1. UGG for tryptophan, AUA for isoleucine, and AUG for methionine. Because the third letter of the codon can be replaced by a different letter (A or G, U or C) without changing the amino acid specified. As a result of the wobble at the third position, one tRNA can bind to at least two different codons that specify the same amino acid.
2.In a nonoverlapping code, the entire polypeptide would be nonfunctional past the site of mutation, while in an overlapping code, only three amino acids would be affected.
3. The curve for absorbance would show peaks at 18 and 28S, whereas the curve for radioactivity would show a single peak at 45S.
4. The radioactivity profile would follow the absorbance profile very closely, showing peaks at 18S and 28S. There might also be very small peaks of radioactivity in the rRNA precursor size ranges, reflecting the fact that rRNA precursors are still being synthesized with 3H-uridine.
5.A polypeptide with alternating amino acids arginine and glutamic acid. Only one type of polypeptide would be produced, because it wouldn't matter where the ribosome first attached to begin reading. There would likely be differences as to the specific amino acid (Arg or Glu) at the two ends of the polypeptides.
6.23 or 8 different codons.
7. It would be similar to that of Figure 11.21 except it would contain two intervening sequences within the coding region.
8. Eight (one for initiation and termination and two for each elongation).
9. The reannealing curve for the oocyte DNA would have a much greater percentage of its DNA reanneal in the highly repeated fraction due to the amplification of the rDNA.
10. It strongly suggests that, but is not definitive since the code could have still been overlapping and the other two codons affected were degenerate ones that did not change their coding properties.
11. They would comprise a population of highly variable length. 16 of them (GAA, CAA, AAA, GAG, CAG, AAG, GGA, CGA, AGA, UCA, UUA, UAC, UAU, UGC, UGU, UGG).
12. It would take 5 nucleotides (25). 24 would only be able to code for 16 different amino acids, whereas 25 could code for 32 amino acids.
13.Ancient life forms would have required some mechanism for self-replication of their genetic information. This is the type of catalytic activity that would allow an RNA genome to be copied accurately in the same way that DNA polymerases are able to replicate DNA in a modern cell. Presumably, it will never be possible to prove the existence of an ancient RNA world, particularly using ribozymes generated in the laboratory, because there is no direct physical evidence that such molecules ever arose in primitive living organisms. The strongest evidence for such a world would probably be the discovery of an organism living today that utilized RNA as both its genetic material and its catalysts. Such an organism would presumably be a relic of an ancient RNA world that had survived for four billion years
14. Because it has such a short half-life. Although rRNAs and tRNAs are synthesized at a much slower rate, they have a much longer lifetime, which causes them to accumulate while the mRNA is being degraded.
15. 5'-GCU-3'. This same tRNA could be used to incorporate serine when the codon was AGU instead of AGC.
16. Evolution progresses using materials that are available in the organism at the time. The fact that protein synthesis utilizes RNA catalysts suggests that RNA preceded proteins and were responsible for the appearance of proteins. The fact that DNA transcription utilizes only proteins, suggests that proteins were present as the cell's primary catalysts at the time that DNA evolved as the genetic material and that RNAs no longer had much of a basic catalytic role.
17. Yes, because frameshift mutations are very likely to generate premature termination codons in their new reading frame.
18. That one strand can serve as the template for transription of one gene, and the other strand can serve as the template for transcription of another gene.
19. Because they are only about 20-25 nucleotides long, it is very unlikely one of them will be affected by a mutagenic agent (e.g., UV irradiation) that strikes the genome randomly. Similarly, their small size and lack of distinguishing features (e.g., splicing sequences) makes them virtually impossible to identify in a background of millions of noncoding sequences present in the genome. They were eventually identified by purifying RNAs of very small size (less than 25 nucleotides), reverse transcribing them, sequencing them, and searching genome databases for their presence. That's when it was discovered they were encoded by precursors with fold-back structures.
