Ribonucleic acid, or RNA, is a fundamental molecule performing various cellular functions. Unlike DNA, which primarily stores genetic information, RNA is highly versatile, acting as a messenger, an adapter, a structural component, and even a catalyst in biological processes. This adaptability allows RNA to participate in gene expression and protein synthesis, translating genetic instructions into functional components in the cell.
Within the Nucleus
The nucleus is the initial site for RNA synthesis and processing. Here, genetic information stored in DNA is transcribed into precursor RNA molecules through transcription. RNA polymerases create these nascent RNA strands using a DNA template.
One significant type of RNA produced in the nucleus is precursor messenger RNA (pre-mRNA). This molecule is an unprocessed transcript that contains both coding regions, called exons, and non-coding regions, known as introns. Before leaving the nucleus, pre-mRNA undergoes extensive processing, including the removal of introns and the joining of exons, a precise operation known as splicing.
Additionally, the nucleus is home to small nuclear RNAs (snRNAs), which are components of the spliceosome, the molecular machinery responsible for pre-mRNA splicing. Ribosomal RNA (rRNA) precursors are also synthesized and processed within a specialized nuclear region called the nucleolus. This region is dedicated to the assembly of ribosomal subunits, which are then exported to the cytoplasm.
In the Cytoplasm
Once processed in the nucleus, many RNA molecules, particularly messenger RNA (mRNA), are transported to the cytoplasm to carry out their functions. The cytoplasm is the jelly-like substance filling the cell, where much of the cell’s metabolic activity occurs. Here, mRNA acts as a template, carrying the genetic code from the DNA to the ribosomes, the cellular machinery responsible for protein production.
Protein synthesis, also known as translation, involves two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). Transfer RNA molecules are relatively small and act as adapters, each carrying a specific amino acid to the ribosome. They recognize corresponding three-nucleotide sequences on the mRNA, called codons, ensuring the correct amino acid sequence is built into the growing protein chain.
Ribosomal RNA, in combination with various proteins, forms the ribosomes themselves. Ribosomes are composed of two subunits, a large and a small one, which come together on the mRNA to facilitate protein synthesis. The rRNA within the ribosome plays a direct role in forming the peptide bonds that link amino acids together, effectively catalyzing the protein assembly process.
Ribosomes exist in two forms within the cytoplasm: free ribosomes and those bound to the endoplasmic reticulum (ER). Free ribosomes synthesize proteins that remain within the cell’s cytosol. In contrast, ribosomes attached to the ER produce proteins destined for secretion outside the cell, insertion into cellular membranes, or delivery to organelles like lysosomes.
Inside Mitochondria
Mitochondria are unique organelles that also contain their own RNA molecules. These organelles possess their own small, circular DNA genome, distinct from the cell’s main nuclear DNA. This mitochondrial DNA (mtDNA) encodes a limited number of genes for mitochondrial function, including specific RNA molecules.
Within the mitochondrial matrix, a specialized machinery exists to synthesize mitochondrial messenger RNA (mt-mRNA), mitochondrial transfer RNA (mt-tRNA), and mitochondrial ribosomal RNA (mt-rRNA). These RNAs produce a small subset of proteins directly within the mitochondria. These proteins are components of the electron transport chain, important for cellular energy production.
While most proteins required for mitochondrial function are encoded by nuclear genes and imported into the organelle, the presence of an independent RNA synthesis and protein-making system highlights the semi-autonomous nature of mitochondria. The mitochondrial ribosomes, formed by mt-rRNA and proteins, share similarities with bacterial ribosomes, supporting the evolutionary theory that mitochondria originated from ancient bacteria.