A student recently asked: "I am confused by this explanation because I thought that all eukaryotic cells were monocistronic, meaning that only one protein can be expressed from a gene?"
Eukaryotic mRNA is monocistronic whereas prokaryotic mRNA is polycistronic; however, the question stem is not asking us to consider this. The question stem is asking us to consider the central dogma of biology (i.e. DNA --> RNA --> Protein). The modifications described in the passage and mentioned in the question are referring to covalent modifications of a protein. This means translation has already occurred, qualifying the modifications as post-translational.
Additionally, monocistronic mRNA means only one protein will be translated from one mRNA transcript, not that each gene can only code for one protein. Multiple isoforms of the same protein is possible due to alternative splicing, which is carried out in the nucleus and classified as a post-transcriptional modification.
Hope that helps!
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