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Animation: Post-Transcriptional Control Mechanisms

by Pearson
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Post-Transcriptional Control Mechanisms After transcription, a cell can rapidly adjust to environmental changes by regulating several processes. In RNA processing, a cap and tail are added, introns removed, and remaining exons spliced together. Alternative splicing of RNA may create different mRNA or messenger RNA molecules from the same primary RNA transcript. Passage of mRNA through the nuclear envelope provides additional opportunities for post-transcriptional control. The life span of mRNA molecules helps determine how much of a protein is made. Nucleotide sequences that affect mRNA longevity are often found in the 3 prime untranslated region following the stop codon. Enzymes attack the poly-A tail first, and eventually the rest of the mRNA. Depending on its ability to resist this degradation, a eukaryotic mRNA molecule may last from hours to weeks. The longer an mRNA lasts, the more protein is made. Translation of specific mRNAs may also be blocked by regulatory proteins that bind to sequences within the untranslated region at the 5 prime end of the mRNA molecule, modulating the amount of protein produced. Often, polypeptides must be modified in some way to produce functional proteins. For example, a polypeptide is cleaved in two or more polypeptide chains. Other molecules might be attached to the protein, or the protein might be phosphorylated. Some proteins need to be transported to specific destinations. A cell can get rid of abnormal or damaged proteins, and limit the lifetime of functional proteins, by means of selective degradation. The cell marks the protein for destruction by attaching small ubiquitin proteins. Giant protein complexes called proteasomes recognize ubiquitin and break down the tagged protein.
Post-Transcriptional Control Mechanisms After transcription, a cell can rapidly adjust to environmental changes by regulating several processes. In RNA processing, a cap and tail are added, introns removed, and remaining exons spliced together. Alternative splicing of RNA may create different mRNA or messenger RNA molecules from the same primary RNA transcript. Passage of mRNA through the nuclear envelope provides additional opportunities for post-transcriptional control. The life span of mRNA molecules helps determine how much of a protein is made. Nucleotide sequences that affect mRNA longevity are often found in the 3 prime untranslated region following the stop codon. Enzymes attack the poly-A tail first, and eventually the rest of the mRNA. Depending on its ability to resist this degradation, a eukaryotic mRNA molecule may last from hours to weeks. The longer an mRNA lasts, the more protein is made. Translation of specific mRNAs may also be blocked by regulatory proteins that bind to sequences within the untranslated region at the 5 prime end of the mRNA molecule, modulating the amount of protein produced. Often, polypeptides must be modified in some way to produce functional proteins. For example, a polypeptide is cleaved in two or more polypeptide chains. Other molecules might be attached to the protein, or the protein might be phosphorylated. Some proteins need to be transported to specific destinations. A cell can get rid of abnormal or damaged proteins, and limit the lifetime of functional proteins, by means of selective degradation. The cell marks the protein for destruction by attaching small ubiquitin proteins. Giant protein complexes called proteasomes recognize ubiquitin and break down the tagged protein.