The AIDS phase of HIV infection begins when the concentration of CD4 T cells in the blood drops below what concentration?
A) 2,000 cells per cubic millimeter
B) 1,000 cells per cubic millimeter
C) 500 cells per cubic millimeter
D) 200 cells per cubic millimeter
E) No CD4 T cells are observed.
D
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Why do animal viruses have envelopes and phages rarely do?
A. Since bacteria don't have cell membranes, the bacterial viruses (phages) don't pick them up when they leave the target cells. B. Phages acquire an outer surrounding that is a part of the cell wall of the bacterium they were created in, rather than an outer surrounding of plasma membrane. C. Animal viruses will often utilize the envelope in order to fuse with the plasma membrane of a new target cell, gaining entry into the cytoplasm. D. Animal viruses build the envelope inside of the target cell as they are being replicated, but before the cell breaks open and releases them into the extracellular environment. The envelope is a remnant of this building process.
Which of the following is true when comparing RNA three-dimensional structure to that of DNA?
A. RNA helices are right-handed but not antiparallel like DNA helices. B. RNA helices are stabilized by base stacking but are not right-handed like DNA. C. Unlike DNA helices, RNA helices often have additional stabilization by non–Watson-Crick base pairing. D. RNA helices are less stable than DNA helices because they have regions that are unpaired. E. RNA helices are antiparallel, stabilized by base stacking, and are left-handed like DNA helices.
Where is the electron transport chain of photosynthesis located?
a. cristae b. mitochondria c. chloroplast stroma d. thylakoid membrane e. outer chloroplast membrane
Which form of electromagnetic radiation has the shortest wavelength?
A) Gamma rays and X rays B) Ultraviolet light C) Visible light D) Infrared light