Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 18.3 Noncoding RNAs Play Multiple Roles in Controlling Gene Expression
Noncoding RNAs (ncRNAs) are RNA molecules that are transcribed but not translated into proteins. They regulate gene expression at multiple levels, significantly expanding our understanding of genetic regulation beyond protein-coding genes.
Discovery and Importance of Noncoding RNAs
- Protein-coding DNA constitutes only about 1.5% of the human genome.
- Most of the remaining DNA was previously considered "junk," but recent discoveries show many noncoding regions are transcribed into functional ncRNAs.
- ncRNAs regulate gene expression through mRNA degradation, translation inhibition, and chromatin modification.
MicroRNAs (miRNAs) and Small Interfering RNAs (siRNAs)
- MicroRNAs (miRNAs):
- Short (~22 nucleotides), single-stranded RNA molecules.
- Derived from larger precursors and associate with proteins.
- Bind to complementary sequences in target mRNAs.
- Can degrade mRNA or block its translation, regulating gene expression for about half of all human genes.
- Small interfering RNAs (siRNAs):
- Similar in size and function to miRNAs.
- Formed from double-stranded RNA precursors.
- Primarily involved in RNA interference (RNAi), a natural process that silences genes with complementary sequences.
- Used experimentally to disable specific genes to study their function.
Chromatin Remodeling by ncRNAs
- Small ncRNAs can modify chromatin structure, influencing gene expression:
- In yeast, siRNAs trigger the condensation of chromatin at centromeres, crucial during mitosis.
- In mammals, mechanisms of chromatin remodeling involving ncRNAs are still under investigation.
- Piwi-interacting RNAs (piRNAs):
- 24–31 nucleotides long, derived from single-stranded RNA.
- Induce heterochromatin formation, blocking expression of transposons ("jumping genes") in germ cells.
- Essential for genome integrity during gamete formation.
Long Noncoding RNAs (lncRNAs)
- Range from 200 nucleotides to hundreds of thousands.
- Thousands of lncRNAs are expressed at specific times in specific cells.
- Roles of lncRNAs include:
- X chromosome inactivation (XIST RNA): coats one X chromosome in females, condensing it into inactive heterochromatin (Barr body).
- Serving as scaffolds to organize DNA, proteins, and other RNAs, regulating transcription and chromatin state.
Evolutionary and Biological Roles of ncRNAs
- RNA interference pathways may have evolved as defense mechanisms against viral infections.
- ncRNAs likely play important roles in embryonic development, controlling precise gene expression patterns.
- Ongoing research continues to uncover novel functions and regulatory roles of various ncRNAs.
In a Nutshell
Noncoding RNAs, previously overlooked, play crucial regulatory roles in gene expression. MicroRNAs and siRNAs regulate mRNA stability and translation, while piRNAs and lncRNAs influence chromatin structure. The expanding knowledge of ncRNAs reveals a complex, dynamic layer of genetic regulation critical for organismal development and cellular function.