DNA Replication

DNA replication is the process by which DNA makes a copy of itself during cell division. This ensures that each daughter cell has an identical copy of the genetic material.

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Semi-conservative Replication

The term "semi-conservative" means that each new DNA molecule contains:

• One strand from the original DNA molecule
• One newly synthesized strand

Requirements

For DNA replication to occur, several components are needed:

1. Nucleotides

   • All four types (A, G, C, T)
   • Free nucleotides in the nucleus
   • Must be complementary to template

2. Enzymes

   • DNA helicase
   • DNA polymerase
   • Other supporting enzymes

3. Energy

   • Chemical energy source
   • Drives the process
   • ATP required

4. Template DNA

   • Original DNA strands
   • Acts as pattern for new DNA

The Replication Process

DNA replication occurs in a specific sequence:

1. Unwinding

• DNA helicase breaks hydrogen bonds
• Double helix separates
• Creates replication fork

2. Template Exposure

• Both strands act as templates
• Bases become exposed
• Ready for complementary pairing

3. Nucleotide Addition

• Free nucleotides align
• Match by base pairing rules:
  • A with T
  • G with C
• Forms complementary strands

4. Strand Synthesis

• DNA polymerase joins nucleotides
• Forms phosphodiester bonds
• Works in 5' to 3' direction
• Creates sugar-phosphate backbone

5. Completion

• Two identical DNA molecules form
• Each contains one old and one new strand
• Hydrogen bonds stabilize structure

Directional Synthesis

DNA synthesis has important directional constraints:

1. Antiparallel Strands

   • DNA strands run in opposite directions
   • One 5' to 3', other 3' to 5'
   • Named by carbon numbers in sugar

2. DNA Polymerase

   • Only works 5' to 3' direction
   • Can only add to 3' end
   • Creates directional challenge

3. Leading Strand

   • Continuous synthesis
   • Follows replication fork
   • 5' to 3' direction

4. Lagging Strand

   • Discontinuous synthesis
   • Made in fragments
   • Later joined together

Learning Objectives

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Key Takeaways

• DNA replication is semi-conservative
• Process requires specific enzymes and components
• Synthesis occurs in 5' to 3' direction
• Leading and lagging strands form differently
• Ensures accurate genetic inheritance