First experimental proof for semi-conservative DNA replication was shown in which of the following organism ?

Escherichia coli.

The correct answer is Otion (3)- Escherichia coli.

During replication, the two strands would separate and act as a template for the synthesis of new complementary strands. After the completion of replication, each DNA molecule would have one parental and one newly synthesised strand. This scheme was termed as semiconservative DNA replication. It is now proven that DNA replicates semiconservatively. It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells.