Semi-conservative replication of chromosomes in eukaryotes
Autoradiography experiment in Vicia faba. Autoradiography was also utilized by J.H. Taylor and his co-workers for the study of duplicating chromosomes in the root tip cells of Vicia faba. Results of such experiments were first published in 1957. After incorporation of tritiated thymidine, when root tips were transferred to unlabellcd medium, in the first generation of duplication both chromatids were labelled (interpreted as one DNA double helix in each chromatid, and only one of the two strands labelled). In the second cycle of duplication, in each chromosome, one of the two chromatids was found to be labelled. This was interpreted as showing semi-conservative mode of duplication (Fig. 26.4).

Taylor's experiment is important in so far as it demonstrated the semnconservative mode of replication in chromosomes of a higher plant. But they should not necessary mean semi-conservative mode of replication of DNA, because the organization of DNA in chromosomes of higher plants and animals is rather complex and not as simple as envisaged by Taylor, while interpreting his results.

Use of harlequin chromosomes. Semi-conservative replication of chromosomes can also be visualized through an examination of chromosomes that are allowed two rounds of replication in a medium containing bromodeoxy-uridine. These replicated chromosomes are then stained with a fluorescent dye and Giemsa stain to produce what are described as harlequin chromosomes (Fig. 26.5). The newly synthesized DNA stains differently than the parental DNA, so that the harlequin chromosomes have been found suitable for a study of (i) semi-conservative replication, (ii) crossing over and (iii) sister chromatid exchanges.