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

19. Celery stalks that are immersed in fresh water for several hours become stiff and hard. Similar stalks left in a salt solution become limp and soft. From this we can deduce that the cells of the celery stalks are (A) hypotonic to both fresh water and the salt solution. (B) hypertonic to both fresh water and the salt solution. (C) hypertonic to fresh water but hypotonic to the salt solution. (D) hypotonic to fresh water but hypertonic to the salt solution.

20. Which kind of metabolic poison would most directly interfere with glycolysis? (A) an agent that reacts with oxygen and depletes its concentration in the cell (B) an agent that binds to pyruvate and inactivates it (C) an agent that closely mimics the structure of glucose but is not metabolized (D) an agent that blocks the passage of electrons along the electron transport chain

21. In cats, black fur color is caused by an X-linked allele; the other allele at this locus causes orange color. The heterozygote is tortoiseshell. What kinds of offspring would you expect from the cross of a black female and an orange male? (A) Tortoiseshell females; tortoiseshell males (B) Black females; orange males (C) Orange females; orange males (D) Tortoiseshell females; black males  

22. A young animal has never had much energy. He is brought to a veterinarian for help and is sent to the animal hospital for some tests. There they discover his mitochondria can use only fatty acids and amino acids for respiration, and his cells produce more lactate than normal. Of the following, which is the best explanation of his condition? (A) His mitochondria lack the transport protein that moves pyruvate across the outer mitochondrial membrane. (B) His cells cannot move NADH from glycolysis into the mitochondria. (C) His cells contain something that inhibits oxygen use in his mitochondria. (D) His cells lack the enzyme in glycolysis that forms pyruvate.

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.

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.

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