ANSWERS to practice questions for quiz 1 1. Only coding DNA is ...

ANSWERS to practice questions for quiz 1
1. Only coding DNA is present in exons. T/F
False: an exon may be part of the 5’UTR or the 3’UTR of an mRNA; or, regions of
exon could be spliced out, thus not coding (alternative splicing)
2. A mutation in an intron cannot affect the protein product made from that gene. T/F
False: such a mutation could introduce a new splice site, or could affect a regulatory
region, could cause frameshift that affects open reading frame, etc.
3. microRNAs (small double stranded RNAs) usually work by inhibiting transcription of
specific RNAs. T/F
False: microRNAs inhibit translation usually by cleaving the mRNA—there are only a
few examples of micro RNAs that inhibit transcription by inducing methylation.
4. Which of the movements below is/are critical for the extension of cells, in a sheet,
along the inside of vertebrate embryo during gastrulation?
a. epiboly
b. convergent extension
c. involution
d. ingression
e. delamination
5. Human identical twins are formed when the dividing fertilized egg splits in half, before
the 32-cell stage. Each new embryo contains half the number of cells that the original
embryo contained, and yet develops normally. This is possible in humans because:
a. you only need half the cells you are born with; the rest die by apoptosis anyway
b. the cells that end up in each half-embryo have different content initially, but are
reprogrammed to start development over when separated
c. the remaining cells are all equivalent and can signal to each other to make up for the
missing cells
d. the cells from one embryo signal to the other adjacent embryo to make up for any
deficiencies in each half-embryo
6. (1) Imagine you could take a just-fertilized C. elegans embryo (1-cell stage) and
dictate the plane of its first division so that it was perpendicular to the normal plane of
division (cleavage furrow is horizontally placed rather than vertically). Predict the
consequences of this change on the development of the embryo. (see figures 8.42 and/or
8.43 for division planes). Because the determinants of the 1-cell embryo are
asymmetrically localized, primarily from anterior to posterior, changing the division
plane would likely give you two cells that are equivalent rather than different. At the
very least, you’d have the capacity to make germ line present in both of these cells, so if
the embryo could develop far enough, you’d have more germ line. However, it’s more
likely that the embryo would not be able to develop normally; this change would likely
impact the signaling of the 4 cell embryo, thus leading to fate changes.

7. Convergent extension is an important type of movement during morphogenesis.
Briefly explain how this type of cell rearrangement might be important for a developing
embryo. Convergent extension is particularly important for the spread of notochord
cells, as they have to extend a long ways from the dorsal blastopore lip all the way up to
the future head. Basically, any time you need a chunk of cells to line up in a long orderly
row, convergent extension is the way to go.
8. You decide to investigate whether some known signaling pathways might be involved
in determining ventral vs. dorsal mesoderm fate in Xenopus. You use antibody labeling
to observe the localization of two different signaling ligands, Shh and a TGFβ family
member (BMP4), in the early blastula stage. There are many different possibilities for
how these two ligands might be localized within the embryo. A. Predict their localization
if mesoderm forms due to a morphogen gradient or gradients. B. Predict their
localization if two different kinds of mesoderm require different signals to be specified
îf they act as a morphogen, each molecule must be present in a gradient across the
embryo, and specify fate dependent on concentration. Thus, in this case, you might see
Shh gradient from low on the ventral side to high on the dorsal side, and BMP from high
on the ventral side to low on the dorsal side.
9.The polypeptide hormone prolactin stimulates milk production in female mammary
glands. When the hormone is present, production of the milk protein casein increases,
largely because the casein mRNA degrades much more slowly than it does when
prolactin is absent. The hormone does not enter the mammary gland cells, and the
degradation rates of other mRNAs are not affected. From your knowledge of signaling,
predict the steps in a pathway by which the hormone could cause an increase in the
stability of this specific mRNA.
Prolactin could
1. act directly as a transcription factor (some steroid hormones can be transported
directly to the nucleus –“nuclear hormone receptors”) to trigger casein
production (prolactin doesn’t actually work this way, but it is a reasonable
hypothesis)
2. act as signaling ligand (bind to a receptor) and through signaling cascade,
activate transcription of casein
3. prevent degradation of casein by binding to whatever normally degrades It after it
has been translated
4. activate transcription of another factor which could bind to the 3’UTR of the
casein mRNA, allowing it to stick around longer and thus produce more protein.