Core Concepts
In this tutorial, you will learn all about ribonucleic acid (RNA). We begin with an introduction to nucleic acids, which includes a comparison between their two main types: DNA and RNA. Then, we discuss the synthesis of RNA through the process of transcription, consider the different types of RNA (and their functions), and analyze RNA processing in both eukaryotes and prokaryotes. Lastly, some other interesting facts about RNA are shared!
Topics Covered in Other Articles
- The DNA Tautomer: B-DNA, A-DNA, and Z-DNA
- Protein Folding
- Catalysts & Activation Energy
- GPCR – G Protein-Coupled Receptors
- RNA Interference
Introduction to Nucleic Acids
Nucleic acids are complex, organic substances found in all living things and viruses. They are one of the four major classes of biological macromolecules (along with lipids, proteins, and carbohydrates). Nucleic acids have numerous functions within the cells; these include the storage and transmission of genetic information as well as a substantial role in protein synthesis. Nucleic acids also have importance in research contexts, when implemented with techniques such as molecular cloning and PCR.
A nucleic acid is made up of chemical building blocks known as nucleotides. Each nucleotide consists of a phosphate group, five-carbon sugar molecule, and nitrogenous base. Nucleotides are linked together by phosphodiester bonds, forming a sugar-phosphate backbone. Furthermore, a nucleic acid strand has directionality. The sequence of nucleotides begins at the 5′ end (where a free phosphate group is attached to the 5′ carbon of the sugar molecule) and terminates at the 3′ end (where a free hydroxyl group is attached to the 3′ carbon of the sugar molecule).
The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA and RNA differ from one another in a few key ways. First, DNA contains the sugar deoxyribose, while RNA contains the sugar ribose. Second, DNA and RNA differ in base composition. They each contain the nitrogenous bases adenine, cytosine, and guanine; however, DNA uses the base thymine, while RNA uses the base uracil. Lastly, in most cases, DNA is a double-stranded molecule, while RNA is a single-stranded molecule.

Synthesis of RNA
Ribonucleic acid (RNA) is synthesized from the genetic information encoded by DNA through a process known as transcription. Transcription takes place in three steps: initiation, elongation, and termination.
- Initiation: Transcription begins as the enzyme RNA polymerase binds to a specific sequence of DNA called the promoter. This causes the DNA double helix to unwind and results in the separation of DNA strands (by breaking the hydrogen bonds between complementary base pairs).
- Elongation: One strand of DNA serves as a template for RNA synthesis. RNA polymerase reads the template and adds nucleotides (one by one) using complementary base pairing to build the new strand of RNA. The DNA template is read by RNA polymerase in the 3′ to 5′ direction; however, the RNA transcript is synthesized in the 5′ to 3′ direction.
- Termination: Elongation ends when RNA polymerase encounters a sequence of DNA known as a terminator, signaling that the RNA transcript is complete. RNA polymerase detaches from the DNA template and releases the newly synthesized molecule of RNA.

Types of RNA
The three most well-known types of RNA are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Each of these molecules plays a key role in protein synthesis and the central dogma of biology.

Messenger RNA
Messenger RNA (mRNA) delivers genetic information from DNA to ribosomes in the cytoplasm of the cell. Ribosomes use mRNA to make proteins through a process known as translation. A molecule of mRNA divides into a series of three-nucleotide units known as codons. Each codon corresponds to a specific amino acid, except for the stop codons that signal the end of protein synthesis. The exact correspondence between the mRNA sequence and the protein sequence is determined by the genetic code.
Transfer RNA
Transfer RNA (tRNA) is the smallest of the three main types of RNA, typically possessing only 70-90 nucleotides per molecule. It serves as a link (or bridge) between the molecule of mRNA and the growing chain of amino acids. One end of tRNA contains a sequence of three adjacent nucleotides called an anticodon, while the other end contains the acceptor stem to which a particular amino acid is attached. Molecules of tRNA carry amino acids into ribosomes, where their anticodons bind with complementary mRNA codons. As a result, the amino acids are added to the sequence one at a time in the order dictated by the mRNA transcript.
Ribosomal RNA
Ribosomal RNA (rRNA) is the most abundant type of RNA, making up roughly 80% of the RNA in our cells. Like transfer RNA, rRNA is non-coding (meaning it does not translate into a protein). Furthermore, rRNA is the major component of ribosomes, as it combines with proteins to form the small and large subunits of a ribosome. Ribosomal RNA ensures the proper alignment of mRNA, tRNA, and ribosome during translation. It also helps to catalyze the formation of peptide bonds between amino acids.
RNA Processing
RNA processing refers to the sequence of events through which a newly synthesized molecule of RNA develops into its mature, fully-functioning form. The post-transcriptional modifications of transfer RNA and ribosomal RNA are fairly similar in eukaryotic and prokaryotic cells. However, there are major differences between eukaryotes and prokaryotes when it comes to the processing of messenger RNA.
In prokaryotic cells, transcription and translation both occur in the cytoplasm. As a result, these processes can happen simultaneously, meaning there is little to no processing of mRNA in prokaryotes. In eukaryotic cells, transcription occurs in the nucleus, while translation occurs in the cytoplasm. There are three major processing steps required to help convert the eukaryotic precursor mRNA (pre-mRNA) into a mature mRNA that can be transported out of the nucleus and translated into a protein. These steps (listed and described in further detail below) include the addition of a 5′ cap, the addition of a 3′ poly-A tail, and RNA splicing.
- 5′ Capping: A modified guanine nucleotide (referred to as the 5′ cap) is added to the 5′ end of pre-mRNA. The 5′ cap helps the ribosome attach to mRNA during translation and regulates the export of mRNA from the nucleus. It also protects the 5′ end of mRNA from degradation by exonucleases.
- 3′ Polyadenylation: The 3′ end of pre-mRNA is trimmed, and a stretch of adenine nucleotides is added to form the polyadenylated (poly-A) tail. The 3′ poly-A tail stabilizes mRNA, prevents enzymatic degradation, and facilitates nuclear export.
- RNA Splicing: Pre-mRNA contains introns (non-coding regions) and exons (coding regions). During this process, a large RNA-protein complex known as a spliceosome removes the introns and joins the exons together.

Interesting Facts About RNA
- RNA is more reactive (and less stable) than DNA. This is largely due to the presence of an extra hydroxyl group on the 2′ carbon of the ribose sugar in RNA, making it more susceptible to hydrolysis.
- Although the vast majority of enzymes are proteins, it is also possible for RNA molecules to catalyze biochemical reactions. These RNA catalysts are known as ribozymes and play a key role in RNA splicing, transfer RNA biosynthesis, and many other important processes.
- The RNA world hypothesis proposes that RNA preceded DNA and proteins in evolution. This theory suggests that the earliest life forms on Earth used RNA alone, given the molecule’s ability to carry genetic information (like DNA does) and drive chemical reactions (like proteins do).
- Certain vaccines (such as the Pfizer-BioNTech and Moderna COVID-19 vaccines) utilize messenger RNA to trigger an immune response that will help protect against future infections.
- Retroviruses, such as HIV and HTLV-1, use RNA (rather than DNA) as their genetic material. These viruses can produce a copy of DNA from an RNA template through the process of reverse transcription. They use the enzyme reverse transcriptase to synthesize the DNA, which can then be inserted into the genome of the host cell it invades.
- Stem loops (or hairpin loops) are an essential element of RNA structure; they form when a strand of RNA folds back on itself as a result of intramolecular base pairing. For example, tRNA contains several stem loops, which help to form its unique structure resembling a three-leaf clover.