Chapter 12, Problem 31P

In the 1920s, Barbara McClintock, later a Nobel laureate for her discovery of transposable elements, examined the behavior of chromosomes in wheat cells that had been subjected to X-rays. She noticed that the X-rays produced chromosomal breaks during G1 phase, and that after subsequent chromosome replication in S phase, the broken ends of the two sister chromatids could join together to make a fusion chromosome larger than the original. Even later, during mitotic metaphase and early in mitotic anaphase, the joined sister chromatids would form an unusual bridge structure in which chromatin was stretched between the two spindle poles and could then eventually break. She called this phenomenon the breakage-fusion-bridge cycle. Each of photographs (a) and (b) that follow shows a cell in early mitotic anaphase that has two such chromatin bridges.

 

a.	What ensures that the ends of normal chromosomes do not fuse together as do the ends of the sister chromatids after breakage?
b.	The following figure shows a chromosome with genes A–G; the arrow indicates the location of X-ray induced breakage. Draw the resulting bridge (that is, the large fused chromosome) as it would be seen in mitotic anaphase, and label all the genes and important chromosomal structures the bridge contains. Use arrows to show the forces exerted by the spindle apparatus on this bridge.
c.	If the sister chromatids fuse, why must the fusion chromosome behave as a bridge during mitosis? [Think about the forces pulling on the bridge described in your answer to part (b).]
d.	What is likely to happen to the bridge during mitotic anaphase? What then is likely to happen in the two daughter cells produced by the mitosis just described, and why? (Hint: McClintock’s name for this phenomenon implies it is a cycle.)