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Chapter 13: Problem 4

The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids. Why?

Short Answer

Expert verified
Answer: The mRNA formed from the repeating tetranucleotide UUAC incorporates three amino acids, while the mRNA formed from the repeating tetranucleotide UAUC incorporates four amino acids.

Step by step solution

01

Understand the Genetic Code

The genetic code is a set of rules that define how the four-letter genetic alphabet (A, C, G, and U) is translated into the 20-letter amino acid alphabet. The code is composed of three-letter "codons" in the mRNA, each of which corresponds to a specific amino acid. There are 64 different codons, and three of these (UAA, UAG, and UGA) are known as "stop codons," meaning they do not code for any amino acids and instead signal the end of translation.
02

Analyze the Repeating Tetranucleotide UUAC

When we analyze the repeating tetranucleotide UUAC, we must split it into three-letter codons to see which amino acids are coded for. If we start from the first base, we get the codons UUA and UAC, which code for the amino acids Leucine and Tyrosine, respectively. If we start from the second base, we get UAC and UUA, which are the same codons we obtained from the first frame. Thus, the mRNA formed from UUAC incorporates only three amino acids: Leucine, Tyrosine and a stop signal (the third codon).
03

Analyze the Repeating Tetranucleotide UAUC

Now, let's analyze the repeating tetranucleotide UAUC. If we start from the first base, we get the codons UAU and CUA, which code for Tyrosine and Leucine, respectively. If we shift to the second base, we get the codons AUC and UAU, which code for Isoleucine and Tyrosine, respectively. So, in this case, the mRNA formed from UAUC incorporates four different amino acids: Tyrosine, Leucine, Isoleucine, and a stop signal (the third codon).
04

Conclusion

The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, while the mRNA formed from the repeating tetranucleotide UAUC incorporates four amino acids. This difference is due to the distinct codon combinations produced from the different tetranucleotide sequences and how they are translated in the genetic code.

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Most popular questions from this chapter

Predict the amino acid sequence produced during translation by the following short hypothetical mRNA sequences (note that the second sequence was formed from the first by a deletion of only one nucleotide): Sequence 1: 5'-AUGCCGGAUUAUAGUUGA-3' Sequence \(2: 5^{\prime}-\) AUGCCGGAUUAAGUUGA-3' What type of mutation gave rise to Sequence 2 ?

What was the initial evidence for the existence of mRNA?

Describe the role of two forms of RNA editing that lead to changes in the size and sequence of pre-mRNAs. Briefly describe several examples of each form of editing, including their impact on respective protein products.

M. Klemke et al. (2001) discovered an interesting coding phenomenon in which an exon within a neurologic hormone receptor gene in mammals appears to produce two different protein entities (XL \(\alpha\) s, ALEX). Following is the DNA sequence of the exon's \(5^{\prime}\) end derived from a rat. The lowercase letters represent the initial coding portion for the XL \(\alpha\)s protein, and the uppercase letters indicate the portion where the ALEX entity is initiated. (For simplicity, and to correspond with the RNA coding dictionary, it is customary to represent the noncoding, nontemplate strand of the DNA segment.) \(5^{\prime}-\) gtcccaaccatgcccaccgatcttccgcctgcttctgaagATGCGGGCCCAG (a) Convert the noncoding DNA sequence to the coding RNA sequence. (b) Locate the initiator codon within the XL \(\alpha\) segment. (c) Locate the initiator codon within the ALEX segment. Are the two initiator codons in frame? (d) Provide the amino acid sequence for each coding sequence. In the region of overlap, are the two amino acid sequences the same? (e) Are there any evolutionary advantages to having the same DNA sequence code for two protein products? Are there any disadvantages?

One form of posttranscriptional modification of most eukaryotic RNA transcripts is the addition of a poly-A sequence at the \(3^{\prime}\) end. The absence of a poly-A sequence leads to rapid degradation of the transcript. Poly-A sequences of various lengths are also added to many prokaryotic RNA transcripts where, instead of promoting stability, they enhance degradation. In both cases, RNA secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A sequences. Considering the activities of RNAs, what might be general functions of \(3^{\prime}\) -polyadenylation?
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