1. Parts of a Nucleotide

• Components:

  • Phosphate Group: Attached to the 5' carbon of the sugar.

  • Deoxyribose Sugar: A 5-carbon sugar (lacks an oxygen at the 2' position).

  • Nitrogenous Base: Can be adenine (A), thymine (T), cytosine (C), or guanine (G).

2. Structure of the Double Helix

• Antiparallel Strands: The two strands run in opposite directions (5' to 3' and 3' to 5').

• Hydrogen Bonds: A pairs with T (2 H-bonds), and C pairs with G (3 H-bonds), holding the two strands together.

• Backbone: Composed of alternating sugar and phosphate groups.

• Rungs of the Ladder: Formed by the nitrogenous base pairs connected by hydrogen bonds.

3. Semiconservative Copying Mechanism

• Semiconservative Replication: Each new DNA molecule consists of one original strand and one newly synthesized strand.

• How It Works:

  • When DNA is replicated, the two strands separate, and each serves as a template for a new complementary strand, ensuring that each daughter cell receives one original and one new strand.

4. Steps and Enzymes in DNA Replication

• Step 1: Unwinding the DNA

  • Enzyme: Helicase - unwinds and separates the two DNA strands.

• Step 2: Stabilizing Single Strands

  • Proteins: Single-stranded binding proteins - prevent the separated strands from re-annealing.

• Step 3: Starting Replication

  • Enzyme: Primase - synthesizes a short RNA primer complementary to the DNA template to provide a starting point for DNA synthesis.

• Step 4: Synthesizing New DNA

  • Enzyme: DNA Polymerase - adds nucleotides to the growing DNA strand, synthesizing new DNA in the 5' to 3' direction.

• Step 5: Joining Fragments

  • Enzyme: DNA Ligase - joins Okazaki fragments on the lagging strand by sealing gaps between fragments.

5. Leading vs. Lagging Strand

• Leading Strand:

  • Synthesized continuously in the same direction as the replication fork.

  • Requires only one RNA primer at the origin of replication.

• Lagging Strand:

  • Synthesized discontinuously away from the replication fork in short segments called Okazaki fragments.

  • Requires multiple RNA primers for each fragment.