The ratio of purines to pyrimidines should be constant because purines always bind with pyrimidines, no matter which ones they may be.

In humans, fertilization normally occurs in the fallopian tube. The ovary is the female gonad that contains eggs, so eliminate (A). The uterus is the organ that houses the developing embryo, so eliminate (C). (D) can also be eliminated because the placenta is the structure that nourishes the embryo. Finally, (E) can be eliminated as the urethra is a tube that connects the urinary bladder to the urinary meatus for the removal of urine from the body of both females and males.

The two hormones that are responsible for the maintenance of Ca in the blood are parathyroid hormone and calcitonin. Therefore, eliminate (A), (C), and (E). Now review the effects of these hormones. Parathyroid hormone increases Ca²⁺ in the blood, while calcitonin decreases Ca²⁺ in the blood. A sustained decrease in circulating Ca²⁺ levels might be caused by decreased levels of parathyroid hormone.

All of the choices are correct differences between prokaryotes and eukaryotes.

DNA polymerase, not RNA polymerase, is the enzyme that causes the DNA strands to elongate. DNA helicase unwinds the double helix, so (A) is true and therefore incorrect. Choice (B), DNA ligase seals the discontinuous Okazaki fragments, is also true. Eliminate it. In the presence of DNA polymerase, DNA strands always grow in the 5' to 3' direction as complementary bases attach. Therefore, (D) is also incorrect. Topoisomerase is used to prevent over-winding and underwinding, (E) is incorrect.

Plasmodesmata are cytoplasmic strands that interconnect cells and form an intercellular communication network within a plant body. Gap junctions are used in intercellular communication with animals, but are not found in plants, so eliminate (A). Desmosomes are used in maintaining connectivity of tissues and Hemidesmosomes are special desmosomes used in skin, so eliminate (B) and (C). Tight junctions prevent leakage of transported solutes and water, so eliminate (E).

Arteries are thick-walled vessels that carry blood away from the heart. Blood moves by contracting muscles. Choices (A) and (B) are characteristics of veins. Veins are thin-walled vessels (with valves) that return blood to the heart. Choice (C) is an example of a vein. Choice (D) may or may not be true depending if the blood is being carried in the pulmonary circuit (lungs) or system circuit (rest of the body).

Insulin decreases the level of blood glucose by increasing the storage of glycogen in muscles. The other hormones are correctly paired with their respective function.

Muscle contractions require calcium ions, so (I) is required. The calcium ions bind to troponin which moves tropomyosin allowing for contraction to occur. You need energy in order to have a muscle contraction, so (II) is required. And actin is one of the proteins involved in muscle contractions, so (III) is required. The answer is (D).

Choices (B) and (D) are incorrect: Hemoglobin's affinity for O2 decreases as the concentration of H+ increases (or the pH decreases) and as the concentration of CO2 increases (or the concentration of HCO3- increases). Also eliminate (C), as hemoglobin's affinity for oxygen in tissue muscles does not increase during exercise.

They both contain genetic material (DNA), a plasma membrane, and a cell wall. Unlike fungi, bacteria lack a definite nucleus. Therefore, eliminate (B). Bacteria are unicellular, whereas fungi are both unicellular and multicellular. Therefore, eliminate (C), (D), and (E). All those options contain membrane-bound organelles (mitochondria and golgi)

Animal cells have centrioles while plants do not require them. Both animal and plant cells have an endoplasmic reticulum, membrane-bound organelles, and lysosomes. Only plants have a cell wall made of cellulose and large vacuoles. Neither plants nor animals have circular DNA, this is a characteristic of the Kingdom Monera.

Viruses are made up of nucleic acid surrounded by a protein coat called a capsid. They do not contain a cell wall, proteins, or cell membrane.

The primary site of glucose reabsorption is the proximal convoluted tubule. The glomerulus, (A), is a tuft of capillaries that filters fluid into the Bowman's capsule. Choice (C) and (D), the loop of Henle, is the site of salt reabsorption. Choice (D), the collecting duct, is the site in which urine is concentrated.

