1.5.2 DNA Replication

• Why is semi-conservative replication important and describe its process ?

  • DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix - Both strands act as templates
  • Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
  • Hydrogen bonds form between adenine-thymine and guanine-cytosine
  • DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
  • Forming phosphodiester bonds
  • Semi-conservative - each new DNA molecule consists of one original / template strand and one new strand

  
• Why is semi-conservative replication important?

  • Ensures genetic continuity between generations of cells
• Use your knowledge of enzyme action to suggest why DNA polymerase moves in opposite directions along DNA strands?

  ● DNA has antiparallel strands ● So shapes / arrangements of nucleotides on two ends are different ● DNA polymerase is an enzyme with a specific shaped active site ● So can only bind to substrate with complementary shape (phosphate end of developing strand)
• Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication ?

  Watson and Crick
• Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication?

  - Used bacteria grown in broth with either heavy nitrogen (15N) or light nitrogen (14N) → bacteria use the nitrogen to make bases for DNA → DNA with only 15N settles out near the bottom of a centrifuge tube, DNA with only 14N settles out near the top → if DNA has one 15N strand and one 14N strand it settles out in the middle • Experiment showed that DNA replication is semi-conservative, not conservative

  
• evidence from scientist

  15N- bottom= 2 heavy chain

  add 14Nand divide once- middle= 1 light chain and 1 heavy chain

  14N divides again- 1 heavy chain, 1 light chain (settles in the middle)

  2 light chain ( settle at top)

  
• Exam insight: common mistakes ❌-(1/5)


  ❌-“DNA polymerase forms hydrogen bonds / joins together complementary bases.”

  ✅-DNA polymerase joins adjacent (next to, not opposite) nucleotides, forming phosphodiester bonds.
• Exam insight: common mistakes ❌-(2/5)


  ❌-“Hydrogen bonds are hydrolysed.”

  ✅-Breaking of hydrogen bonds is not a hydrolysis reaction.
• Exam insight: common mistakes ❌-(3/5)


  ❌-“Helicase unzips the double helix.”

  ✅-This is too vague. DNA helicase breaks hydrogen bonds.
• Exam insight: common mistakes ❌-(4/5)


  ❌-“DNA polymerase forms phosphodiester bonds between bases.”

  ✅-DNA polymerase forms phosphodiester bonds between the phosphate group of one nucleotide & the deoxyribose of another.
• Exam insight: common mistakes ❌-(5/5)


  ❌-“Free bases attach to exposed bases.”

  ✅-Free nucleotides attach to exposed nucleotide bases via complementary base pairing, but not the bases on their own.