Yeast are single-cell organisms that divide to reproduce. After twenty rounds of
ID: 84136 • Letter: Y
Question
Yeast are single-cell organisms that divide to reproduce. After twenty rounds of division, a single yeast cell (top of the figure below) produces a round colony comprising over one million daughter cells (bottom). Yeast contain a gene, named ADE6, whose protein product participates in the pathway shown in the upper-left box in the figure. DNA methylation ("me-DNA") alters chromatin structure to regulate the expression of ADE6. Although yeast do not have X chromosomes, assume that they do, and that the ADE6 gene is located on the X. The particular yeast strain you're working with is diploid and has two copies of the X, but only one (XI) contains a wild-type ADE6 genes; ADE6 has been deleted from X2, as shown in the right box. Assume that, at some point during colony growth, random X-chromosome inactivation occurs. just like it does in mammals. a) Complete the pathway diagram in the left box below by completing the arrow leading from methylated DNA ("me-DNA") to the ADE6 gene by adding the appropriate symbol: either an arrowhead or a vertical bar. b) For each of colonies A, B, and C, provide two pieces of information. First based on the colony sectoring color pattern, state the point during the process of colony growth at which random XCI occurred (for example: "one-cell stage") Second: state whether random XCI was random or skewed with respect to which X chromosome was inactivated.Explanation / Answer
From the given information, methylation alters the chromatin expression of the gene. Methylation gives a negative charge to the DNA which helps in the suppression of the gene expression.
a. Completing the box on the left, me-DNA suppresses the ADE6 expression in the yeast. It would get a vertical bar.
b.
Group A: The random XCI occurred at the 1 cell stage due to which the expression is low in the subsequent colonies. The XCI in these cells were skewed.
Group B: The random XCI occurred at the 4 cell stage due to which the expression is found in 1/4th of the final population. The XCI in these cells is skewed.
Group C: The random XCI occurred at the 8 cell stage. The XCI is random in these cells.
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