Gene Expression and Cell Specialization

Albumin is high only in liver, insulin only in islets and myosin only in muscle (others below 1%), while the glycolysis enzyme is 48-100% in every tissue.

• Correct: Albumin, insulin and myosin are each high in one tissue; the glycolysis enzyme is in each of them.: Correct: three tissue-specific genes and one used everywhere.
• Each of the four mRNAs is high in one of the tissues and nearly absent from the other two tissues.: The glycolysis enzyme is 48% or more in every tissue, so it is not limited to one.
• The four mRNAs are found at similar levels in the three tissues, within the error bars of the data.: Albumin is 100% in liver and under 1% elsewhere, far outside the error bars.
• The glycolysis enzyme mRNA is high in one tissue, while the other three are found in each tissue.: This reverses the pattern: the glycolysis enzyme is the one found everywhere.

Almost every cell in the body carries the same genome. The near-zero bar shows the gene is not being transcribed in liver, not that it is missing; testing the liver cells' DNA would find the gene.

• Correct: The graph measures mRNA, not DNA: liver cells have the gene but do not transcribe it.: Correct: absence of mRNA is not absence of the gene.
• The student is right, because a cell that carries a gene transcribes it at least at some level.: Many genes are present but silent; differential gene expression is the rule.
• The graph shows a little insulin mRNA in liver, so liver cells probably secrete some insulin too.: 0.1% is at the level of background, and mRNA alone would not show the hormone is made and released.
• The student is right, because each cell type keeps the genes it uses and discards the others.: Cells do not discard unused genes; nuclei from specialized cells can still direct a whole new animal.

MyoD is a master regulator: it binds enhancers of many muscle genes and switches them on together, and it activates more muscle transcription factors. Skin cells carry those genes, so MyoD can turn many of them into muscle-like cells, as was first shown in 1987.

• Correct: Many skin cells start transcribing myosin and other muscle genes and begin to look like muscle cells.: Correct: one factor can launch a whole coordinated program.
• The skin cells make MyoD protein but transcribe the same genes as before, since their genes are fixed.: A cell's genes are not fixed off forever; a new transcription factor can switch on genes it targets.
• The skin cells transcribe myosin but lose their copies of the genes that skin cells use.: Switching programs does not delete genes; DNA is not lost.
• The skin cells start transcribing albumin and insulin as well as myosin, since MyoD switches on silent genes.: MyoD binds the enhancers of muscle genes; liver and islet genes need their own activators.

A microRNA is loaded into a protein complex and guided by base pairing to its target mRNA, where it reduces translation and speeds the mRNA's breakdown. That fits lower mRNA, even lower protein and unchanged transcription.

• Correct: miR-X pairs with protein P's mRNA, so the mRNA is broken down and translated less.: Correct: post-transcriptional regulation by base pairing.
• miR-X binds to the protein P gene's promoter and stops RNA polymerase from copying it.: Transcription is unchanged, so miR-X does not act at the promoter.
• miR-X is translated into a protein that breaks down protein P after it is made.: MicroRNAs are not translated; they act as RNA.
• miR-X adds methyl groups to the protein P gene, packing it into heterochromatin.: Methylation would silence transcription, which did not change.

Extra miR-X has almost no effect (0.97, 0.95) on an mRNA it cannot pair with. So the effect requires complementary pairing at that site; it is not a side effect of adding RNA to cells.

• Correct: It shows the effect depends on miR-X pairing with that sequence, not on adding RNA in general.: Correct: a control for specificity.
• It shows that changing three bases of an mRNA changes the amino acids of the protein it codes for.: The stem says the protein is unchanged; the three bases lie outside the coding part.
• It shows that the cells' own miR-X keeps protein P lower than it would be without it.: That is shown by the row where the cells' own miR-X is blocked.
• It shows that miR-X lowers the transcription of each gene in the cell by about the same amount.: Transcription of the protein P gene is unchanged in every row.

With twice as much Bicoid, the level stays above 15 units farther back: the four-copy curve is 18.1 at 60% and 12.2 at 70%, so the edge moves to about 65%. Real embryos from such mothers have enlarged head regions shifted backward.

• Correct: The hunchback region extends back to about 65%, so front structures take up more of the embryo.: Correct: more morphogen moves each threshold away from the source.
• The hunchback region shrinks toward the front end, so front structures take up less of the embryo.: More Bicoid raises the level at every position, so the region grows rather than shrinks.
• The hunchback region stays near 47%, because each cell carries the same genes in both kinds of embryo.: Cells respond to the Bicoid level they experience, not just to which genes they carry.
• Hunchback is switched on along the whole length, so no back structures form anywhere in the embryo.: The four-copy curve falls below 15 units by about 65%, so the back part still lacks hunchback.