telomere telomere Ade2 gene at normal location white colony of yeast cells Ade2 gene moved near telomere red colony of yeast cells with white sectors Figure Q4-3 Position effect on expression of the yeast Ade2 gene (Problem 4-13). The Ade2 gene codes for one of the enzymes of adenosine biosynthesis, and the absence of the Ade2 gene product leads to the accumulation of a red pigment. Therefore a colony of cells that express Ade2 is white, and one composed of cells in which the Ade2 gene is not expressed is red.
Look at the two yeast colonies in Figure Q4–3. Each
of these colonies contains about 100,000 cells descended
from a single yeast cell, originally somewhere in the mid-
dle of the clump. A white colony arises when the Ade2 gene
is expressed from its normal chromosomal location. When
the Ade2 gene is moved to a location near a telomere, it
is packed into heterochromatin and inactivated in most
cells, giving rise to colonies that are mostly red. In these
largely red colonies, white sectors fan out from the middle
of the colony. In both the red and white sectors, the Ade2 gene is still located near telomeres. Explain why white sec-
tors have formed near the rim of the red colony. Based on
the patterns observed, what can you conclude about the
propagation of the transcriptional state of the Ade2 gene
from mother to daughter cells in this experiment?
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