A second year medical student looking to match into a competitive residency joins a laboratory studying gene regulation in a mouse model of hepatocellular carcinoma. He isolates nucleic acids from the cells after exposure to a known carcinogen and has the sequence analyzed. He is surprised to find that some of the nucleotides are Pseudouridine and ribothymidine. Which type of nucleic acid has the student likely isolated?
A. t RNA
B. r RNA
C. hn RNA
D. m RNA
E. sn RNA
Answer- The correct answer is- A- t RNA.
Transfer RNA s are post transcriptionally modified. Ribothymidine and Pseudouridine are synthesized post transcriptionally.
All tRNA molecules contain four main arms. The acceptor arm terminates in the nucleotides CpCpAOH. These three nucleotides are added posttranscriptionally by a specific nucleotidyl transferase enzyme. The tRNA-appropriate amino acid is attached, or “charged” onto, the 3′-OH group of the A moiety of the acceptor arm ( Figure). The DHU, TΨC, and extra arms help define a specific tRNA. t-RNA is the only RNA to contain these modified nucleotides.
Figure- secondary structure of t RNA
As regards other options
r RNA are structural components of ribosomes. They do not contain these modified nucleotides.
hn RNA (heterogeneous nuclear RNA) is the primary transcript that undergoes further modifications like capping, tailing, base modifications and splicing to become the mature m RNA.
mRNAs are the carriers of genetic information from DNA to the translation machinery. They also do not contain these modified nucleotides.
Sn RNAs (small nuclear RNAs) help in splicing of hn RNA to form mature messenger RNA. These RNAs also do not contain these modifies nucleotides.
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