mode of replication where each replicated DNA molecule consists of one original strand and one newly synthesized strand?
A3. Semiconservative replication.

Q4. What is the name of the short RNA strand required as an initial starting block for DNA polymerase to begin synthesis?
A4. Primer.