Transcription, Translation, And Regulation - USMLE Step 1

1. This specific post-translational modification is critical for which of the following steps in collagen biosynthesis?
   Answer: Formation of a stable triple helix The hydroxylation of specific proline and lysine residues occurs within the rough endoplasmic reticulum. The resulting hydroxyproline and hydroxylysine residues are essential for the formation of hydrogen bonds that stabilize the triple-helical structure of procollagen. Without this modification (e.g., in vitamin C deficiency or certain genetic defects), the triple helix is unstable and is degraded within the cell, leading to impaired collagen synthesis. Cross-linking occurs later in the extracellular space and is mediated by lysyl oxidase.
2. The pathogenic mechanism in this patient's disorder involves silencing of the affected gene primarily through which of the following molecular events?
   Answer: Hypermethylation of the promoter region This patient has Fragile X syndrome, which is caused by a large expansion of CGG repeats in the promoter region of the FMR1 gene. This expansion leads to excessive DNA methylation (hypermethylation) of the promoter's CpG islands. DNA methylation is an epigenetic modification that silences gene expression by recruiting proteins that condense chromatin, making the gene inaccessible for transcription. Histone hyperacetylation is associated with gene activation. The other options describe different mechanisms of mutation.
3. The normal function of this miRNA is most likely to cause which of the following?
   Answer: Repression of translation of the oncogene mRNA MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression post-transcriptionally. They bind to complementary sequences, typically in the 3' UTR of target mRNAs. This binding recruits a protein complex (RISC) that either leads to the degradation of the mRNA (if binding is perfectly complementary) or, more commonly in animals, represses translation (if binding is imperfect). Therefore, the normal function of this tumor-suppressive miRNA is to decrease the production of the oncoprotein by repressing translation.
4. This genetic alteration is best classified as which type of mutation?
   Answer: Nonsense The genetic code includes start and stop codons. The codon UGG codes for the amino acid tryptophan. The codon UGA is one of the three stop codons (along with UAA and UAG). A mutation that changes an amino acid-coding codon into a stop codon is called a nonsense mutation. This results in the premature termination of translation and the production of a truncated, usually nonfunctional, protein.
5. The low level of cAMP directly results in which of the following?
   Answer: Catabolite activator protein (CAP) does not bind to the promoter region High levels of transcription from the lac operon require two conditions: the presence of lactose (to inactivate the repressor) and the absence of glucose. Glucose presence leads to low cAMP. Cyclic AMP binds to the catabolite activator protein (CAP), and this cAMP-CAP complex binds to the promoter region of the lac operon, greatly enhancing the affinity of RNA polymerase for the promoter. When cAMP is low, CAP is inactive and does not bind, resulting in low levels of transcription even if lactose is present and the repressor is inactive. This mechanism is called catabolite repression.
6. The binding of which of the following components to the 40S subunit is directly mediated by a GTP-bound initiation factor?
   Answer: Initiator tRNA (Met-tRNAi) During eukaryotic translation initiation, the initiator tRNA (Met-tRNAi) is brought to the 40S ribosomal subunit by eukaryotic initiation factor 2 (eIF2). eIF2 is a G-protein that is active when bound to GTP. The ternary complex of eIF2-GTP-Met-tRNAi binds to the 40S subunit to form the preinitiation complex. The binding of mRNA is mediated by the eIF4F complex. The joining of the 60S subunit occurs later and requires GTP hydrolysis by eIF5B.
7. This mutation most likely impairs which of the following steps of mRNA processing?
   Answer: Addition of the poly(A) tail The sequence AAUAAA is the consensus polyadenylation signal sequence in eukaryotes. It is recognized by an endonuclease and polyadenylate polymerase, which cleave the pre-mRNA and add a long chain of adenine nucleotides (the poly(A) tail) to the 3' end. This poly(A) tail is crucial for protecting the mRNA from degradation and facilitating its export from the nucleus and translation in the cytoplasm. Disruption of this signal impairs polyadenylation, leading to mRNA instability and lower cytoplasmic levels.
