Chapter 7, Problem 24P

Bromodeoxyuridine (BrdU) is a synthetic nucleoside that can be incorporated into newly synthesized DNA in place of thymine (T). Researchers can add BrdU to medium in which cells are growing, and then they can detect its presence in chromosomes by staining with an antibody specific to BrdU.

a.	The following figure at the left shows metaphase chromosomes isolated from somatic cells grown for some time in the presence of BrdU. Darker staining indicates more BrdU. For how many cell generations have these cells been growing in the presence of BrdU?
b.	So-called harlequin chromosomes (similar to those at the right of the figure) can be isolated from cells grown in exactly the same way as those at the left, if the cells are exposed to X-rays a few hours before their chromosomes are prepared for analysis. The X-rays induce double-strand breaks in the DNA. What kind of repair process are you witnessing in these harlequin chromosomes?
c.	At what point in the cell cycle did the repair process described in part (b) take place? Explain.
d.	Bloom syndrome is an autosomal recessive disease in humans characterized by short stature, distinctive facial features, and an increased risk of cancer. The preceding figure at the right actually shows cells of Bloom syndrome patients grown in BrdU for the proper number of cell generations; almost all of these cells display harlequin chromosomes even though the cells were not subjected to X-rays. What do you think is the function of the protein specified by the wild-type allele of the Bloom syndrome gene (called BLM +)?