1. Look at the figure on the first page (Synthesis of Protein Molecules) and relate it to the table on the third page (Genetic Code).
1a. What specifically determines that Ala-tRNA (difficult to read in the figure) is bound to the ribosome? What determines that Val-tRNA is bound?
1b. The figure shows the process of valine (Val) attaching to alanine (Ala) to increase the size of the growing protein chain by one amino acid. Redraw the figure one moment into the future, after the transfer of valine to the chain has been accomplished.
1c. Redraw the figure one moment into the past, before the transfer of alanine to the chain has been accomplished.
1d. What triplet codons were used to encode Ser, Gly, Leu, and Ser upstream from the alanine residue? What base (C, U, A, or G) do you think precedes "CUCUUGGG" at the left hand side of the figure? (The RNA extends well beyond to the left and right what is shown on the page).
1e. Continue translating the message beyond Ala-Val-Phe- as far as you can.
2. What is the relationship between DNA, RNA, and protein?
3. Crick states that the genetic code translates in one direction only: nucleic acid to protein. Why is this? Try to translate in the reverse direction, using the protein sequence shown in the figure on the first page: Tyr-Thr-Pro-Ser-Leu-Gly-Ser-Ala-Val-Phe. Remember, in doing the translation, you can see only the protein sequence. You don't know the messenger RNA sequence.
4. Crick speaks of both tRNA (transfer RNA) and mRNA (messenger RNA). What is the function of each in translating the information contained in nucleic acids to protein?
5. Compare the table on the second page (Twenty Amino Acids) to the table on the third page (Genetic Code). Why aren't "OCHRE" and "AMBER" listed in the first table?
6. Translate the following message, beginning with the first base: AUGUUUCACUAAGUG.
7. Consider the table shown on the fourth page (Variety of Synthetic RNA's). What amino acid sequences would you expect if the genetic code were doublet instead of triplet?
8. Why does poly-AUC make polyisoleucine, polyserine, and polyhistidine? What enables Crick (top right of p.114) to assign specific amino acids to specific codons?
9. What is a mutation? Give an example of a base substitution mutation and a phase shift mutation of the following sequence: AUGUUUCACUAAGUG.
10. The PAP-S gene encodes Pokeweed Antiviral Protein, a protein that is able to suppress infection by certain viruses. Unfortunately, the protein also tends to suppress life by humans (1st UR Symposium on Transgenic Tobacco). Suppose you found a mutation near the beginning of the gene that made the protein less toxic towards humans while retaining its effectiveness towards viruses. Would you expect this mutation to be a base substitution or a phase shift?
11. Which of the following changes in protein could be caused by a single base substitution?
11a. leucine _ phenylalanine
11b. proline _ histidine
11c. arginine _ methionine
11d. serine _ aspartate
12. Base substitutions takes place in humans at the rate of about one substitution every billion bases per cell generation. Cancers are caused by mutations in specific genes. Perhaps a dozen mutations are required to get a full-blown tumor. What (quantitatively) is the likelihood that 12 specific base substitutions occur simultaneously?
13. Yet cancer happens. How can we reconcile the gross improbability that 12 mutations occur with the relatively high frequency of cancer?