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NBME 16 Answers

nbme16/Block 1/Question#5 (reveal difficulty score)
Poliovirus mRNA lacks a 5' m7G cap but is ...
Presence of an internal ribosome entry site ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–

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submitted by โˆ—cassdawg(1779)

In eukaryotes, the 5' methylguanosine (m7G) cap is important in the initiation of translation because eukaryotic initiation factors (eIFs) identify the cap and help assemble the ribosome at that site (FA2020 p45).

The absence of a 5' m7G cap would thus require a different sequence to allow translation initiation and ribosomal entry (i.e. an internal ribosomal entry site).

The 5' and 3' untranslated regions are more important in regulation of translation and termination of translation, respectively, and their absence would not aid translation of the viral mRNA. The 3' poly-A tail is important for protection of eukaryotic mRNA from degradation and aiding in exportation from the nucleus; its absence would not aid translation. A very short open reading frame would not make a significant difference in ability to be translated (the open reading frame is the part of the mRNA able to be translated).

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