Evidence For Semi-conservative Replication

Question 1

Describe the difference in mass between DNA containing 14-N and 15-N.

Answer 1

DNA containing the heavy isotope 15-N is slightly heavier than DNA containing the normal isotope 14-N.

Question 2

Describe and explain the results for the evidence of semi-conservative replication.

Answer 2

1. Initially all DNA contains 15-N
   —> mass at bottom of test tube
2. After 1st centrifugation, all DNA are 15-N/14-N hybrids
   —> mass in middle of test tube
3. After 2nd centrifugation, 50% DNA are hybrids and 50% contains 14-N only
   —> mass of 14-N at top of test tube and mass of hybrids in middle of test tube
4. After 3rd centrifugation, 25% are hybrids and 75% are 14-N
   —> mass of 14-N at top of test tube and mass of hybrids in middle of test tube.