8. This condition is most likely caused by a defect in the post-translational addition of which of the following to lysosomal enzymes?
   Answer: Mannose-6-phosphate This is a classic presentation of I-cell disease (Mucolipidosis II), a lysosomal storage disorder. It is caused by a deficiency in N-acetylglucosaminyl-1-phosphotransferase. This enzyme catalyzes the first step in adding a mannose-6-phosphate (M6P) tag to lysosomal enzymes in the Golgi apparatus. This M6P tag acts as a molecular address label, targeting these enzymes for transport to the lysosome. Without the tag, the enzymes are incorrectly secreted from the cell, leading to their high levels in the serum and deficiency within lysosomes.
9. How will this omission affect the expression of the protein when the gene is transfected into a human cell line?
   Answer: Translation will proceed normally. The Shine-Dalgarno sequence is a ribosomal binding site in prokaryotic mRNA, generally located around 8 bases upstream of the start codon AUG. It is essential for the initiation of translation in bacteria. Eukaryotic cells use a different mechanism for translation initiation, where the 40S ribosomal subunit typically binds to the 5' cap of the mRNA and scans downstream until it encounters the first AUG codon (Kozak sequence context). Therefore, the absence of a Shine-Dalgarno sequence will have no effect on translation in a human (eukaryotic) cell line.
10. The proteins that are targeted for degradation by the cellular machinery inhibited by this drug are typically tagged with which of the following?
    Answer: A polyubiquitin chain The proteasome is a large protein complex responsible for degrading damaged or unneeded proteins. The primary signal that targets a protein for proteasomal degradation is the covalent attachment of a polyubiquitin chain. Proteasome inhibitors block this degradation pathway, causing toxic accumulation of these tagged proteins and triggering apoptosis, an effect that is particularly potent in rapidly dividing cancer cells like myeloma cells.
11. The cellular machinery targeted in this patient's condition is primarily responsible for which of the following processes?
    Answer: Removal of introns from pre-mRNA This patient's presentation is classic for Systemic Lupus Erythematosus (SLE). Anti-Smith antibodies, which are highly specific for SLE, target small nuclear ribonucleoproteins (snRNPs). These snRNPs are core components of the spliceosome, the complex responsible for removing non-coding sequences (introns) from pre-mRNA transcripts in the nucleus. The other options describe different, unrelated cellular processes.
12. The bacterial toxin responsible for this patient's symptoms inactivates which of the following eukaryotic targets via ADP-ribosylation?
    Answer: Elongation factor 2 (eEF-2) Diphtheria toxin catalyzes the ADP-ribosylation of eukaryotic elongation factor 2 (eEF-2). This modification inactivates eEF-2, which is required for the translocation step of polypeptide chain elongation. By blocking this essential step, the toxin halts protein synthesis, leading to cell death and the characteristic necrosis seen in diphtheria.
13. Treatment with this agent would most likely lead to which of the following changes in the chromatin of target cells?
    Answer: Decreased affinity of histones for DNA and increased transcription Histone acetylation removes the positive charge from lysine residues on histone tails, reducing their affinity for negatively charged DNA. This leads to a more relaxed chromatin structure (euchromatin), which is accessible to transcription factors and RNA polymerase, thus promoting gene expression. HDACs remove these acetyl groups, promoting condensation. An HDAC inhibitor would therefore cause an accumulation of acetylated histones, leading to decreased histone-DNA affinity and increased transcription of target genes, such as tumor suppressors.
14. The DNA sequences that the activated androgen receptor normally binds to are best described as which of the following regulatory elements?
    Answer: Enhancers Steroid hormone receptors, such as the androgen receptor, are ligand-activated transcription factors. Upon binding their hormone, they translocate to the nucleus and bind to specific DNA sequences called hormone response elements (HREs). These HREs function as enhancers or silencers to modulate the transcription of target genes. Promoters are regions where RNA polymerase binds. Operators and Shine-Dalgarno sequences are features of prokaryotic gene regulation and translation, respectively.
