C) If you know the sequence of one codon for a particular amino acid, and you ha
ID: 207312 • Letter: C
Question
C) If you know the sequence of one codon for a particular amino acid, and you had to guess what the sequence of the other codons were, what positions would you have a better than random chance of being able to predict what the sequence was? 2. You are studying the tryptophan synthetase gene that Yanofsky also examined to determine the relationship between the nucleotide sequence and the amino acid sequence of the gene, as illustrated in Figure 8.3). A) Yanofsky found a large number of mutations that affected the tryptophan synthetase gene. How did he find these mutations and why do you think he probably not recover missense mutations in a big stretch of the gene that encodes the part of the protein between amino acids 49 and 175? B) You start by looking at the mutations that Yanofsky recovered. One of these mutations affected amino acid number 49 and changed it from Glutamic acid to Valine. If you took this mutant E. Coli line and exposed it to a mutagen that could potentially change bases, insert bases, or delete bases, what type of second mutation could you discover that would create a wild-type tryptophan synthetase gene and where would it be located? C) Most of the mutations that Yanofsky recovered were missense mutations. However, Yanofsky also recovered a nonsense mutation that changed amino acid number 15 into a stop codon. This codon normally encodes Lysine. Does the recovery of this mutation support the hypothesis that this Lysine residue is critical in the function of the tryptophan synthetase protein? Why or why not? 3. What does it mean that the genetic code is degenerate? 4.In eukaryotes and prokaryotes, what is the sequence of the initiator codon?Explanation / Answer
3.A code in which several code words have the same meaning.
Of 64 codons, 61 represent amino acids, and three are stop signals.
Although each codon is specific for only one amino acid (or one stop signal), the genetic code is described as degenerate, or redundant, because a single amino acid may be coded for by more than one codon.
4.An initiator codon in DNA initiates the translation of the first amino acid in the polypeptide chain.
The first three bases of the coding sequence of mRNA to be translated into proteins, is where the initiation codon is located.
This is an important structure, because the actual protein sequence that is translated is defined by a start codon.
The initiation codon is almost always preceded by an untranslated region called 5' UTR, which is also known as the leader sequence.
It is a particular section of mRNA, which starts at the +1 position. This is the region where transcription begins and ends, just before the codon start of the coding region.
The start codon always codes for methionine in eukaryotes and a modified Met (fMet) in prokaryotes. The most common start codon is AUG.
The start codon is often preceded by a 5' untranslated region (5' UTR). In prokaryotes this includes the ribosome binding site.
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