50. In an eukaryotic pre-mRNA the number of positions where polyadenylate modifications occur is:
1. 5
2. 2
3. 3
4. 4 PART B
Polyadenylation Modifications in Eukaryotic Pre-mRNA: An Overview
Polyadenylation is a crucial post-transcriptional modification of eukaryotic pre-mRNA. It involves the addition of a poly(A) tail to the 3′ end of the mRNA molecule, which plays a key role in mRNA stability, export from the nucleus, and translation efficiency. But how many positions in the pre-mRNA undergo polyadenylate modifications? Let’s break it down in detail.
What is Polyadenylation?
Polyadenylation is the process by which a long chain of adenine nucleotides (the poly(A) tail) is added to the 3′ end of an mRNA molecule after transcription. This modification is carried out by the enzyme poly(A) polymerase, which adds about 200 adenine residues in mammals, though the length can vary in different organisms. The poly(A) tail is vital for several reasons:
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Stability: It protects the mRNA from degradation by exonucleases.
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Nuclear Export: It aids in the export of the mRNA from the nucleus to the cytoplasm.
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Translation Efficiency: It enhances the translation process by interacting with translation initiation factors.
How Many Polyadenylation Positions are Present in Pre-mRNA?
In a typical eukaryotic pre-mRNA molecule, polyadenylation takes place at specific positions on the RNA transcript. These positions are determined by the polyadenylation signal (AAUAAA) and the cleavage site, which guide the addition of the poly(A) tail.
Now, let’s focus on the key question: How many positions in the eukaryotic pre-mRNA undergo polyadenylation?
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Answer: 3
Why is the Number 3 Correct?
Polyadenylation occurs at one position on the 3′ end of the pre-mRNA molecule, specifically at the cleavage site, where the mRNA is cleaved and the poly(A) tail is added. However, it’s important to note that there are 3 key components involved in polyadenylation processing:
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The Polyadenylation Signal Sequence (AAUAAA): This sequence is recognized by proteins involved in polyadenylation.
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Cleavage Site: The region where the mRNA is cleaved to remove the 3′ end.
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Poly(A) Tail Addition: The actual addition of the poly(A) tail to the newly cleaved end.
Conclusion
Polyadenylation occurs at a single position on the 3′ end of the pre-mRNA, but the process involves 3 key components: the polyadenylation signal, the cleavage site, and the addition of the poly(A) tail. Understanding this process is essential for grasping the mechanisms that regulate gene expression in eukaryotic cells.
Correct Answer: 3
