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1. One way to think about the genetic code is to compare it to the International Morse Code, a system of dots, dashes, and spaces used to send messages by telegraph and shortwave radio. In the Morse code, each letter of the alphabet is represented by a unique combination of two, three, or four dots and dashes. For example, dot-dash represents the letter A, dash-dot-dot-dot represents the letter B, dot-dash-dot represents the letter C, and so forth. Based on this information and your knowledge of the genetic code, explain each of the following terms as it relates to coding, and indicate with an M if it is true of the Morse code, with a G if it is true of the genetic code, and with both letters if it is true for both.

a) degenerate

b) unambiguous

c) triplet

d) universal

e) nonoverlapping

2. A portion of a polypeptide produced by bacteriophage T4 was found to have the following sequence of amino acids:

Deletion of a single nucleotide in one location on the T4 DNA template strand with subsequent insertion of a different nucleotide nearby changed the sequence to

a) what was the nucleotide sequence of the segment of the mRNA that encoded this portion of the original polypeptide?

b) What was the nucleotide sequence of the mRNA encoding this portion of the mutant polypeptide?

c) Can you determine which nucleotide was deleted and which was inserted? Explain your answer.

3. In a cell-free protein synthesis system from E. coli, the polyribonucleotide AUGUUUUUUUUUUUU directs the synthesis of the oligopeptide met-phe-phe-phe-phe. In the presence of Rambomycin, a new antibiotic just developed by Macho Pharmaceuticals, only the dipeptide met-phe is made.

Which step in polypeptide synthesis does Rambomycin inhibit? Explain your answer.

4. To enable it to be transmitted intact from one cell generation to the next, the linear DNA molecule of a eukaryotic chromosome must have appropriate nucleotide sequences making up three special kinds of regions: origins of replication (at least one), a centromere, and two telomeres. What would happen if such a chromosomal DNA molecule somehow lost

a) all of its origins of replication?

b) all of the DNA constituting its centromere?

5. Carefully inspect the double-stranded DNA molecule shown here, and notice that it has twofold rotational symmetry:

Label each of the following statements as T if true or F if false.

a) There is no way to distinguish the right end of the double helix from the left end.

b) If a solution of these molecules were heated to denature them, every single-stranded molecule in the solution would be capable of hybridizing with every other molecule.

c) If the molecule were cut at its midpoint into two halves, it would be possible to distinguish the left half from the right half.

d) If the two single strands were separated from each other, it would not be possible to distinguish one strand from the other.

e) In a single strand from this molecule, it would be impossible to determine which is the 3' end and which is the 5' end.

1. These problems are intended to be challenging. To answer them, you should expect to make reference to your text (and perhaps your Bio200 notes) and to spend some time thinking them through. DON'T PANIC if answers don't come to you right away!

2. You may consult with your colleagues in solving these problems. Each of you, however, should turn in your own answers in your own words.

3. These are only a beginning! You should also review your Bio200 materials (especially assignments and exams) and chapter 4 in Lodish. Use the CD-ROM question and answer guide. (An extra credit point to each student who sends me an e-mail explaining which chapter 4 question on the CD-ROM is miskeyed--one of them is.) Come and ask me questions about whatever you're unsure of.

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