DNA structure, functions, replication

Key Notes: DNA Structure, Function, and Replication

DNA Structure

• Shape: Double helix, resembling a twisted ladder.

• Components:

  • Rails of the ladder: Sugar-phosphate backbone.

  • Steps of the ladder: Nitrogenous base pairs (A-T, G-C).

• Nucleotides: DNA is made of nucleotide monomers, each consisting of:

  • Sugar (deoxyribose).

  • Phosphate group.

  • Nitrogen base (A, T, G, or C).

Chargaff’s Rules

• Erwin Chargaff (mid-1900s) studied nitrogen bases in DNA.

• Findings:

  • Adenine (A) ≈ Thymine (T).

  • Guanine (G) ≈ Cytosine (C).

• Significance: Helped confirm complementary base pairing in DNA structure.

Watson & Crick’s Double Helix Model

• Discovered DNA’s double-helix shape (1953).

• Based on Rosalind Franklin’s X-ray crystallography images.

• Base pairing rules maintain the uniform structure of DNA:

  • A pairs with T (2 hydrogen bonds).

  • G pairs with C (3 hydrogen bonds).

• Hydrogen bonds hold the two strands together.

DNA Replication

• Occurs during the S phase of the cell cycle.

• Steps:

  1. DNA helicase unwinds the double helix by breaking hydrogen bonds.

  2. DNA polymerase reads the DNA strands and builds new complementary strands.

  3. Each new DNA molecule contains one original strand and one new strand (semi-conservative replication).

• DNA polymerase also proofreads and corrects mistakes during replication.

Key Takeaways

• Chargaff’s Rules: A=T, G=C.

• DNA is a double helix, discovered by Watson & Crick.

• Complementary base pairing ensures structural stability.

• DNA replication is semi-conservative, meaning each new DNA molecule retains one strand from the original.