Aresearcher attempts to use a segment of ribosomal rna as a molecular clock to calculate the date of the last common ancestor of two species. he finds that the two species diverged from a common ancestor 150 million years ago. these particular species have abundant fossil records that strongly suggest that these species diverged 100 million years ago. which of the following answer choices best describes a reason for the discrepancy in the date of the last common ancestor of these species? ribosomal rna is not a reliable molecular clock and should not be used for dating. the neutral mutation rate used in calculating the last common ancestor date may not have been accurate. it is highly likely that the two organisms did not share a common ancestor, so the results are invalid. a large portion of the ribosomal rna is not vital to survival, so it is not likely to be well preserved.

Rease is an enzyme used by plants to break down urea (a nitrogen-containing compound) into carbon dioxide and ammonia. urease urea > > > carbon dioxide and ammonia ammonia is broken down by plants into a nitrogen source plants need to grow. thus, plants could not use urea as a nitrogen source unless it was first converted to ammonia. in soybean plants there are two different kinds of urease, one produced in the seeds and the other produced in the leaves of the plant. three types of soybean plants were used in a set of experiments: normal soybeans and two mutant strains, one lacking the urease in the seeds only (strain 1) and one lacking urease in the leaves only (strain 2). experiment 1 separate areas in a field were planted with normal, strain 1, and strain 2 soybeans. all types of soybeans appeared to grow, flower, and produce seeds equally well. there were no externally detectable differences among the strains. experiment 2 small pieces of plant leaves of equal weight were obtained from each type of soybean plant and separately placed on media in culture dishes. tissue growing in this way will become an unorganized clump of cells referred to as callus. to provide a controlled nitrogen source, half the tissue samples of each type were placed on media containing urea, and the other half of the samples were placed on media containing ammonia. after 30 days, the weight gain for each of the callus samples was determined. results are shown in the table below.

Acell in the human nervous system whose primary function is to form myelin and the blood-brain barrier, respond to injury, remove debris, and enhance learning and memory is called a(n) cell.

Which is a function of the central nervous system