27. Which of the following is a correct statement about nitrogen fixation in root nodules?
(A) The plant contributes the nitrogenase enzyme.
(B) The process is relatively inexpensive in terms of ATP costs.
(C) Leghemoglobin helps maintain a low O2 concentration within the nodule.
(D) The bacteria of the nodule are autotrophic.

23. The frequency of heterozygosity for the sickle cell anemia allele is unusually high, presumably because this reduces the frequency of malaria. Such a relationship is related to which of the following? (A) Mendel's law of independent assortment (B) Mendel's law of segregation (C) Darwin's explanation of natural selection (D) Darwin's observations of competition

24. The enzyme telomerase solves the problem of replication at the ends of linear chromosomes by which method? (A) causing specific double strand DNA breaks that result in blunt ends on both strands (B) causing linear ends of the newly replicated DNA to circularize (C) adding numerous short DNA sequences such as TTAGGG, which form a hairpin turn (D) adding numerous GC pairs which resist hydrolysis and maintain chromosome integrity

25. When DNA is compacted by histones into 10 nm and 30 nm fibers, the DNA is unable to interact with proteins required for gene expression. Therefore, to allow for these proteins to act, the chromatin must constantly alter its structure. Which processes contribute to this dynamic activity? (A) DNA supercoiling at or around H1 (B) methylation and phosphorylation of histone tails (C) hydrolysis of DNA molecules where they are wrapped around the nucleosome core (D) accessibility of heterochromatin to phosphorylating enzymes

26. Which of the following is correct for both prokaryotic and eukaryotic gene expression? (A) After transcription, a 3' poly-A tail and a 5' cap are added to mRNA. (B) Translation of mRNA can begin before transcription is complete. (C) RNA polymerase binds to the promoter region to begin transcription. (D) The mRNA transcript is the exact complement of the gene from which it was copied.

28. In animals, embryonic stem cells differ from adult stem cells in that (A) embryonic stem cells are pluripotent, and adult stem cells are totipotent. (B) embryonic stem cells are totipotent, and adult stem cells are pluripotent. (C) embryonic stem cells have more genes than adult stem cells. (D) embryonic stem cells have fewer genes than adult stem cells. 

29. Which of the following best describes siRNA? (A) a short double-stranded RNA, one of whose strands can complement and inactivate a sequence of mRNA (B) a single-stranded RNA that can, where it has internal complementary base pairs, fold into cloverleaf patterns (C) a double-stranded RNA that is formed by cleavage of hairpin loops in a larger precursor (D) a molecule, known as Dicer, that can degrade other mRNA sequences

30. Which of the following series best reflects what we know about how the flu virus moves between species? (A) An avian flu virus undergoes several mutations and rearrangements such that it is able to be transmitted to other birds and then to humans. (B) The flu virus in a pig is mutated and replicated in alternate arrangements so that humans who eat the pig products can be infected. (C) A flu virus from a human epidemic or pandemic infects birds; the birds replicate the virus differently and then pass it back to humans. (D) An animal such as a pig is infected with more than one virus, genetic recombination occurs, the new virus mutates and is passed to a new species such as a bird, the virus mutates and can be transmitted to humans.

31. Biofuels are mainly produced by: (A) plants that convert hemicellulose into gasoline. (B) the breakdown of cell wall biopolymers into sugars that can be fermented. (C) the genetic engineering of ethanol generating genes into plants. (D) plants that are easy to grow in arid environments.

32. A plant will recognize a pathogenic invader (A) if it has many specific plant disease resistance (R) genes. (B) when the pathogen has an R gene complementary to the plant's antivirulence (Avr) gene. (C) when the pathogen secretes Avr protein. (D) if it has the specific R gene that corresponds to the pathogen molecule encoded by an Avr gene.

33. All of the following are responses of plants to cold stress except: (A) the production of a specific solute "plant antifreeze" that reduces water loss. (B) excluding ice crystals from the interior walls. (C) conversion of the fluid mosaic cell membrane to a solid mosaic one. (D) an alteration of membrane lipids so that the membranes remain flexible.