15. This toxin most likely exerts its effect by directly inhibiting which of the following enzymes?
    Answer: RNA polymerase II The toxin described is α-amanitin, found in Amanita phalloides mushrooms. It is a potent inhibitor of eukaryotic RNA polymerase II, the enzyme responsible for synthesizing mRNA. Inhibition of mRNA synthesis halts the production of new proteins, leading to cell death, particularly in metabolically active cells like hepatocytes. RNA polymerase I (synthesizes most rRNA) and RNA polymerase III (synthesizes tRNA and 5S rRNA) are much less sensitive to α-amanitin. DNA polymerase III is a prokaryotic enzyme involved in DNA replication.
16. Doxycycline exerts its bacteriostatic effect by binding to the 30S ribosomal subunit and directly blocking which of the following steps?
    Answer: Binding of aminoacyl-tRNA to the A site Tetracycline antibiotics, including doxycycline, work by binding to the 30S ribosomal subunit of prokaryotes. This binding sterically hinders the attachment of aminoacyl-tRNA to the acceptor (A) site of the ribosome, thereby preventing the addition of new amino acids to the growing polypeptide chain and halting protein synthesis. Formation of the peptide bond is inhibited by chloramphenicol. Translocation is inhibited by macrolides and clindamycin.
17. Which of the following would be a direct consequence of p53 binding to its target DNA sequences?
    Answer: Increased recruitment of RNA polymerase II to target gene promoters p53 is a transcription factor, a protein that controls the rate of transcription of genetic information from DNA to messenger RNA. When p53 binds to its specific DNA response elements in the regulatory regions of its target genes (like p21), it recruits co-activator proteins and general transcription factors. This complex then facilitates the binding and activity of RNA polymerase II at the promoter, leading to increased transcription of the target gene. The other options describe post-transcriptional or epigenetic silencing mechanisms.
18. The presence of this intronic sequence in the mature mRNA is most likely to result in which of the following?
    Answer: A frameshift and premature termination of translation Mutations that create new splice sites (cryptic splice sites) lead to incorrect splicing of pre-mRNA. In this case, including a portion of an intron in the mature mRNA will alter the reading frame because the number of inserted nucleotides is unlikely to be a multiple of three. This frameshift will change the amino acid sequence downstream of the insertion and almost always introduces a premature stop codon, leading to the synthesis of a truncated and nonfunctional β-globin protein. This is a common mechanism in some forms of β-thalassemia.
19. This phenomenon is best described as an alteration in which of the following processes?
    Answer: Alternative splicing Alternative splicing is a regulated process during gene expression that results in a single gene coding for multiple proteins. In this process, particular exons of a gene may be included within or excluded from the final, processed messenger RNA (mRNA). The production of two different mRNA molecules (one with exon 3 and one without) from a single pre-mRNA transcript is a classic example of alternative splicing. In this case, a mutation likely altered a splice site, causing exon 3 to be skipped during processing in a portion of the transcripts.
20. The susceptibility of mitochondrial ribosomes to this class of antibiotics is because they are structurally most similar to which of the following?
    Answer: Prokaryotic ribosomes According to the endosymbiotic theory, mitochondria evolved from aerobic prokaryotes that were engulfed by ancestral eukaryotic cells. As a result, mitochondria retain many prokaryotic features, including their own circular DNA and ribosomes (70S, composed of 50S and 30S subunits) that are structurally similar to bacterial ribosomes. Aminoglycosides target the bacterial 30S subunit. Their toxicity in humans is partly due to their ability to also bind to the mitochondrial 30S subunit, disrupting mitochondrial protein synthesis and leading to cellular dysfunction, especially in energy-intensive tissues like the inner ear and kidney.

Start this quiz