Essential Cell Biology 5th edition

Questions
295
specialized cells is based on limited rearrangements of the
genome.) Describe an experiment that substantiates the
first sentence of this question, and explain why it does.
QUESTION 8–11
Figure 8−17 shows a simple scheme by which three
transcription regulators are used during development
to create eight different cell types. How many cell types
could you create, using the same rules, with four different
transcription regulators? As described in the text, MyoD is
a transcription regulator that by itself is sufficient to induce
muscle-specific gene expression in fibroblasts. How does
this observation fit the scheme in Figure 8−17?
QUESTION 8–12
Imagine the two situations shown in Figure Q8–12. In
cell I, a transient signal induces the synthesis of protein
A, which is a transcriptional activator that turns on many
genes including its own. In cell II, a transient signal induces
the synthesis of protein R, which is a transcriptional
repressor that turns off many genes including its own. In
which, if either, of these situations will the descendants of
the original cell “remember” that the progenitor cell had
experienced the transient signal? Explain your reasoning.
(A) CELL I
OFF
gene activator
(B) CELL II
A
transient
signal
turns on transcription
of activator mRNA
OFF transient
signal
R
R
gene repressor turns on transcription
of repressor mRNA
Figure Q8–12
A
A
A
A
activator protein
turns on its own
transcription
R
R
R
repressor protein
turns off its own
transcription
QUESTION 8–13
Discuss the following argument: “If the expression of every
gene depends on a set of transcription regulators, then the
expression of these ECB5 regulators eQ8.12/Q8.12
must also depend on the
expression of other regulators, and their expression must
depend on the expression of still other regulators, and so
on. Cells would therefore need an infinite number of genes,
most of which would code for transcription regulators.”
How does the cell get by without having to achieve the
impossible?