4 Types of RNA

Rotavirus particle, illustration
KATERYNA KON/SCIENCE PHOTO LIBRARY / Getty Images

RNA (or ribonucleic acid) is a nucleic acid that is used in making proteins inside of cells. DNA is like a genetic blueprint inside of every cell. However, cells do not “understand” the message DNA conveys, so they need RNA to transcribe and translate the genetic information. If DNA is a protein “blueprint,” then think of the RNA as the “architect” that reads the blueprint and carries out the building of the protein.

There are different types of RNA that have different functions in the cell. These are the most common types of RNA that have an important role in the functioning of a cell and protein synthesis.

Messenger RNA (mRNA)

A strand of messenger RNA being translated
mRNA is translated into a polypeptide. (Getty/Dorling Kindersley)

Messenger RNA (or mRNA) has the main role in transcription, or the first step in making a protein from a DNA blueprint. The mRNA is made up of nucleotides found in the nucleus that come together to make a complementary sequence to the DNA found there. The enzyme that puts this strand of mRNA together is called RNA polymerase. Three adjacent nitrogen bases in the mRNA sequence is called a codon and they each code for a specific amino acid that will then be linked with other amino acids in the correct order to make a protein.

Before mRNA can move on to the next step of gene expression, it first must undergo some processing. There many regions of DNA that do not code for any genetic information. These non-coding regions are still transcribed by mRNA. This means the mRNA must first cut out these sequences, called introns, before it can be coded into a functioning protein. The parts of mRNA that do code for amino acids are called exons. The introns are cut out by enzymes and only the exons are left. This now single strand of genetic information is able to move out of the nucleus and into the cytoplasm to begin the second part of gene expression called translation.

Transfer RNA (tRNA)

Molecular model of transfer RNA
tRNA will bind an amino acid to one end and has an anticodon on the other. (Getty/MOLEKUUL)

Transfer RNA (or tRNA) has the important job of making sure the correct amino acids are put into the polypeptide chain in the correct order during the process of translation. It is a highly folded structure that holds an amino acid on one end and has what is called an anticodon on the other end. The tRNA anticodon is a complementary sequence of the mRNA codon. The tRNA is therefore ensured to match up with the correct part of the mRNA and the amino acids will then be in the right order for the protein. More than one tRNA can bind to mRNA at the same time and the amino acids can then form a peptide bond between themselves before breaking off from the tRNA to become a polypeptide chain that will be used to eventually form a fully functioning protein.

Ribosomal RNA (rRNA)

Ribosomal RNA and transcription factors
Ribosomal RNA (rRNA) helps facilitate the bonding of amino acids coded for by the mRNA. (Getty/LAGUNA DESIGN)

Ribosomal RNA (or rRNA) is named for the organelle it makes up. The ribosome is the eukaryotic cell organelle that helps assemble proteins. Since rRNA is the main building block of ribosomes, it has a very large and important role in translation. It basically holds the single stranded mRNA in place so the tRNA can match up its anticodon with the mRNA codon that codes for a specific amino acid. There are three sites (called A, P, and E) that hold and direct the tRNA to the correct spot to ensure the polypeptide is made correctly during translation. These binding sites facilitate the peptide bonding of the amino acids and then release the tRNA so they can recharge and be used again.

Micro RNA (miRNA)

Molecular Model of microRNA
miRNA is thought to be a control mechanism leftover from evolution. (Getty/MOLEKUUL)

Also involved in gene expression is micro RNA (or miRNA). miRNA is a non-coding region of mRNA that is believed to be important in the either promotion or inhibition of gene expression. These very small sequences (most are only about 25 nucleotides long) seem to be an ancient control mechanism that was developed very early in the evolution of eukaryotic cells. Most miRNA prevent transcription of certain genes and if they are missing, those genes will be expressed. miRNA sequences are found in both plants and animals, but seem to have come from different ancestral lineages and are an example of convergent evolution.