Erwin Chargaff investigated the nucleotide composition of DNA. He analyzed DNA from various organisms and measured the relative amounts of adenine (A), guanine (G), cytosine (C), and thymine (T) present in the DNA of each organism. Table 1 contains a selected data set of his results. Table 1. Nucleotide composition of selected organismsample DNA from Organism %A %G %C %T %(G+C) %(A+T) Octopus 33.2 17.6 17.6 31.6 35.2 64.8 Chicken 28.0 22.0 21.6 28.4 43.7 56.4 Rat 28.6 21.4 20.5 28.4 42.9 56.0 Grasshopper 29.3 20.5 20.7 29.3 41.2 58.6 Wheat 27.3 22.7 22.8 27.1 45.5 54.4 Which of the following statements best explains the data set?

Phytoplankton (black line) are important because they occupy the bottom of the food web absorbing CO2, functioning as a carbon sink. A scientist suggests that an El Niño event in early 1998 caused a significant decrease in the abundance of phytoplankton that year in the equatorial Pacific Ocean. The horizontal axis is labeled Year, and the years 1990 through 2000, in increments of 2, are indicated. The vertical axis is labeled, Relative Plankton Density. A key indicates that one line represents zooplankton and the other line represents phytoplankton. The data represented in the graph are as follows. Note, all data are approximate. Data for zooplankton. The line begins approximately one third of the way up the vertical axis above the origin and trends slightly upwards and to the right, fluctuating up and down across the length of the horizontal axis with upward spikes approximately every two years, and a significant downward spike at the year 1998 that falls to just above the horizontal axis. Data for phytoplankton. The line begins just below the midpoint on the vertical axis, above the origin, and trends slightly upwards and to the right, fluctuating up and down across the length of the horizontal axis, staying above the line for zooplankton and with a significant downward spike at the year 1998 that falls one third of the way up the vertical axis, comes back up by 1999, and remains fairly constant after that. Figure 1. Relative abundance of plankton over time in the eastern equatorial Pacific Ocean Which of the following observations would best support this alternative hypothesis?

There is a horizontal rectangle that is labeled D N A. The rectangle is divided into 9 boxes. From left to right, the first box is labeled P sub r, the second box is labeled trp R, the third box is labeled promoter, the fourth box is labeled Operator, the fifth box is labeled trp E, the sixth box is labeled trp D, the seventh box is labeled trp C, the eighth box is labeled trp B, and the ninth box is labeled trp A. Above the rectangle on the left is a large oval shape labeled R N A Polymerase. An arrow extends from the oval shape to the right and points to an X that is positioned above the boxes labeled Promoter and Operator. An irregular shape that looks like half of a circle and which is labeled R is positioned on top of the boxes labeled Promoter and Operator. A smaller oval shape labeled T is positioned partly on top of the shape labeled R. Underneath the rectangle, an arrow points downward from the box labeled trp R to a small, horizontal rectangle labeled trp R m R N A. On the left side of the rectangle is a 5 prime and a dash. On the right side of the rectangle is a dash and a 3 prime. An arrow points from the rectangle to a large oval shape labeled R. The large oval shape has a smaller oval shape cut out of it. To the left of the large oval shape are the words Inactive Repressor. A horizontal arrow points from the large oval shape to an irregular shape that looks like half of a circle and which is labeled R. A smaller oval shape labeled T is positioned partly on top of the shape labeled R. To the left of the irregular shape are the words Active Repressor. Above and to the left of the irregular shape is a small oval shape labeled T. A curved arrow extends from the shape labeled T down and to the right, eventually merging with the horizontal arrow that extends from the large oval shape on the left to the irregular shape on the right. A curved arrow extends from the irregular shape up and counterclockwise to point to the same shape that is positioned on top of the boxes labeled Promoter and Operator. There is a key below the figure that reads R equals Repressor molecule, T equals Tryptophan molecule, P sub R equals Promoter for the trp R gene, trp R equals Repressor gene, and Operator equals Binding site for repressor molecule. The Trp operon is a coordinately regulated group of genes (trpA–trpE) that are required for tryptophan biosynthesis in E. coli. Based on the figure above, which of the following correctly describes the regulation of the Trp operon?

