Meselson and Weigle's experiment using lambda (λ) phage

Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
Coupling and repulsion hypothesis
A testcross in maize
Crossing over and meiosis 
Crossing over and chiasma formation
Mechanism of genetic recombination
Crossing over and linkage maps 
Recombination frequencies from a test-cross
Recombination frequencies from F2 data
Interference and coincidence
Linkage maps
Mapping function and poisson distribution
Linkage groups
Chi-square test 
Cytological basis of crossing over
Creighton and McClintock's experiment in corn
Meselson and Weigle's experiment using lambda (λ) phage
Crossing over at four strand stage

Meselson and Weigle's experiment using lambda (λ) phage
In 1961, Matthew Meselson and Jean Weigle

reported results of an experiment, which proved that crossing over also involves breakage and reunion of DNA molecules (rather than of chromosomes, as shown for fruitfly and corn in two experiments described earlier). In their experiment, Meselson and Weigle used the following two strains of λ phage, (i) One strain had the genetic markers c and mi at one end of the chromosome, and the DNA of this chromosome was ‘heavy’ because the phage strain was produced from E. coli cells grown on medium containing 13C and 15N (heavier isotopes), (ii) The second strain was ++ for the markers and had light DNA.

The above two strains of λwere used for simultaneous double infection of E. coli cells grown on a medium containing 12C and 14N. After lysis phage particles were analysed by spinning in a caesium chloride density gradient.

A wide band was observed, which indicated that the phage DNA ranged in density from the heavy parental value to the light parental value, with several intermediate densities (Fig. 10.18). Recombinant phages c + were also recovered, which had a density close to heavy parental value. This could be explained because only a small tip of chromosome carrying + allele of mi would come from the light parental chromosome. In the reciprocal cross, when heavy ++ phages were crossed with light c mi, heavy recombinants were + mi and light recombinant phages were c + as expected. These experiments suggested physical breakage and reunion of DNA leading to recombination.
Evidence of breakage and reunion of DNA in λ phage, during recombination.
Fig. 10.18. Evidence of breakage and reunion of DNA in λphage, during recombination.