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RNA and the Genetic Code

Chapter 7: RNA and the Genetic Code

5 min read Updated Apr 18, 2026
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🎯 Diagnostic: Test Your Starting Level 22 questions (2 per section). No prior reading required - see what you already know.

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1. (7.1) Which RNA type carries amino acids to the ribosome?
B. tRNA: "transfer" - physically carries each amino acid to the ribosome and pairs its anticodon with the mRNA codon. mRNA = message; rRNA = ribosome scaffold and catalyst; snRNA = splicing.
2. (7.1) Which RNA is the catalytic component of the ribosome (a ribozyme)?
D. Ribosomal RNA (rRNA) catalyzes peptide bond formation - the ribosome is a ribozyme, not a protein enzyme. The large subunit's 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes) contains the peptidyl transferase center.
3. (7.2) Transcription differs from DNA replication in that:
C. RNA polymerase uses ribonucleotides (with U), transcribes only the template strand, and initiates without a primer. DNA replication uses dNTPs (T not U), copies both strands, and needs an RNA primer.
4. (7.2) If the template (antisense) strand of DNA reads 3'-TACGGA-5', the mRNA produced is:
A. RNA polymerase reads the template 3'→5' and synthesizes mRNA 5'→3' with complementary bases (A↔U, G↔C). So template 3'-TACGGA-5' → mRNA 5'-AUGCCU-3'.
5. (7.3) In eukaryotes, RNA polymerase II transcribes:
B. Eukaryotes have three polymerases: Pol I = rRNA (except 5S); Pol II = mRNA and many regulatory RNAs; Pol III = tRNA + 5S rRNA. Prokaryotes use a single RNA polymerase for everything.
6. (7.3) The TATA box is:
C. TATA-binding protein (TBP) recognizes the TATA box and recruits the rest of the Pol II machinery. Prokaryotes have analogous -10 (Pribnow box, "TATAAT") and -35 elements.
7. (7.4) Post-transcriptional processing of eukaryotic pre-mRNA includes:
D. The 5' cap (7-methylguanosine) protects the mRNA and helps ribosome binding. The poly(A) tail (~100-250 A's) stabilizes the mRNA. Splicing removes introns. All happen in the nucleus before mRNA exits.
8. (7.4) The 5' cap of mRNA is:
B. m7G is attached to the first transcribed nucleotide via a 5'-5' triphosphate bridge - an orientation different from normal 5'-3' phosphodiester bonds. The cap protects from 5' exonucleases and is recognized by the translation initiation machinery (eIF4E).
9. (7.5) Introns are:
A. Exons "Express" → kept in final mRNA. Introns "In Trash" → spliced out. "EX-cellent IN the trash." Prokaryotes typically lack introns; most eukaryotic protein-coding genes have multiple introns.
10. (7.5) Alternative splicing allows:
C. Different combinations of exons are selected → different proteins from one gene. Human genes average 8-10 alternative transcripts each; this is how ~20,000 genes encode ~100,000 proteins. Drosophila Dscam is a famous extreme (38,000 isoforms).
11. (7.6) The genetic code has how many codons total?
D. Three-nucleotide codons over 4 bases give 43 = 64 codons. 61 code for amino acids; 3 are stop signals (UAA, UAG, UGA). Because there are 20 amino acids but 61 coding codons, the code is redundant (degenerate).
12. (7.6) "Wobble" refers to:
B. The 3rd codon base is read loosely - e.g., inosine at the tRNA's wobble position can pair with U, C, or A. This is why fewer than 61 tRNAs can decode all 61 sense codons. Most degeneracy is at the 3rd codon position.
13. (7.7) Translation initiation in eukaryotes requires:
A. 40S + eIFs + Met-tRNAiMet form the 43S pre-initiation complex that binds the 5' cap. Scans along mRNA to the first AUG, then 60S joins to form the 80S ribosome. Prokaryotes use a Shine-Dalgarno sequence (complementary to 16S rRNA) to position the ribosome.
14. (7.7) Translation elongation requires which cofactors?
C. GTP powers elongation. EF-Tu/eEF1A delivers tRNAs; EF-G/eEF2 translocates the ribosome. Each round hydrolyzes 2 GTPs. Add the 1 ATP for aminoacyl-tRNA charging, and each residue costs 4 high-energy phosphate bonds.
15. (7.8) The tRNA molecule has what shape:
B. Cloverleaf secondary structure with 3 hairpin loops (D, anticodon, TΨC). In 3D the molecule folds into an L shape: anticodon at one tip reading the mRNA, CCA-acceptor end at the other tip carrying the amino acid.
16. (7.8) The ribosome has three tRNA-binding sites, named:
D. Incoming charged tRNA enters the A site. The growing peptide is held in the P site. Deacylated tRNA exits through the E site before leaving the ribosome.
17. (7.9) Which is an example of a post-translational modification?
C. PTMs are covalent changes made to proteins AFTER translation: phosphorylation, glycosylation, ubiquitination, methylation, acetylation, cleavage of signal peptides, proteolytic activation of zymogens.
18. (7.9) Ubiquitination typically targets a protein for:
A. A polyubiquitin (K48-linked) chain signals "destroy me" - the proteasome recognizes it, unfolds the target, and chops it into small peptides. Other ubiquitin linkages (K63) have non-degradative signaling roles.
19. (7.10) In the lac operon, lactose (or its derivative allolactose) induces gene expression by:
B. Allolactose is the inducer. Without lactose: repressor binds operator, blocks transcription. With lactose: allolactose binds repressor, it detaches, transcription proceeds. Glucose separately regulates via CAP/cAMP (catabolite repression) - a classic example of dual control.
20. (7.10) The trp operon is repressed when tryptophan is abundant because:
D. Trp is a repressible operon: the repressor is inactive by default but activated when Trp binds (as corepressor). When tryptophan is plentiful, the cell stops making the synthesis machinery. Attenuation provides a second layer of control.
21. (7.11) Eukaryotic gene regulation includes all of these EXCEPT:
C. Eukaryotes use chromatin remodeling, DNA methylation (generally silencing), histone modifications (opening or closing chromatin), tissue-specific transcription factors, enhancers/silencers, miRNAs, etc. Polycistronic transcripts (multiple genes on one mRNA) are prokaryotic.
22. (7.11) Histone acetylation generally:
A. HATs (histone acetyltransferases) add acetyl to lysine side chains; the +1 charge on -NH3+ is replaced by neutral -NHCOCH3. Weaker DNA-histone binding = open chromatin (euchromatin) = more transcription. HDACs reverse it to silence genes.

If DNA is the blueprint, RNA is the printout the ribosome actually reads. The chapter starts with how DNA is copied into RNA (transcription), how RNA is processed into a mature message, and how that message is decoded into protein (translation). The final sections cover the regulatory on-off switches that decide which genes get expressed when.

Almost every passage on the MCAT’s Bio/Biochem section touches this chapter. Learn the flow from gene to protein cold.

The Central Dogma

Keep in mind that information in cells moves from DNA → RNA → Protein. Transcription copies a stretch of DNA into mRNA. Translation decodes mRNA into a polypeptide using the genetic code. Regulation happens at every step.

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