DNA Replication

What Is DNA Replication?

• DNA replication: The process of copying DNA.

• Purpose: Makes an identical copy of DNA before cell division.

• Location: Nucleus of eukaryotic cells.

• Semiconservative: Each new DNA molecule has:

  • 1 old/original strand

  • 1 new strand

• Direction: New DNA is always made 5’ → 3’.

Base Pairing

• Adenine (A) → Thymine (T)

  • 2 hydrogen bonds

• Cytosine (C) → Guanine (G)

  • 3 hydrogen bonds

Nitrogenous Bases

• Purines = 2 rings

  • Adenine (A)

  • Guanine (G)

• Pyrimidines = 1 ring

  • Thymine (T)

  • Cytosine (C)

Important Enzymes

Helicase

• Unzips DNA

• Breaks hydrogen bonds between bases.

Topoisomerase

• Relieves DNA tension

• Prevents too much twisting/supercoiling ahead of the replication fork.

SSBPs

• Hold DNA strands apart

• Prevent the strands from joining back together.

Primase

• Makes a short RNA primer.

• Gives DNA polymerase a place to start.

DNA Polymerase

• Adds new DNA nucleotides.

• Builds DNA 5’ → 3’.

• Proofreads and fixes mistakes.

• Replaces RNA primers with DNA.

DNA Ligase

• Glues DNA pieces together.

• Joins Okazaki fragments on the lagging strand.

• Seals breaks in the sugar-phosphate backbone.

4. Important Structures

Replication Fork

• Y-shaped area where DNA is opened and copied.

Leading Strand

• Made continuously.

• Made toward the replication fork.

• Always built 5’ → 3’.

Lagging Strand

• Made in pieces.

• Pieces are called Okazaki fragments.

• Fragments are made 5’ → 3’.

• DNA ligase joins the fragments together.

Phosphodiester Bonds

• Bonds that connect nucleotides together in the DNA backbone.

Steps of DNA Replication

Step 1 — Unzip

• Helicase separates the DNA strands.

• SSBPs keep the strands apart.

• Topoisomerase reduces twisting/tension.

Step 2 — Start

• Primase adds RNA primers.

• Free nucleotides match the template:

  • A → T

  • C → G

Step 3 — Build

• DNA polymerase adds nucleotides.

• DNA is always built 5’ → 3’.

• Leading strand = continuous.

• Lagging strand = Okazaki fragments.

Step 4 — Finish

• DNA polymerase replaces RNA primers with DNA.

• DNA ligase joins Okazaki fragments.

• Result = 2 identical DNA molecules.

Historical DNA Discoveries

Frederick Griffith

• Discovered a transforming principle in bacteria.

Oswald Avery

• Showed that DNA was responsible for bacterial transformation.

Hershey & Chase

• Confirmed that DNA is genetic material using bacteriophages.

Erwin Chargaff

• Discovered:

  • A = T

  • C = G

Rosalind Franklin & Maurice Wilkins

• Used X-ray crystallography.

• Franklin’s famous image was Photograph 51.

Watson & Crick

• Built the 3D double-helix model of DNA.

• Used Chargaff’s rules and Franklin’s X-ray data.