Examine Figure 21.14. If you wanted to use a molecular clock to date a relatively recent divergence event—say, one that occurred in the last 100 million years or so—which of the four types of genes would make the best clock? Why?
A. Fibrinopeptides, because they have
changed the most over that time period.
B. A histone gene, because it has changed
relatively little over that time.
C. Fibrinopeptides, because they have
changed relatively little over that time.
D. A histone gene, because it has changed
the most over that time period.
E. Hemoglobin, because of its moderate rate
of change.
A
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Researchers pulsed rapidly dividing cultured cells with radioactive thymidinefor 30 minutes. The cells were then exposed to a solution containing non-radiolabeled thymidine. Cells were analyzed at 2-hour intervals At the 2-hour time point, no cells appeared to be dividing. Only after 4 hours did some labeled cells appear to be in M phase. This result can be explained in the following way:
A) Radiolabeled compounds are somewhat cytotoxic, and cell division was initially inhibited. B) The cells were arrested in a nondividing state because of the treatment and could not enter M phase until several hours after the label was removed. C) The synthesis (S) phase is lengthy, about 12 hours in most cell types, and the radioactive thymidine was not present long enough for most cells to be labeled. D) There seems to be a gap or a lag in the cell cycle, between the synthesis of DNA and cell division.
Bifidobacterium bifidum is a major pathogen in breast-fed babies.
Answer the following statement true (T) or false (F)
Antidiuretic hormone and oxytocin are
products of a. exocrine glands. b. secretory neurons. c. blood capillaries. d. the anterior pituitary. e. kidney and uterine wall cells, respectively
Within the nitrogen cycle, which organism both coverts and releases nitrogen?
a. bacteria b. viruses c. fungi d. plants e. humans