DNA REPLICATION

Helicase

Unwinds and separates the two parental DNA strands by breaking hydrogen bonds.

Topoisomerase

Prevents supercoiling and tension ahead of the replication fork so replication can continue smoothly.

Primase

Synthesizes short RNA primers to provide a starting point for DNA polymerase.

DNA Polymerase

Synthesizes new DNA strands by adding complementary nucleotides in the 5′ → 3′ direction and proofreads for errors.

Prokaryotes (E. coli) Pol III

main synthesis enzyme

Prokaryotes (E. coli): Pol I

replaces RNA primers with DNA

Eukaryotes Pol α

starts replication

Eukaryotes Pol δ / ε

elongation

DNA Ligase

Seals gaps between DNA fragments by forming phosphodiester bonds, creating a continuous strand.

Single-Strand Binding Proteins (SSBs)

Stabilize single DNA strands and prevent them from re-annealing or breaking.

Leading Strand

Allows continuous DNA synthesis toward the replication fork.

Lagging Strand

Enables DNA synthesis in the opposite direction through short segments.

5′ End

Indicates the direction of the strand; synthesis starts from this end.

3′ End

Provides the hydroxyl group where DNA polymerase adds nucleotides.

Origin of Replication

Starting site where DNA replication begins.

Replication Fork

Region where DNA is unwound and new strands are synthesized.

Okazaki Fragments

Allow DNA synthesis on the lagging strand in short, manageable segments.

Primer

Provides a free 3′-OH group required for DNA polymerase to begin synthesis.

Parental Strand

Serves as the template for complementary base pairing.

Daughter Strand(s)

Form the new DNA molecules that will be passed to daughter cells.

DNA Bases (A, T, C, G)

Store genetic information and allow accurate copying through complementary base pairing.

Hydrogen Bonds

Hold complementary bases together while allowing easy strand separation.

Hydrogen Bonds A-T

2 bonds

Hydrogen Bonds G-C

3 bonds

Deoxyribose

Forms part of the sugar-phosphate backbone, giving DNA structure and stability.

Phosphate Group

: Links nucleotides together to form the backbone of DNA and provides structural strength.