A Level Biology Practice Exam

What is the outcome of semi-conservative replication?

1. Both daughter DNA molecules consist of two new strands

2. Each daughter DNA molecule contains one old strand and one new strand

3. The original DNA strand is completely destroyed

4. Only one new strand is formed in the daughter DNA molecules

The correct answer is: Each daughter DNA molecule contains one old strand and one new strand

The outcome of semi-conservative replication is that each daughter DNA molecule contains one old strand and one new strand. This process was demonstrated by the Meselson-Stahl experiment and is fundamental to DNA replication in all living organisms. During replication, the double-stranded DNA unwinds and separates into two single strands. Each of these original strands serves as a template for the formation of a new complementary strand, resulting in two DNA molecules, each composed of one parental (old) strand and one newly synthesized strand. This method of replication ensures that the genetic information is accurately passed on while minimizing the chances of errors or mutations in the new strands. In contrast, if both daughter DNA molecules consisted of two completely new strands, that would imply a mechanism of replication that is not semi-conservative. Similarly, the complete destruction of the original DNA strand would contradict the semi-conservative nature, as this process relies on retaining half of the original DNA. Furthermore, forming only one new strand in the daughter molecules would not align with the semi-conservative requirement of pairing each original strand with a new complementary strand.