Question

When John Cairns first visualized the replication of E. coli chromosomes in 1963 what did he conclude?

1. The chromosomes of E. coli are circular structures that exist as θ-shaped intermediates during replication.
2. The unwinding of the two complementary parental strands and their semiconservative replication occur simultaneously or are closely coupled.
3. Semiconservative replication of the E. coli chromosome started at a specific site, and proceeded sequentially and unidirectionally around the circular structure

Answer 1

John Cairns visualized E. coli chromosome replication in 1963 and determined that the chromosomes are circular with theta-shaped intermediates during semiconservative replication, starting from a single origin and proceeding bidirectionally.

Step-by-step explanation:

When John Cairns first visualized the replication of E. coli chromosomes in 1963, he concluded several key points about the nature of DNA replication in this organism. Firstly, he observed that the chromosomes of E. coli are circular structures that exist as theta-shaped intermediates during replication, which led to the definition of these structures as 'theta images.' Cairns identified that replication starts at a single origin of replication on the bacterial chromosome and proceeds around the circle to completion. The experiments done by others, such as Matthew Meselson and Franklin Stahl, supported the idea of semiconservative replication, where each strand of the DNA serves as a template for a new complementary strand, resulting in two DNA molecules that each include one parental strand and one new strand.

In summary, Cairns' work and subsequent experiments elucidated that the DNA replication process in E. coli is semiconservative, has a single origin of replication, and the replication machinery utilizes specific nucleotide sequences at the origin to begin the replication process, proceeding bidirectionally.