The sympathetic division is active during emergency situations. This leads to a decrease in peristalsis in your gastrointestinal tract. (Your stomach shuts down.) Stimulation of the sympathetic nervous system leads to (A), pupils dilating, (B), peripheral blood vessels constricting, (C), sweating, and (E), dilation of the trachea.

Crossing-over and synapsis occur during meiosis, which produces haploid gametes. Separation of homologous chromosomes occurs during meiosis I, while separation of sister chromatids does not occur until meiosis II.

This question tests your ability to associate what happens when enzymes are denatured and what would happen in the synaptic cleft. Acetylcholinesterase is an enzyme that degrades acetylcholine in the synaptic cleft. If acetylcholinesterase is denatured, acetylcholine will still be released from the presynaptic membrane and continue to diffuse across the synaptic cleft and bind to the postsynaptic membrane because acetylcholine is not degraded.

Pheromones act as sex attractants, alarm signals, or territorial markers. Auxins are plant hormones that promote growth, so eliminate (A). Hormones are chemical messengers that produce a specific effect on target cells within the same organism, so eliminate (B). Enzymes are catalysts that speed up reactions, so eliminate (D). Urea is a waste product that must be excreted from the body, so eliminate (E).

The development of an egg without fertilization is known as parthenogenesis. Parthenogenesis is a form of asexual reproduction found in insects and lizards. Choice (A), meiosis, is a form of sexual reproduction that produces gametes. Choice (C), embryogenesis, refers to the early stages of embryo development. Choice (D), vegetative propagation, is a form of asexual reproduction by which plants produce identical offspring from stems, leaves, or roots. Choice (E), is a phenomenon in genetics in which one gene influences two or more seemingly unrelated phenotypic traits.

 This is an example of mutualism. Both organisms benefit. Choice (A), parasitism, is an example of a symbiotic relationship in which one organism benefits and the other is harmed. Choice (B), commensalism, is when one organism benefits and the other is unaffected. Choice (D), endosymbiosis, is the idea that some organelles originated as symbiotic prokaryotes that live inside larger cells.

The primary difference between bone and cartilage is that cartilage is flexible and lacks blood vessels. They are both parts of the skeletal system; Therefore (A) is incorrect. Bone and cartilage are connective tissues made up of collagen and calcium salts, so eliminate (B). Both cartilage and bones secrete a matrix, making (D) incorrect as well.

There is no union of gametes in mitosis. Choices (A) and (C) are incorrect: Asexual reproduction involves the production of two new cells with the same number of chromosomes as the parent cell. If the parent cell is diploid, then the daughter cells will be diploid. The daughter cells are identical to each other and to the parent cell, so eliminate (B) and (E).

The respiratory rate in humans will be affected by an increase in the amount of CO2 and a drop in pH levels, which is the same as an increase in hydrogen ion levels. Strenuous exercising will also modify the respiratory rate. O2 concentration generally does not play an important role in regulating respiration.

Peroxisomes catalyze reactions that produce hydrogen peroxide, ribosomes are involved in protein synthesis, and mitochondria contain enzymes involved in cellular respiration. Eliminate (B), (D), and (E) because lysosomes are the sites of degradation; they contain hydrolytic enzymes but do not produce hydrogen peroxide. Choice (C) is incorrect, as the Golgi apparatus sorts and packages substances that are destined to be secreted out of the cell. Interestingly, O2 concentration generally doesn't play an important role in regulating respiration.

Speciation occurred in the Galapagos finches as a result of the different environments on the islands. This is an example of divergent evolution. The finches were geographically isolated. Choice (A), convergent evolution, is the evolution of similar structures in distantly related organisms. Choice (C), disruptive selection, is a selection that favours both extremes at the expense of the intermediates in a population. Choice (D), stabilizing selection, is a selection that favours the intermediates at the expense of the extreme phenotypes in a population. Choice (E), sympatric speciation, is a type of speciation that occurs when organisms are not split by geographical barriers.

