24. In humans, identical twins are possible because
(A) of convergent extension.
(B) of the heterogeneous distribution of cytoplasmic determinants in unfertilized eggs.
(C) of interactions between extraembryonic cells and the zygote nucleus.
(D) the gray crescent divides the dorsal-ventral axis into new cells.
(E) early blastomeres can form a complete embryo if isolated.

20. Which of these statements about human evolution is CORRECT? (A) Different species of the genus Homo have coexisted at various times throughout hominin evolution. (B) Mitochondrial DNA analysis indicates that modern humans are genetically very similar to Neanderthals. (C) The evolution of upright posture and enlarged brain occurred simultaneously. (D) Human evolution has proceeded in an orderly fashion from an ancestral anthropoid to Homo sapiens. (E) The ancestors of Homo sapiens were chimpanzees.

21. Arthropods invaded land about 100 million years before vertebrates did so. This most clearly implies that (A) extant terrestrial arthropods are better adapted to terrestrial life than are extant terrestrial vertebrates. (B) arthropods evolved before vertebrates did. (C) vertebrates evolved from arthropods. (D) arthropods have had more time to co-evolve with land plants than have vertebrates. (E) ancestral arthropods must have been poorly adapted to aquatic life, thus experienced a selective pressure to invade land.

22. The number of MHC protein combinations possible in a given population is enormous. However, an individual in that population has only a couple of MHC possibilities. Why? (A) Once a B cell has matured in the bone marrow, it is limited to two MHC response categories. (B) Once a T cell has matured in the thymus, it can only respond to two MHC categories. (C) MHC proteins from one individual can only be of class I or class II. (D) The MHC proteins are made from several different gene regions that are capable of rearranging in a number of ways. (E) Each of the MHC genes has a large number of alleles, but each individual only inherits 2 for each gene.

23. Which of the following statements about the adrenal gland is CORRECT? (A) During stress, ACTH stimulates the adrenal cortex, and neurons of the sympathetic nervous system stimulate the adrenal medulla. (B) At all times, the anterior portion secretes ACTH, while the posterior portion secretes oxytocin. (C) At all times, the adrenal gland monitors calcium levels in the blood and regulates calcium by secreting the two antagonistic hormones, epinephrine and norepinephrine. (D) During stress, the alpha cells of islets secrete insulin and simultaneously the beta cells of the islets secrete glucagon. (E) During stress, TSH stimulates the adrenal cortex and medulla to secrete acetylcholine.

25. DDT was once considered a "silver bullet" that would permanently eradicate insect pests. Today, instead, DDT is largely useless against many insects. Which of these would have been required for this pest eradication effort to be successful in the long run? (A) The frequency of DDT application should have been higher. (B) None of the individual insects should have possessed genomes that made them resistant to DDT. (C) DDT application should have been continual. (D) All habitats should have received applications of DDT at about the same time. (E) Larger doses of DDT should have been applied.

26. Assume that excessive consumption of ethanol increases the influx of negative chloride ions into "common sense" neurons whose action potentials are needed for you to act appropriately and not harm yourself or others. Thus, any resulting poor decisions associated with ethanol ingestion are likely due to (A) increased membrane depolarization of "common sense" neurons. (B) more action potentials in your "common sense" neurons. (C) decreased membrane depolarization of "common sense" neurons. (D) fewer IPSPs in your "common sense" neurons. (E) more EPSPs in your "common sense" neurons.

27. Which is one of the two laws of inheritance identified by Mendel? (A) The law of priority. (B) The law of independent assortment. (C) The law of neutralism. (D) The law of paternity. (E) The law of polarity.

28. In glycolysis, for each molecule of glucose oxidized to pyruvate (A) 4 molecules of ATP are used and 2 molecules of ATP and 4 molecules of NADH are produced. (B) 2 molecules of ATP are used and 4 molecules of ATP and 2 molecules of NAD are produced. (C) 2 molecules of ATP are used and 4 molecules of ATP and 2 molecules of NADH are produced. (D) 2 molecules of ATP are used and 4 molecules of ATP and 1 molecules of NADH are produced. (E) 2 molecules of ATP are used and 2 molecules of ATP and 2 molecules of NADH are produced.

29. In vertebrate animals, brown fat tissue's color is due to abundant mitochondria. White fat tissue, on the other hand, is specialized for fat storage and contains relatively few mitochondria. Brown fat cells have a specialized protein that dissipates the proton-motive force across the mitochondrial membranes. Which of the following might be the function of the brown fat tissue? (A) to allow other membranes of the cell to perform mitochondrial function. (B) to increase the rate of oxidative phosphorylation from its few mitochondria. (C) to allow the animals to regulate their metabolic rate when it is especially hot. (D) to regulate temperature by converting energy from NADH oxidation to heat. (E) to increase the production of ATP synthase.

30. Which of the following represents a difference between viruses and viroids? (A) Viruses contain introns; viroids have only exons. (B) Viruses infect many types of cells, whereas viroids infect only prokaryotic cells. (C) Viruses always have genomes composed of DNA, whereas viroids always have genomes composed of RNA. (D) Viruses have capsids composed of protein, whereas viroids have no capsids. (E) Viruses cannot pass through plasmodesmata; viroids can.