The chromosomes of eukaryotes cells can be observed at the metaphasis. They show no sign of fluorescence. (Figure 1A).

Eukaryotes cells are grown in a medium containing a fluorescent label. During the first metaphase, some of the cells are mounted on a microscope's slide and the chromosomes are observed. As shown on Figure 1B, the chromosomes are all totally fluorescent. The rest of the culture is put in a 'cold' medium (without fluorescent dye) for further divisions and observation (Figure 1C). The observation shows that all the chromosomes after the second cell division have one fluorescent chromatid and one non fluorescent chromatid.

In the conservative replication, one strand of the native DNA is copied, and this copy is used as the template for the synthesis of the second strand. As a result, the original strand is conserved.

If we observe the chromosomes after the first cell division (in the hypothesis of a conservative replication), they should be half fluorescent.

If the model of conservative replication was correct we would never observe a chromosome with both of its arms labelled. What would happen is that fluorescence would be incorporated only in the sister DNA, and would only be found in one of the arms.

The illustration shows what happens in the hypothesis of a conservative replication, at the molecular level.

Figure 2: When DNA is replicated in presence of a fluorescent dye, in the case of conservative replication hypothesis, the newly formed DNA has both strands labelled (B), and the original DNA is conserved (not fluorescent). If we observe the chromosomes, they have one fluorescent chromatid and a non fluorescent one (C).

According to the model of semi-conservative replication, each DNA strand is used as a template for the synthesis of a new strand of DNA. The two newly formed DNA strands are made of one original strand and one newly synthetised strand.

If the semi-conservative model is correct then, during the experiment it would be possible to have fluorescence incorporated into both sister DNA molecules at stage B, in which case both arms of the chromosome will be labelled (Figure 3C).

When the experiment is performed, the observations confirm the semi-conservative replication model. One can observe the 'totally labelled' chromosome predicted by this model.