2.2 - DNA replication

Question 1

Results of the Meselson and Stahl experiment (first and second replication)

Answer 1

• original and heavy strands separate
• each heavy strand acts as a template for a new complementary strand
• new, light strands form alongside the original strands
• each one results in a DNA molecule of one old, heavy and one new, light strand
• the second replication has original and 2 new strands act as templates
• light strands form
• half of them have one original and one new strand and other are light

Question 2

Meselson and Stahl experiment

Answer 2

1. 2 different isotopes of nitrogen were used (14 and 15 N)
2. Bacteria were grown in 15 N medium (heavy DNA)
3. and they were transferred to a medium with 14N for a round of replication
4. DNA was extracterd and centrifuged
5. steps 3-4 were repeated for another replication round
6. distribution of heavy and light DNA was analysed

Question 3

Process of semi-conservative replication

Answer 3

1. DNA molecule is broken down by DNA helicase - breaks hydrogen bonds between complementary bases - unwinds the double helix and separates the strands
2. Each strand acts as a template as free nucleotides attract to their complementary bases
3. DNA polymerase joins the free nucleotides together via condensation reactions in the 5’ to 3’ direction - forms phosphodiester bonds to create a sugar phosphate backbone of the new DNA strand
4. Two identical copies of DNA are made - each one has an original strand of the DNA and a new strand

Question 4

How is DNA replication done?

Answer 4

Through semi-conservative replication