A 30-year-old African American woman develops a facial rash in a "butterfly" pattern over her face and complains of feeling tired and achy in her joints. In the course of a full rheumatologic workup you note that she has anti-snRNP antibodies. Which of the following do snRNPs affect?

Protection of mRNA from degradation

Polyadenylation of the 3' end of mRNA

Addition of the 5' 7-methylguanosine cap of mRNA

An investigator is studying the genotypes of wingless fruit flies using full exome sequencing. Compared to wild-type winged fruit flies, the wingless fruit flies are found to have a point mutation in the gene encoding wing bud formation during embryogenesis. The point mutation in the gene causes the mRNA transcript to have a 'UUG' segment instead of an 'AUG' segment. Which of the following processes is most likely affected by this mutation?

Binding of met-tRNA to 40S complex

Catalyzation of peptide bond formation

Dissociation of mRNA from ribosome complex

Shift of peptidyl-tRNA from A to P site

An investigator is studying the function of the endoplasmic reticulum in genetically modified lymphocytes. A gene is removed that facilitates the binding of ribosomes to the endoplasmic reticulum. Which of the following processes is most likely to be impaired as a result of this genetic modification?

Production of secretory proteins

E. coli has the ability to regulate its enzymes to break down various sources of energy when available. It prevents waste by the use of the lac operon, which encodes a polycistronic transcript. At a low concentration of glucose and absence of lactose, which of the following occurs?

Increased cAMP levels result in binding to the catabolite activator protein

Decreased cAMP levels result in poor binding to the catabolite activator protein

Increased allolactose levels bind to the repressor

Repressor releases from lac operator

Transcription of the lac Z, Y, and A genes increase

Expression of an mRNA encoding for a soluble form of the Fas protein prevents a cell from undergoing programmed cell death. However, after inclusion of a certain exon, this same Fas pre-mRNA eventually leads to the translation of a protein that is membrane bound, subsequently promoting the cell to undergo apoptosis. Which of the following best explains this finding?

Post-translational modifications

Post-transcriptional modifications for USMLE Step 2 CK: High-Yield Biochemistry Lessons

5' Capping - The Starting Cap

What: Addition of a 7-methylguanosine (m7G) cap to the 5' end of nascent pre-mRNA in the nucleus.
How: An unusual 5'-to-5' triphosphate linkage is formed, catalyzed by guanylyltransferase.
When: Occurs co-transcriptionally, shortly after transcription initiation by RNA polymerase II.
Functions:
- Protects mRNA from degradation by 5' exonucleases.
- Essential for export from the nucleus to the cytoplasm.
- Acts as a binding site for the cap-binding complex (CBC).

⭐ The 5' cap is recognized by eukaryotic initiation factor eIF4E, which recruits the small ribosomal subunit (40S) to the mRNA, marking a critical step for initiating translation.

7-methylguanosine 5' cap structure on mRNA

3' Polyadenylation - The Protective Tail

Function: A long chain of adenine nucleotides (~250 A's) is added to the 3' end of pre-mRNA, creating the poly(A) tail.
Mechanism:
- Triggered by the polyadenylation signal sequence (AAUAAA) near the 3' end.
- An endonuclease cleaves the pre-mRNA downstream of this signal.
- Polyadenylate polymerase (PAP) adds the adenine residues without a template.
Purpose:
- ↑ mRNA stability by protecting it from 3' exonuclease degradation.
- Facilitates nuclear export.
- Required for efficient translation initiation.

⭐ The poly(A) tail interacts with Poly(A)-Binding Protein (PABP). PABP helps circularize the mRNA by binding to initiation factors on the 5' cap, significantly boosting translation efficiency.

mRNA processing, translation, and deadenylation diagram

Splicing - The Cutting Room Floor

Process of removing non-coding regions (introns) and joining coding regions (exons) from pre-mRNA. Occurs in the nucleus.
Carried out by the spliceosome, a large complex of small nuclear ribonucleoproteins (snRNPs).
Key Sites: Spliceosome recognizes consensus sequences.
- 5'-GU...AG-3': The intron begins with GU and ends with AG.
- 📌 Mnemonic: Got Under, After Going.

⭐ Autoantibodies against snRNPs (anti-Smith antibodies) are highly specific for Systemic Lupus Erythematosus (SLE).

Alternative Splicing: A single gene can yield multiple mature mRNAs and proteins by selectively including or excluding exons. This greatly increases proteomic diversity.

Splicing Variants - One Gene, Many Proteins

Alternative Splicing: A regulated process where a single pre-mRNA transcript is processed in different ways, leading to distinct mature mRNA molecules.
Mechanism:
- Spliceosomes selectively remove introns and join exons.
- By including or excluding certain exons, different combinations are made from the same gene.
- This vastly increases the proteomic diversity from a limited number of genes.
Example: The tropomyosin gene produces multiple protein isoforms specific to different tissues (e.g., smooth vs. striated muscle) through alternative splicing.

⭐ The calcitonin gene produces calcitonin in thyroid C-cells but produces calcitonin gene-related peptide (CGRP) in neural tissue due to alternative splicing.

Alternative splicing mechanism and protein isoforms

Eukaryotic mRNA is processed in the nucleus before translation.

5' capping with 7-methylguanosine is crucial for translation initiation and mRNA protection.

3' polyadenylation (poly-A tail) stabilizes mRNA and facilitates its export from the nucleus.

Splicing removes introns (non-coding regions) via the spliceosome; key sites are GU (5' end) and AG (3' end).

Alternative splicing allows a single gene to code for multiple protein isoforms.

Anti-Smith antibodies in SLE target snRNPs of the spliceosome.

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