MIT Department of Biology 7.014 Introductory Biology, Spring 2005 Solutions to 7.014 Quiz II Class Average = 79 Median = 82 Grade Range % A 90 - 100 27 B 75 - 89 37 C 59 – 74 25 D 41 – 58 7 F 0 – 40 2Name:________________________________________ TA:____________________ Question 1 (29 points) Barney is an alien. On his ship, hidden in the Stata building, you find alien bacteria that metabolize wood. You call this species A. termiticus, and call your original strain BLT (for “Barney’s little termiticus”). You subject a sample of BLT to mutagens, and isolate a new strain that no longer metabolizes wood. You conclude that you have succeeded in disrupting at least one gene necessary to metabolize wood. You call the mutant strain M. You mix a sample of M with a sample of heat-killed wild type BLT, and the resulting strain metabolizes wood. You summarize your data in the following table: Strains Metabolizes wood? BLT Yes Heat-killed BLT No M No Heat-killed BLT +M Yes a) Did the content of any of the BLT or M cells change in the experiment? If yes, which cells underwent the change, and what change occured? If not, explain why there was no change. Some cells of type M underwent the change. They acquired some of the genetic material from the heat-killed BLT cells. In particular, some part of the genetic material they acquired encoded for the gene that was mutated in M. Having received this gene, M cells now behave like the wild-type BLT cells because they can now produce the agent (like the Earth proteins) that restores the wood metabolism pathway. You plan to characterize the alien genetic material. You start by breaking some A. termiticus cells open to determine their molecular composition. You find that they contain various small molecules, carbohydrates, lipids, and two other macromolecules, A and B. In order to determine which macromolecule is the carrier of genetic information, you repeat your previous experiment, but include test tubes where you treat the sample of the heat-killed BLT with either an agent that destroys macromolecule A (A-ase) or macromolecule B (B-ase). You find the following results (including the repeat of your previous experiment in the first 4 lines): Strains and agents Metabolizes wood? BLT Yes Heat-killed BLT No M No Heat-killed BLT +M Yes A-ase treated heat-killed BLT +M Yes B-ase treated heat-killed BLT +M No b) Which molecule is the carrier of genetic information in A. termiticus? Justify your answer. B is the carrier of genetic information in the alien organism. This is because when the BLT cells are treated with B-ase, no B remains in the sample, and, as a result, no transformation occurs. On the other hand, treatment with A-ase has no bearing on the transformation ability of the heat-killed BLT cells. We, therefore, must conclude that B is the carrier of genetic information. 2Name:________________________________________ TA:____________________ Question 1, continued Next, you set out to determine the structure of the alien genetic material molecule. You first determine that it has six types of bases that you name S, V, W, X, Y, Z. You further determine that the alien cell’s content of S is the same as its content of each of X and Z; and that the content of V is the same as its content of each of W and Y. When you determine the structure of this molecule by X-ray crystallography, you are not surprised to find that the molecule consists of 3 interacting strands. c) What base interaction combinations do you expect for this molecule? Based on the data, we expect S on one strand to interact with X on another strand and Z on the third strand. We would also expect V on one strand to interact with W on another strand and Y on the third strand. (You want to investigate the mechanism of replication of the alien genetic material. You decide to repeat the Meselson-Stahl experiment. Recall that labeled strands are “heavy” (low in the gradient) and unlabeled strands are “light” (high in the gradient). Also recall that in the experiment, the culture is grown on heavy Nitrogen (15N), and is switched to light Nitrogen 14N) at time=0. Before proceeding you define three possible models of alien replication: • conservative, in which, after replication, old strands stay together, and new molecules are made entirely of new strands; • semi-conservative, in which, after replication, each molecule has one old strand; and • asymmetric, in which replication creates a molecule with one new strand, and a molecule with two new strands. Each column in the table below reflects the predictions one of these models makes about the outcome of the experiment. The outcomes are described using symbols a-d to indicate the levels on the gradient as depicted in the figure on the right. d) Label each column with the name of the appropriate model. a b c d # cycles of replication Asymmetric Conservative Semi-conservative 0 d d d 1 b, c a, d b 2 a, b, c a, d a, b 3 a, b, c a, d a, b You determine that each A. termiticus mother cell completes one round of genetic material replication, but gives rise to three daughter cells. e) Which of the above models is not consistent with this data? Why? In order to create three daughter cells, replication must produce three molecules of genetic material per cycle. Asymmetric model predicts creation of only two molecules of genetic material in each replication cycle. Therefore, asymmetric model is not consistent with data. 3X Name:________________________________________ TA:____________________ Question 1, continued Below are the structures of an alien nucleotide X and the earth DNA nucleotide cytosine (C). ‐O ‐O ‐O S O S O S ‐O O ‐O ‐O ‐O NH2 NH2 N O O O NO O‐P O P O P 5’ O O‐O‐O‐ 3’ OH f) For both structures, i. box the sugar ii. circle the base iii. underline the part of the structure used to power the addition of the nucleotide onto the growing chain. g) For the DNA nucleotide, i. label the 5’ end ii. label the 3’ end iii. draw an arrow to the part of the molecule that identifies it as a nucleotide used in DNA rather than in RNA. 4Name:________________________________________ TA:____________________ Question 2 (30 points) a) Transcription i. is the process that transfers information from ___DNA____ to ____RNA___. ii. in eukaryotic organisms, transcription occurs in the Nucleus Ribosome Ribosome
View Full Document
Unlocking...