The experiment of figure 17.3 is based upon harlequin staining technique. In this experiment the chromosome is treated with BrdU (5-bromodeoxyuridine). This nucleotide analog binds to different degree with the stains as compared to the normal chromosomes. When the BrdU treated chromosome are stained then the BrdU containing strand fluoresce brightly than the normal chromatid because it gets less stained due to the presence of BrdU.
Hence, the BrdU inhibits the binding of Giemsa to the chromatids.
In the observations of figure 17.11, the solid red phenotype was due excision of the transposable element (which is Ds locus in this case). The Ds was when present next to the C allele, it produced colorless kernels by suppressing the property of C allele of producing red phenotype. But when it was excised off to some other site then the kernel again became reD.We came to this conclusion by the following When the crossing of female eggs (containing Ds bound C allele) was done with male eggs (containing free C allele) then one of the resultant daughter chromosomes had changes. The change was that in one of the chromosome, the Ds locus has left its site near C allele and has excised off to some other site. This made C allele again active and it expressed its property of providing red color to the kernel. This observation is shown below in the diagram:
Hence, from the above explanation we concluded that the solid red phenotype is due to the excision of the transposable element.
Differences between the Holliday model and the double strand break model :
• In Holliday model, during strand invasion and intertwining of the complementary strands of two homologs with similar chromatids occurs. An X-shaped connection is formed between them that is called Holliday junction.
While in case of double strand break model, the homologous chromatids are intertwined by forming a D loop in between them.
• During the formation of Holliday junction, one strand of each homologous chromatid is nicked at similar points, which then undergo Holliday junction formation.
While in case of double strand break model, both the strands of the same chromatid (recessive chromatid) are nicked, which then undergo initiation of D loop formation by the unnicked strand of the dominant chromatiD.• The Holliday junction formed migrate from the point of its formation to the opposite end of the strand leading to the complete reversal of the base pairs of one strand each of the two homolog chromatids. But the D loop forms at a certain point and do not migrate from its generation point.