Researchers occasionally find a gene whose coding sequences fail to match its protein product. In some instances, the cause has been traced to RNA editing, in which certain bases in the RNA are chemically modified after transcription. The modification is carried out by enzymes. One of these editing enzymes is a cytosine deaminase, which converts a 'C' in the RNA into a 'U'. This type of editing occurs at some 'C' residues in RNA transcripts from plant mitochondria and chloroplasts, and also in the apolipoprotein B RNA transcript in mammals. Consider the mRNA sequence shown here, which is translated in a reading from starting with the first codon at the left:
5' - GUA CCA CGC UCG UCU CAU - 3'
In this sequence, the cytosines accessible to RNA editing are the first C in the 2nd and 3rd sequence and second C in the 4th sequence and the first C in the 6th sequence
A. What is the amino acid sequence if none of the accessible cytosines are edited?
B. What is the amino acid sequence if all of the accessible cytosinces are eduted?
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Protein sequencing is a technique to determine the amino acid sequence of a protein, as well as which conformation the protein adopts and the extent to which it is complexed with any non-peptide molecules. Discovering the structures and functions of proteins in living organisms is an important tool for understanding cellular processes, and allows drugs that target specific metabolic pathways to be invented more easily.
The two major direct methods of protein sequencing are mass spectrometry and the Edman degradation reaction. It is also possible to generate an amino acid sequence from the DNA or mRNA sequence encoding the protein, if this is known. However, there are a number of other reactions which can be used to gain more limited information about protein sequences and can be used as preliminaries to the aforementioned methods of sequencing or to overcome specific inadequacies within them.
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