T2L8 DNA Replication + Meselson and Stahl

Question 1

Describe the process of DNA replication

Answer 1

• DNA Helicase enzyme unwinds the double helix - breaking the hydrogen binds between complimentary pairs
  -2 strands are now separate
• Free nucleotides in the nucleus come and line up with the two strands
• DNA polymerase joins up adjacent nucleotides forming phosphodiester bonds (condensation reactions)
  -Opposite nucleotides form hydrogen bonds
  -Two DNA molecules - each with one old strand and one new strand - this is called semi conservative replication

Question 2

Name the three possible theories for DNA replication

Answer 2

Conservative
Semiconservative
Fragmentary

Question 3

Describe the process in the meselson and stahl experiment

Answer 3

E coli grown in N15 medium
Sample taken

E coli transferred into N14 for one division
Sample taken

E coli left for another division
Sample taken

N14 control sample

Added to caesium chloride solution and spun in a centrifuge

Question 4

Describe the results from the M+S experiment

Answer 4

Higher = lighter

x1 has middle because 1 strand is n14 and one is n15 (supports semi conservative

x2 has middle and high because some are half and half and some newer DNA molecules are only N14