RNA Processing, Noncoding RNA, and Epigenetic Regulation
RNA processing
Maturation steps
Primary transcripts in eukaryotes undergo capping, splicing, editing, polyadenylation, export, localization, and regulated decay. Alternative splicing expands proteomic diversity, while RNA editing can alter coding potential or RNA structure. These steps are regulated and often cell-type specific.
Which process most directly creates multiple mRNA isoforms from a single gene?
Alternative splicing joins exons in different combinations to produce distinct isoforms.
Correct answer: Alternative splicing
Explain why RNA processing is a genetic control layer rather than only a housekeeping step.
Processing changes gene output and can be developmentally regulated.
Correct answer: Because processing choices determine RNA abundance, localization, translation, and protein isoform identity.
Noncoding RNA
Functional classes
MicroRNAs, small interfering RNAs, long noncoding RNAs, and piRNAs regulate gene expression through transcript degradation, translational repression, chromatin targeting, and transposon silencing. Noncoding RNA pathways are central to development, genome defense, and epigenetic memory.
Selected noncoding RNA classes
microRNA
- Typically represses translation
- Often induces mRNA destabilization
lncRNA
- Can scaffold chromatin regulators
- Can guide locus-specific regulation
Which small RNA class is most strongly linked to transposon silencing in the germ line?
piRNAs guide silencing of transposable elements in germ cells.
Correct answer: piRNA
Epigenetic mechanisms
Heritable chromatin states
Epigenetics refers to stable changes in gene function that do not alter DNA sequence. Major mechanisms include DNA methylation, histone modification, chromatin remodeling, and three-dimensional nuclear organization. Some epigenetic states are mitotically heritable and can contribute to imprinting, X-inactivation, and cell memory.
Conceptual caution
Not every environmentally responsive gene expression change is truly epigenetic; durability across cell divisions is a key criterion.
Why is DNA methylation often associated with transcriptional repression?
Methylation commonly contributes to stable silencing at promoters and repetitive regions.
Correct answer: Because methylation can reduce factor binding and recruit proteins that compact chromatin and inhibit transcription.