20. What are prions?
(A)mobile segments of DNA
(B)tiny molecules of RNA that infect plants
(C) misfolded versions of normal brain protein
(D)viruses that invade bacteria

16. Alternative RNA splicing (A)is a mechanism for increasing the rate of transcription. (B)is a mechanism for increasing the rate of translation. (C) can allow the production of different proteins from a single pre-mRNA. (D)is due to the presence or absence of particular snRNPs.

17. Which of the following statements about microRNAs is false? (A)MicroRNAs are short RNA molecules. (B)MicroRNAs are formed by cleavage of hairpin loops in a larger precursor. (C) MicroRNAs have internal complementary base pairs, folding into cloverleaf patterns. (D)MicroRNAs silence the expression of specific mRNAs. (E)MicroRNAs are found in plants and animals.

18. Which of the following statements describes proto-oncogenes? (A)Their normal function is to suppress tumor growth. (B)They are introduced to a cell initially by retroviruses. (C) They are produced by somatic mutations induced by carcinogenic substances. (D)They often encode proteins that stimulate the cell cycle. (E)They are underexpressed in cancer cells.

19. Muscle cells differ from nerve cells mainly because they (A)contain different genes. (B)express different genes. (C) use different genetic codes. (D)have unique ribosomes. (E)have different chromosomes.

21. The reason for using Taq polymerase for PCR is that (A)only minute amounts are needed for each cycle of PCR. (B)it binds more readily than other polymerases to primer. (C) it is heat stable and can withstand the high temperature of PCR. (D)it can use RNA as template to synthesize DNA.  

22. Why might the cricket genome have 11 times more base pairs than that of fruit fly? (A)The two insect species evolved at very different geologic eras. (B)Fruit flies are more complex organisms. (C) Crickets must make many more proteins. (D)Crickets must have more noncoding DNA.

23. What characteristic of short tandem repeat DNA makes it useful for DNA fingerprinting? (A)The number of repeats varies widely from individual to individual. (B)The sequence of repeats varies widely from individual to individual. (C) The sequence variation is acted upon differently by natural selection in different environments. (D)Every racial group has inherited different short tandem repeats.

24. What is the correct sequence of the following events, under the influence of natural selection? 1. Well-adapted individuals leave more offspring than do poorly adapted individuals.  2. Genetic variation is present in a population.  3. Genetic frequencies within the population change. 4. A change occurs in the environment. (A)4 → 1 → 3 → 2(B)4 → 3 → 1 → 2(C)2 → 1 → 4 → 3(D)2 → 4 → 3 → 1(E)2 → 4 → 1 → 3

25. In peas, a gene controls flower color such that R = purple and r = white. In an isolated pea patch, there are 36 purple-flowering plants and 64 white-flowering plants. Assuming Hardy-Weinberg equilibrium, what is the value of q for this population? (A)0.25(B)0.36(C)0.64(D)0.80

26. The recessive allele that causes phenylketonuria (PKU) is harmful. What maintains the presence of this harmful allele in a population's gene pool? (A)heterozygote advantage(B)diploidy(C)stabilizing selection(D)balancing selection