Q51.Arrange the following in sequence of molecular events.
A. Transcription
B. Nuclear Export
C. Translation
D. Splicing
E. Glycosylation
Choose the correct answer from the options given below :
(1) A, D, B, C, E
(2) A, B, C, D, E
(3) C, D, E, A, B
(4) B, A, E, C, D
The correct sequence of molecular events in eukaryotic gene expression is A, D, B, C, E (option 1). This follows the standard flow from DNA to mature protein in the cell.
Correct Sequence
In eukaryotes, gene expression proceeds through these ordered steps:
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A. Transcription: RNA polymerase II transcribes DNA into pre-mRNA (hnRNA) in the nucleus.
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D. Splicing: Introns are removed and exons joined by the spliceosome to form mature mRNA.
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B. Nuclear Export: Mature mRNA exits the nucleus via nuclear pores to reach the cytoplasm.
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C. Translation: Ribosomes in the cytoplasm translate mRNA into a polypeptide chain.
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E. Glycosylation: Post-translational modification where sugars are added to proteins (often in ER/Golgi), occurring after translation.
This order ensures pre-mRNA is processed before export and translation, with glycosylation as a final modification.
Why Other Options Fail
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Option 2 (A, B, C, D, E): Wrong because nuclear export (B) cannot precede splicing (D); splicing must mature the mRNA first.
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Option 3 (C, D, E, A, B): Wrong; translation (C) cannot start before transcription (A) produces mRNA.
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Option 4 (B, A, E, C, D): Wrong; nuclear export (B) before transcription (A) is impossible, and glycosylation (E) is post-translation.
Introduction to Sequence of Molecular Events
Understanding the sequence of molecular events in eukaryotic gene expression—from transcription and splicing to nuclear export, translation, and glycosylation—is crucial for biology students preparing for exams like NEET. This precise order ensures DNA information becomes functional proteins efficiently.
Step-by-Step Breakdown
Gene expression follows a linear pathway in eukaryotes:
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Transcription (A): DNA unwinds; RNA polymerase synthesizes pre-mRNA in the nucleus.
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Splicing (D): Spliceosome removes non-coding introns, ligates exons into mature mRNA.
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Nuclear Export (B): mRNA passes through nuclear pores to cytoplasm.
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Translation (C): Ribosomes read mRNA codons to assemble amino acids into proteins.
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Glycosylation (E): Sugars attach to proteins in ER/Golgi for stability/function.
Common Exam Traps
Options mixing orders (e.g., export before splicing) test knowledge of nuclear processing. Glycosylation always follows translation as it’s post-translational.
Why This Matters
This sequence of molecular events underpins cellular function, disease (e.g., splicing errors in cancer), and biotech. Practice with MCQs reinforces it for competitive exams.