There are two small black, shaded circles on the plate. The one on the left is labeled A and is surrounded by an unshaded circle on the agar. There is also another small black dot next to the circle. The other circle is labeled P and has no other visible change on the agar. A sterile agar plate, I, is streaked with a pure culture of bacteria by means of aseptic techniques. Paper discs treated with the antibiotics Aureomycin (A) and penicillin (P) are placed at opposite sides of the plate, as shown in the diagram above. The plate is examined after a 24-hour incubation period, and a clear ring is discovered around disc A, but not around disc P. Within the clear ring around disc A, a single bacterial colony with physical characteristics like those of the pure culture is observed. A second sterile agar plate, II, is then streaked with this single colony and also incubated with antibiotics. Which of the following would most likely be observed in plate II after 24 hours?

All of the following were likely present on the primitive Earth during the evolution of self-replicating molecules EXCEPT

In plants, members of the euphorbia family and the cactus family have separate origins, with the euphorbia evolving in the desert regions of Asia and Africa and the cacti in the desert regions of the Americas. Both have fleshy stems adapted for water storage, protective spines, and leaves that are greatly reduced or absent. The similarities between these two families of plants represent an example of

The food web consists of the following 9 organisms or groups of organisms, from bottom to top and left to right: Plants, Squirrels, Mice, Insects, Spiders , Insectivorous Birds, Foxes, and Hawks and Owls, Snakes. From bottom to top, the food web is organized as follows. From Plants, arrows extend to Squirrels, Mice, and Insects. From Squirrels, arrows extend to Foxes, and Hawks and Owls. From Mice, arrows extend to Hawks and Owls, and Snakes. From Insects, arrows extend to Spiders, and Insectivorous Birds. From Spiders, an arrow extends to Insectivorous Birds. From Insectivorous Birds, arrows extend to Hawks and Owls, and Snakes. Which of the following changes to the environment will most likely lead to more energy entering the meadow community represented above?

A model that represents a process occurring in a cell of a particular organism is shown in Figure 1. where the two strands in the middle portion of the helix have separated from each other. Twelve bases are indicated on each of the two strands. Seven complementary bases, with the sequence A U G A U C U, are shown bound to the bottom of the two DNA strands, starting from the left end of the strand. An arrow above the seven bases points from left to right. Figure 1. Process occurring in a cell Which of the following correctly explains the process shown in Figure 1?

A fish population of 250 in a pond has a maximum annual rate of increase of 0.8. If the carrying capacity of the pond is 1,500 for this species, what is the expected population size after one year?

Iron is an essential nutrient that is acquired by organisms from the environment. When intracellular levels of iron are relatively high, living cells synthesize an iron-storage protein called ferritin. The induction of ferritin synthesis by iron was investigated in rats. Figure 1 shows the results of an experiment in which cellular levels of ferritin protein were measured in the presence or absence of iron and actinomycin D, a drug that inhibits transcription. Figure 2 shows the results of an experiment in which cellular levels of ferritin protein were measured in the presence or absence of iron and cycloheximide, a drug that inhibits translation. The horizontal axes are labeled Treatment, and the vertical axes are labeled Relative Protein Level. Five tick marks appear on the vertical axis, from bottom to top, the first tick mark is labeled zero, the third tick mark is labeled one point zero, and the fifth tick mark is labeled two point zero. The graph on the left is labeled Figure one. Ferritin Protein Levels. Four vertical bars appear along the horizontal axis, and the presence or absence of Iron and Actinomycin D is indicated above each bar. The data in the graph, from left to right, are as follows: Bar 1: Absence of Iron; Absence of Actinomycin D; Relative Protein Level one point zero. Bar 2: Presence of Iron; Absence of Actinomycin D; Relative Protein Level two point two. Bar 3: Absence of Iron; Presence of Actinomycin D; Relative Protein Level zero point nine. Bar 4: Presence of Iron; Presence of Actinomycin D; Relative Protein Level two point zero. The graph on the right is labeled Figure 2. Ferritin Protein Levels. Four vertical bars appear along the horizontal axis, and the presence or absence of Iron and Cycloheximide is indicated above each bar. The data in the graph, from left to right, are as follows: Bar 1: Absence of Iron; Absence of Cycloheximide; Relative Protein Level one point zero. Bar 2: Presence of Iron; Absence of Cycloheximide; Relative Protein Level two point five. Bar 3: Absence of Iron; Presence of Cycloheximide; Relative Protein Level zero point nine. Bar 4: Presence of Iron; Presence of Cycloheximide; Relative Protein Level zero point nine. In individuals with iron-rich diets, intracellular levels of iron are higher than in individuals with iron-poor diets. Which of the following best explains how organisms regulate ferritin protein levels in response to available iron?