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NBME 24 Answers

nbme24/Block 1/Question#31 (43.3 difficulty score)
An experimental study is conducted to examine ...
Post-translational modificationπŸ”
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 +5  upvote downvote
submitted by nor16(49),
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OPMC is a ohnrroompe eepiptd ichna.

tI si tdenasrtla nad lrtae yeszmne ctu isht itedppe ni het pb.esdesptiu

cniptainsrTro si orwgn i( hda tsih to...)o cb/ tsi ont eth AmRN ttah akesm dfeeifntr eptpied sroh.enmo orroeM,ve wtha I ese nwo ,is ttha pitrrlsicantnaptsoo moidcontafii is orem or elss gic.i.nps.l so sltanrta.spto- iitonc.mfaoid enxt iemt ew ekam ti trecorc ;-)

medguru2295  This is a perfect explanation. I never knew it was the protein modified (I initially though alternate splicing too) +  
nerdstewiegriffin  Is this an example of polycistronic mRNA? +  
paperbackwriter  @nerdstewiegriffin possibly (because it says that the original mRNA results in proteins plural, BUT the mechanism for POMC derived hormones specifically is not bc of polycystronic mRNA. Polycystronic means that you get multiple protein products from one mRNA, but in this case we get multiple PROTEIN DERIVATIVES from one PROTEIN, implying that there are some modifications/reactions taking place once we've already made ONE protein which change the protein into multiple diff. products. +1  



 +2  upvote downvote
submitted by link981(134),
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Keswyord mfor rD. rocTu mrof lKa:npa

  • lnaoipeicR-t AND
  • Tsoitarnicrp-n NAR
  • taionraslT-n oPtneri
hungrybox  bruh this is like bio 101 lol +3  



Even if you didn't know anything about the gene mentioned they tell you the PROTEIN serves as a precursor for making the hormones thus it has already undergone translation and the only choice that makes sense is Post-translational modification. Additionally, mRNA means it has already undergone splicing and post-transcriptional modification as it would be called hnRNA otherwise. (FA2020 pg 41)




 +1  upvote downvote
submitted by suckitnbme(116),

POMC is a prohormone peptide chain that gets cleaved into gamma-MSH, ACTH, gamma-lipotropin, and beta-endorphin. There's a nice figure of this in Costanzo (Fig 9-10).

It may help to remember that pathologies with increased ACTH (ie Addison's disease) can present with hyperpigmentation since MSH (melanocyte stimulating hormone) is produced alongside ACTH.




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Wyh is antaetrviel sginilcp ro scsianirrtotonpptal- oimiinafotcd n?crtoriec

tea-cats-biscuits  You just have to know that POMC is a pro-protein that must be cleaved; not sure if there’s anything in the stem that would really have given it away. +1  
mcl  Dunno if this helps, but it says "this protein" (singular) is the precursor of two different protein products. This must mean that the modification occurs after the protein is made, which means after transcription and splicing has already happened. +28  
ngman  Also I believe mRNA refers to after the splicing already occurs. If the protein products are from the same mRNA then it can't be alternate splicing. +1  
medschul  They're cleaved by tissue-specific proteases +  
duat98  I think: Alternative splicing occurs with hnRNA not mRNA. You get mRNA from alternatively splicing the hnRNA. an mRNA can only make 1 type of protein. Since the question says the 2 proteins comes from the same mRNA it cannot be alternative splice or post transcriptional mod. FA 2018 page 43 has a good illustration. +6  
tadki38097  Just general testing taking strategy i guess, but for this question i was torn between post-transcriptional and post-translational, but then i saw that alternative splicing was also a choice...because alternative splicing IS a post-transcriptional modification you know it has to be post-translational because you can't select two answer choices. +  



 +0  upvote downvote
submitted by asharm10(16),

It's saying a particular protein is made from this mrna, this protein is used as a precursor to produce those two hormones. so proteins once made, they need post translational trimming of N and C propeptides to be active. like trypsinogen becomes trypsin else they are just inactive protein. So any protein to work they need to go through post translational modifications.




So if a long polypeptide string is generated from signle mRNA then cleaved β†’ post-translational modification.

If modification happens at level of RNA which is cleaved into smaller mRNA that then give proteins β†’ alternative splicing / post-tranSCRIPTIONAL modification.