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post translational modification glycosylation 5 prime capping methylation endonuclease cleavage biochemistry
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Answer: Option 1 — Glycosylation
Correct Answer: 1. Glycosylation
Glycosylation is a post-translational modification (PTM).
Explanation
Post-translational modifications (PTMs) are chemical changes that occur to a protein after it has been synthesized (translated) by the ribosome. They alter the structure, function, localization, or activity of the final protein.
As confirmed by Emery's Elements of Medical Genetics and Genomics:
"Many proteins, before they attain their normal structure or functional activity, undergo post-translational modification, which can include chemical modification of amino-acid side chains (e.g., hydroxylation, methylation), the addition of carbohydrate or lipid moieties (e.g., glycosylation) or proteolytic cleavage of polypeptides (e.g., the conversion of proinsulin to insulin)."
Why the Other Options Are Wrong
| Option | Process | Correct Category |
|---|---|---|
| 1. Glycosylation ✅ | Addition of sugar moieties to a protein after translation | Post-translational modification |
| 2. 5' capping ❌ | Addition of 7-methylguanosine to the 5' end of mRNA | Post-transcriptional modification (of RNA, not protein) |
| 3. Methylation ❌ | In this context, methylation of DNA/histones refers to epigenetic modification; methylation of mRNA (m⁶A) is a post-transcriptional modification. Methylation of amino acid side chains is also a PTM — but glycosylation is the more classic, unambiguous textbook answer for this INICET question. | |
| 4. Endonuclease cleavage ❌ | Refers to cutting of nucleic acids (DNA or RNA) — this is a DNA/RNA processing event, not a protein modification |
Note on Option 3 (Methylation): While methylation can be a post-translational modification (e.g., methylation of lysine/arginine on histones), in standard biochemistry MCQs it is more classically categorized as an epigenetic/DNA modification or post-transcriptional RNA modification. Glycosylation is the unambiguous, universally accepted PTM among the choices.
Key PTMs to Remember (for exam)
| PTM | Example |
|---|---|
| Glycosylation | IgG antibodies, collagen, erythropoietin |
| Phosphorylation | Kinase signaling |
| Ubiquitination | Protein degradation |
| Hydroxylation | Proline → hydroxyproline in collagen |
| Proteolytic cleavage | Proinsulin → Insulin |
| Acetylation | Histone modification |
5' capping, splicing, and polyadenylation are all post-transcriptional modifications of pre-mRNA — not PTMs.
Answer: 1. Glycosylation ✅
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