Ovulation is the first to occur of those mentioned in the question. Ovulation is the release of an egg from the ovary into the fallopian tube. It is followed by fertilization (C) in the fallopian tubes. The zygote will then undergo a series of steps before implantation into the endometrium (B), neurulation (D), and finally organogenesis (E).

Trypsin is an enzyme, which are proteins and organic catalysts that speed up reactions without altering them.  One of its reactions involves the conversion of chymotrypsinogen to chymotrypsin, eliminate (E). They are not consumed in the process. Therefore, you can eliminate (A) and (B). The rate of reaction can be affected by the concentration of the substrate up to a point, so (D) can be eliminated.

The liver does not break down peptides into amino acids. It performs all of the following functions: stores amino acids absorbed in the capillaries, as in (A); detoxifies harmful substances, as in (B);  makes bile, as in (C); and store glycogen, as in (E).

A symbiotic relationship is a relationship among two organisms in which both benefit. Choice (A) describes parasitism, and (E) describes commensalism. Choice (D) is an example of altruistic behaviour. Choice (C) described habituation.

A voltage change from +50 to -70 is called repolarization. You can eliminate (A) because a voltage change from -70 to +50 is called depolarization. Choice (C) is incorrect; a voltage change from -70 to -90 is called hyperpolarization. Choice (D) can be eliminated as well; an action potential is a travelling depolarized wave. It refers to the whole thing, from depolarization, repolarization, hyper-polarization, and back to resting potential. Eliminate choice (E) since this is what occurs at the end of a neuron and involves the release of neurotransmitters.

Crossing-over (exchange of genetic material) occurs in prophase I of meiosis. Eliminate (B) because during metaphase I, the tetrads line up at the metaphase plate. Choice (C) and (E) are also incorrect; during anaphase, the tetrads/chromatids separate. Finally, you can eliminate (D) because, during prophase II, the chromosomes become visible again.

 The mRNA is modified before it leaves the nucleus. It becomes smaller when introns (intervening sequences) are removed. A poly(A) tail and a cap are added to the mRNA and would therefore increase the length of the mRNA, so you can eliminate (A) and (B). Choice (C) is also incorrect, as exons are the coding sequences that are kept by the mRNA.

The set of genotypes that represents a cross that could produce offspring with silver fur from parents that both have brown fur is Bb and Bb. Complete a Punnett square for this question. In order for the offspring to have silver fur, both parents must have the silver allele.

Beta cells secrete insulin. Destruction of beta cells in the pancreas will halt the production of insulin. Therefore, eliminate (A), (B), and (C). This will lead to an increase in blood glucose levels. Therefore, eliminate (E).

Electrons passed down along the electron transport chain from one carrier to another lose energy and provide energy for making ATP, eliminate (E). Glucose is decomposed during glycolysis, but this process is not associated with energy given up by electrons; eliminate (A). Glucose is made during photosynthesis, so eliminate (B). NADH is are energy-rich molecule, which accepts electrons during the Krebs cycle. Therefore, (D) is incorrect as well.

All of the choices are examples of hydrolysis except the conversion of pyruvic acid to glucose. Hydrolysis is the breaking of a covalent bond by adding water. In all of the correct examples, complex compounds are broken down to simpler compounds. The conversion of pyruvic acid to acetyl CoA is an example of decarboxylation—a carboxyl group is removed as carbon dioxide and the 2-carbon fragment is oxidized.

Mutations produce genetic variability. All of the other answer choices are forms of asexual reproduction. Gastrulation is a step in embryonic development in which three distinct germ layers are formed.

Choice (A), (B), and (E) are incorrect: These organisms exhibit the same behaviour because they were subjected to the same environmental conditions and similar habitats. This is an example of convergent evolution. They are analogous, so (D) is also incorrect. They exhibit the same function but are structurally different. However, they are not genetically similar, so (C) is the answer. (One is an insect and the other a bird.)