In semi-conservative replication, how many old strands are present in each new DNA molecule?

In semi-conservative replication, how many old strands are present in each new DNA molecule?

• A. One old strand
• B. Two old strands
• C. No old strands
• D. Three old strands

Answer: (A)

In semiconservative replication, each new DNA molecule contains one old strand from the original double helix and one newly synthesized strand. The two parental strands separate, and each serves as a template for building a complementary strand. So every daughter molecule ends up with one old strand and one new strand. That’s why one old strand is the correct count. The other ideas would imply a different replication pattern (two old strands would be a fully conservative model, no old strands isn’t possible with template-based synthesis, and three old strands can’t occur in a two-stranded molecule).

In semiconservative replication, each new DNA molecule contains one old strand from the original double helix and one newly synthesized strand. The two parental strands separate, and each serves as a template for building a complementary strand. So every daughter molecule ends up with one old strand and one new strand. That’s why one old strand is the correct count. The other ideas would imply a different replication pattern (two old strands would be a fully conservative model, no old strands isn’t possible with template-based synthesis, and three old strands can’t occur in a two-stranded molecule).