D The first organisms to colonize a barren area are known as the pioneer organisms.

The insertion of two nucleotides will shift the reading frame of all the codons "downstream" of the mutation, resulting in a series of mis-sense codons and eventually a nonsense (stop) codon.

A disorder that is only found in males must be Y linked. An X-linked trait would occur in a small proportion of females.

Phloem is the plant vascular tissue responsible for conducting water and dissolved sugars throughout the plant, downward from the leaves. It is always exterior to xylem, from which it is separated by the vascular cambium.

Sister chromatids line up at the metaphase plate, and spindle fibers attach to kinetochore proteins at the centromeres. All sister chromatids must be present and attached to spindle fibers in order to prevent nondisjunction during anaphase. An unattached kinetochore protein generates a signal that prevents the transition from metaphase to anaphase.

During transcription, an mRNA transcript is copied from a DNA template. This transcript transports genetic information out of the nucleus.

A somatic cell is diploid. If the organism's diploid chromosome number is 56, then the haploid gamete will possess half of this number of chromosomes, which is 2

Enzymes decrease the activation energy required for a chemical reaction to take place, allowing a reaction to proceed without a large energy input and thereby greatly increasing the reaction rate.

E Remember your LEAST/EXCEPT/NOT technique. There are four features present in all chordates-dorsal nerve cords (choice A), notochords (choice B), gill slits (choice C), and postanal tails (choice D). Note that some of these features may be found only in embryonic or larval stages. The "wrong" answer is choice E-not all chordates (for example, sea squirts and lancelets) have a bony endoskeleton.

A lack of consistency in environmental conditions will favor semelparity. The offspring's genetic variation ensures that at least some will survive the changing conditions.

Reverse transcriptase is an enzyme used to transcribe RNA sequences to DNA sequences. Its function is the reverse of the transcriptase that produces mRNA from DNA.

B Some animals do not have an instinctive sense for who their mother is and will bond with any object they are exposed to during a certain time period after their birth. The object "imprints" on their minds, and thereafter, even if exposed to their real mother, they will still treat the object as Mom.

Speciation requires that two populations be reproductively isolated from each other, allowing natural selection to act independently on each.

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The nucleic acid in mRNA carries genetic information from the nucleus to the ribosomes in the cytoplasm, where it is translated into a polypeptide.

Convergent evolution occurs when two unrelated species over time adapt the same structure. This is the reverse of what has occurred with tails in lobsters and crabs. An unused evolutionary relic can be termed a vestigial structure and also a homologous structure. This is an example of divergent evolution, where close relatives have begun to adapt differently from one another.

During transcription, an mRNA transcript is copied from a DNA template. This transcript transports genetic information out of the nucleus.

The corpus luteum, which forms after an egg is released from a follicle in the ovary, secretes estrogen and progesterone. These hormones cause the uterine lining to thicken. If a fertilized egg does not implant, the corpus luteum shrivels and the uterine lining is shed.

C These are the characteristics of tropical rain forest.

A silent mutation is not phenotypically visible because the protein product remains unchanged.

Pepsin is an enzyme that digests protein in the stomach. It is produced by cells as an inactive precursor to avoid damaging the proteins within the cells.

In order for an allele to be acted on by natural selection, it must result in an increase in fitness. Fitness is defined as the ability to pass traits on to offspring. Therefore, individuals that produce the greatest number of offspring that grow to become fertile adults have the greatest fitness.

Eutrophication, or the growth of algae, is caused by an excess of dissolved nutrients. The source of these nutrients is often agricultural or residential runoff.

B You should again be able to eliminate choices E and A, as above. The type of RNA that carries amino acids is tRNA. Remember t for "transport."

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The liver secretes lipase, which emulsifies fats and allows them to be digested by pancreatic lipase. However, the liver does not produce digestive enzymes.

Freshwater aquatic animals require less ability to reabsorb water than do terrestrial or saltwater species, which require a greater capacity for water reabsorption and thus a longer loop of Henle.

A If an organism's environment remains absolutely constant, and that organism still exhibits regular rhythms of activity, there must be some internal "clock" that keeps it on schedule (C and E are wrong). Roosters vary the time of their crow as the sun varies the time it rises (B is wrong). The magnetic field has nothing to do with internal clocks (D is wrong).

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Nondisjunction occurs when two homologous chromosomes or chromatids fail to segregate; if this were to occur in meiosis I or II, the resulting zygote could have three copies of a chromosome.

The phyla Mollusca and Arthropoda do not exhibit radial symmetry; they are bilaterally symmetrical. Phylum Porifera demonstrates an asymmetrical body plan. Phylum Cnidaria does exhibit radial symmetry; however, this group lacks any skeletal form, leaving the only possible correct answer to be Echinodermata.

An enzyme speeds up the rate of a reaction by lowering the activation energy, making the transition state less stable. It does not affect the energy of the products or the reactants.

C Whenever you're trying to determine the genotype of an organism that has a dominant phenotype, the best thing to do is something called a test cross. A test cross is a mating between the organism with the unknown genotype and an organism with the recessive phenotype. (Organisms that display the recessive phenotype MUST have the homozygous recessive genotype; the genotype of this organism is known.) Then, you just look at the offspring produced. If the organism with the unknown genotype were homozygous dominant, all the offspring would have the dominant phenotype. If, however, the organism with the unknown genotype were heterozygous, half the offspring would have the dominant phenotype and half would have the recessive phenotype. Look at the two possible Punnett squares:

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E Maples are deciduous trees (they lose their leaves); this is the best tip-off for choice E. You should be able to eliminate choice A (tundra has only small shrubby plants) and choice D (taiga is coniferous forest) easily. Tropical rain forest (choice B) and temperate grasslands (choice C) do not have black bears.

In a trophic pyramid, primary producers form the base, creating all the energy that will travel up through the pyramid. Just above the producers are primary consumers, which are herbivores that obtain all their energy directly from producers. To obtain that energy also requires energy, and so the amount of energy created by producers is not the same amount that moves up. Approximately 10% of energy is lost for every move up the pyramid, so in this example for the 800,000 kJ of energy produced, only 80,000 kJ will be present for primary consumers, the next level. Secondary consumers are the next level up, feeding on primary consumers, so another 10% of that level's energy will be lost, putting 8,000 kJ at this trophic level.

C A change in a population that occurs over a long period of time is evolution. This alone is a good tip-off that choice C, the only choice that mentions evolution, is correct. Speciation has not occurred, only a change in the characteristic of the birds, thus choices A and D can be eliminated. Any change in the fitness of the insects would change the characteristics of the insect population, not the bird population (B is wrong), and random mutation would not produce a specific, directed effect (E is wrong). Birds with large beaks had greater fitness because they could more easily obtain food, thus they had an advantage over birds with smaller beaks, which died out as time passed.

The pancreas secretes glucagon in response to low blood glucose levels. This causes the liver to release glucose into the bloodstream.

Protists include unicellular, eukaryotic organisms that digest food inside the cell. (Fungi, in contrast, carry out extracellular digestion.)

A type III survivorship curve indicates an organism that produces many offspring, many of which do not survive over time. A type II curve represents organisms that produce several offspring, a decent proportion of which survive over time. A type I curve represents an organism that produces very few offspring, most of which survive over time.