Topic 6.2 Notes – DNA Replication

1. How DNA Replication Preserves Genetic Information

Every time a cell divides, it must pass on an accurate copy of its DNA. Replication makes that possible.

Complementary Base Pairing

DNA has specific pairing rules:

• A pairs with T
• C pairs with G

Because of this, if you know one strand’s sequence, you can determine the other. Each strand carries the information needed to rebuild its partner.

Semiconservative Replication

Replication is semiconservative, meaning:

• Each new DNA molecule contains:
  • One original (parental) strand
  • One newly synthesized strand

Each parental strand acts as a template for building a complementary strand. The result is two DNA molecules that are identical in sequence.

This is why replication preserves genetic continuity. The original information guides the creation of the new strand.

Students sometimes confuse semiconservative with “completely new” copies. Remember, half of each molecule is old DNA.

2. The Direction and Structure That Control Replication

The structure of DNA determines how replication works.

Antiparallel Strands

DNA strands run in opposite directions:

• One strand runs 5’ → 3’
• The other runs 3’ → 5’

DNA polymerase can only add nucleotides to the 3’ end of a growing strand. That means all new DNA is synthesized 5’ → 3’.

This single rule explains why one strand is copied smoothly and the other in pieces.

The Replication Fork

When replication begins, the double helix opens up, creating a Y-shaped region where copying occurs.

DNA replication fork with leading and lagging strands

The Y-shaped opening is called the replication fork. Each separated strand serves as a template.

At the fork, one new strand is synthesized continuously toward the opening, called the leading strand. The other is synthesized in short segments away from the fork, called the lagging strand.

Replication moves outward from this fork as enzymes coordinate the copying process.

3. The Key Enzymes of DNA Replication

You should know what each enzyme does and how they work together.

Helicase

• Breaks hydrogen bonds between base pairs
• Unwinds the double helix
• Creates the replication fork

Topoisomerase

• Relieves tension ahead of the fork
• Prevents DNA from becoming overly twisted (supercoiled) as it unwinds

If this didn’t happen, the DNA would tighten up like a twisted rubber band.

Primase

• Lays down short RNA primers on the DNA template (primase is a specialized RNA polymerase, but it is not the RNA polymerase used in transcription)
• DNA polymerase cannot start from nothing
• Provides a free 3’ OH group for DNA polymerase to extend

This is a common quiz question. DNA polymerase always needs a primer.

DNA Polymerase

• Adds DNA nucleotides to the 3’ end
• Synthesizes DNA 5’ → 3’
• Uses complementary base pairing
• Proofreads and corrects mismatches

DNA polymerase is central to the entire process.

Ligase

• Seals gaps in the sugar-phosphate backbone
• Joins DNA fragments on the lagging strand

4. Leading and Lagging Strands

Because DNA polymerase only works 5’ → 3’, the two strands are synthesized differently.

Okazaki fragments are short DNA segments that must be joined together by ligase.

On exams, you might see a diagram with arrows and have to determine which strand is leading. Always check the direction of synthesis, not just left versus right on the page.

5. Accuracy, Proofreading, and Mutations

DNA replication is extremely accurate because:

• Complementary base pairing reduces mistakes
• DNA polymerase proofreads and removes incorrect bases

This accuracy maintains genetic stability across cell divisions.

When errors escape proofreading, they become mutations. These can:

• Have no effect
• Alter protein structure or function
• Cause disease
• Create genetic variation for evolution

Replication balances stability with the possibility of change. That balance is central to inheritance and evolution.