Part IV

1. The bridge-breakage-fusion cycle occurs because the stickly end of sister chromatids will fuse to produce dicentric chromosomes. In embyro tissue, the sticky ends heal and do not fuse. The bridge-breakage-fusion cycle only occurs in endosperm tissue and not in embryo or sprophyte tissue.
   
   
2. Transposable elements can relocate within the nuclear genome from one location on a chromosome to another location on that same chromosome. They also can relocate to a non-homologous chromosome. McClintock used genetic markers that were known to be located on specific chromosomes and linkage analysis to determine that either the Ac or the Ds elements had moved to a new location.
   
   
3. The Ac element can relocate within a gene and disrupt the normal transcription of that allele. This can alter gene expression. At some future generation, the Ac element can re-locate out of that gene and restore normal alleleic expression in the progeny.
   
   
4. Transposable elements have now been identified in fruit flies, bacteria, peas, maize, and humans.