Ribonucleic acid, or RNA, is a molecule found in the cells of all living things. It is a polymeric molecule important for many biological roles, such as the regulation and expression of genes. This nucleic acid is a major factor in the creation of new cells in the body. Functioning as a type of cellular software, RNA carries out the instructions encoded in a cell’s genetic material.
Comparing RNA and DNA
While both are nucleic acids that carry genetic information, RNA and DNA have distinct structural differences that influence their functions. The primary distinction lies in their five-carbon sugar component. RNA contains ribose, which has a hydroxyl group on the second carbon, making it more reactive and susceptible to hydrolysis. DNA, on the other hand, contains deoxyribose, which lacks this hydroxyl group, contributing to its greater stability as a long-term storage medium for genetic information.
Another difference is found in their nitrogenous bases. Both molecules use adenine, guanine, and cytosine. However, RNA utilizes a base called uracil, while DNA uses thymine. Uracil is structurally similar to thymine but lacks a methyl group, which requires less energy for the cell to produce.
The structure of these molecules also sets them apart. DNA is known for its double helix, a structure where two strands wind around each other. In contrast, RNA is a single-stranded molecule. This single-stranded nature allows RNA to fold into complex three-dimensional shapes, enabling it to perform a wider range of functions within the cell, from carrying messages to catalyzing chemical reactions.
The Primary Types of Cellular RNA
Within the cell, there are several distinct types of RNA, each with a specialized role. The most prominent of these are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
Messenger RNA acts as the intermediary between the cell’s genetic blueprint and its protein-building machinery. It carries genetic instructions from the DNA, which is housed in the nucleus, out to the cytoplasm where proteins are made. The sequence of the mRNA molecule dictates the sequence of amino acids in the resulting protein, making it a messenger of genetic information. This process ensures that the original DNA remains protected within the nucleus.
Transfer RNA functions as a molecular adapter, responsible for deciphering the message carried by mRNA. It has a unique, cloverleaf-like structure that allows it to perform this role. One end of the tRNA molecule reads a specific three-nucleotide code on the mRNA, while the other end carries the corresponding amino acid.
Ribosomal RNA is a structural component of ribosomes, the cellular factories where proteins are synthesized. In fact, rRNA makes up about 80% of the RNA in a typical eukaryotic cell. Within the ribosome, rRNA helps to position the mRNA and tRNA molecules correctly and catalyzes the formation of peptide bonds that link the amino acids together into a protein.
How RNA Builds Proteins
The process of building proteins from the instructions in DNA is a two-step process involving transcription and translation. In transcription, a segment of DNA is used as a template to create a molecule of messenger RNA. This process takes place in the cell’s nucleus, where the DNA is stored. The enzyme RNA polymerase unwinds a section of the DNA double helix and synthesizes a complementary strand of RNA.
Once the mRNA molecule is complete, it undergoes some processing and then travels from the nucleus to the cytoplasm. There, it attaches to a ribosome, which is made of ribosomal RNA and proteins. The ribosome then moves along the mRNA, reading its sequence of nucleotides in groups of three, called codons. Each codon specifies a particular amino acid that should be added to the protein chain.
This is where transfer RNA comes into play. As the ribosome moves along the mRNA, the appropriate tRNA molecules bind to the codons, delivering their amino acids to the ribosome. The ribosome then links these amino acids together, forming a growing protein chain.
This process, known as translation, continues until the ribosome reaches a “stop” codon on the mRNA. At this point, the completed protein is released from the ribosome. The protein then folds into its final three-dimensional shape and goes on to perform its specific function in the cell.
Modern Applications of RNA
The understanding of RNA’s functions has led to significant advancements in medicine and biotechnology. One of the most well-known applications is in the development of mRNA vaccines. These vaccines work by introducing a piece of mRNA into the body’s cells. This mRNA contains the instructions for making a harmless piece of a virus, such as a spike protein.
The cells then use this mRNA to produce the viral protein, which the immune system recognizes as foreign. This triggers an immune response, leading to the production of antibodies and memory cells that can protect the body from future infection by the actual virus. This approach has proven to be a rapid and effective way to develop vaccines for new and emerging infectious diseases.
Another area of RNA-based technology is RNA interference (RNAi). This is a natural process in cells that regulates gene expression. Scientists have learned to harness this process to “silence” or turn off specific genes. This is done by introducing small RNA molecules that are complementary to the mRNA of the target gene.
These small RNAs bind to the target mRNA, leading to its degradation and preventing it from being translated into a protein. This has therapeutic potential for treating a wide range of diseases. For example, RNAi could be used to turn off genes that cause cancer or to block the production of proteins that are responsible for genetic disorders.