1.) BrdU is used to trick cells into using it, rather than thymidine, in their DNA. This occurs during the S phase of the cell cycle. The DNA formed with BrdU has unique staining properties. BrdU causes light and dark regions to appear on chromosomes and we can then see crossing over. One round of duplication with BrdU causes no discernible differences in chromosomes. The second round of duplication is where the differences are seen. One of the sister chromatids will look visually normal while the other does not.
2.) A small number of changes in DNA sequence can be beneficial. Sometimes the changes go unnoticed for several generations but eventually accumulate to create a unique trait in an individual. These changes are part of what leads to diversity in organisms. The random mutation in DNA could possibly code for such a unique trait that the organism can benefit from it and aid in the organisms survival.
3.) Protein folding disorders could potentially cause two diseases from one protein. If the protein has a site that is prone to change its folding pattern then t...