Recent developments in science have allowed researchers to more accurately describe the functions of genes. One such method is by looking at the role a gene plays in producing RNA.
Genes can produce mRNA, tRNA, rRNA, snRNA, and other types of RNA. Each type of RNA carries out a specific function that contributes to the function of the cell or cell type.
RNA is a molecule that functions as both a transcript and an encoded sequence. This means that it can either carry information from DNA to protein (as mRNA does) or act as a guide for the production of other molecules (as tRNA and other RNAs do).
This article will discuss the different types of RNA and what they do. More specifically, this article will list and describe each type of RNA and what it encodes for, who discovered it, and some examples of cells that contain it.
rRNA
The largest category of RNAs is known as rRNA. These are transcribed from what are called RNA genes.
RNA genes are composed of sequences of nucleotides that encode for proteins. Proteins are responsible for most functions in your body, so it makes sense that RNA that encodes for proteins is a large category!
There are five different types of rRNA and each one describes a different part of the cell. These include the mitochondrial rRNA, the chloroplast (or cp) rRNA, the nuclear chromosomal (or ncr) rRNA, the small nuclear (or sn) rRNA, and the viral (or vi) rRNAs.
The first three describe parts of the cell that cannot be separated from the rest so they are contained within the cell cytoplasm. The last two describe parts of the cell that can be separated from the rest so they exist outside of the cell nucleus.
tRNA
tRNA stands for transfer RNA. These molecules are part of the machinery that creates proteins from genes.
tRNA molecules carry amino acids to the cell’s protein-making machinery. There, they join other tRNA and amino acids to form a new protein chain.
There are tens of thousands of possible tRNA molecules, and each one carries a different amino acid. This specificity is what makes them so important for building proteins.
tRNAs are part of what is called the epigenome. The epigenome refers to things outside of the DNA sequence that influence how it functions. For example, chemical modifications on the DNA or RNA can influence how it is expressed.
These chemical modifications act as switches, turning the gene expression on or off.
snRNA
snRNA stands for small nuclear RNA. These molecules do not encode for proteins, and instead act as regulators of DNA expression.
There are two main types of snRNA: U1 and U2. U1 snRNA binds to the 5’ end of the genome’s RNA primer, which is a short sequence of RNA that starts the synthesis of DNA.
U2 snRNA complexes with proteins to form the spliceosome, which is a complex that catalyzes the removal of introns, or non-coding parts of the gene sequence. This process is called splicing.
Other types of snRNAs function to stabilize specific RNAs or guide the process of methylation on other RNAs. These functions help regulate gene expression by either inhibiting or allowing RNA expression, respectively.
miRNA
MicroRNA, or miRNA, is a type of RNA that functions as a post-transcriptional regulator.
MiRNAs function by directly targeting mRNA transcripts for degradation and/or translational repression.
Because miRNAs can regulate hundreds of genes, they are considered key regulators in many cellular processes. Due to this widespread influence, changes in miRNA levels or functionality can alter cell function and phenotype.
MiRNAs have been identified in all kingdoms of life, including Eukaryotes, Prokaryotes, Fungi and Bacteria. This makes them an important consideration when examining the regulation of cellular processes and functions.
There are many ways to identify miRNA within a sample. The most common way is through gel electrophoresis, which identifies length differences in molecules. Other methods include using PCR to amplify the RNA and then gel electrophoresis to identify the size difference or northern blotting to identify the type of RNA.
shRNA
As mentioned before, shRNA is a type of small RNA. These RNAs can be used to target and silence specific genes.
ShRNA is short hairpin RNA. Hairpin RNA refers to the structure of the small RNA. It has a sequence of bases at the top that matches another sequence of bases on a gene it targets. The other strand of the hairpin is a complementary sequence that matches the non-hairpin gene strand.
ShRNA can be used to target almost any gene. The only exceptions are transfer RNA (tRNA) and ribosomal RNA (rRNA). tRNAs carry amino acids into cells, and rRNAs are part of the machinery that makes proteins.
There are many ways to use shRNA in cells. One way is to introduce synthetic shRNA into cells using viruses like adeno-associated viruses (AAV) or lentiviruses like HIV. Another way is to introduce non-synthetic shRNA extracted fromcells like blood cells or immune cells.
cDNA
The next type of RNA is called complementary DNA, or cDNA. This type of RNA is made from mRNA.
As mentioned before, mRNA carries information from the DNA to the protein-making machinery in the cell. cDNA is made when an enzyme called reverse transcriptase copies the RNA sequence for a particular gene onto a single-stranded piece of DNA.
This new piece of DNA has the same sequence as the mRNA, which means it carries the information for one specific protein. cDNA can be used to directly make a specific protein using what are called transcription factors.
These are molecules that help activate genes, making this a good term to use when discussing cDNA with your peers! Using transcription factors that activate genes that make proteins is a common way to test if something works in an experiment.
hnRNA
Unlike mRNAs, hnRNAs do not code for a protein. Rather, they act as guides to direct RNA polymerases to specific locations on the genome to either replicate the DNA or modify it.
hnRNAs are part of what is called “junctional DNA”- regions of the genome that do not encode for proteins but rather join two parts of the chromosome.
There are several types of hnRNA, and they can be classified by what type of RNA they are. The five main types of hnRNA are: piRNA, siRNA, miRNA, ta-siRNA, and tasi-miRNA.
piRNAs are one type of hnRNA that act as messengers in a cell’s immune system called “cellular immunity”. piRNAs detect foreign invaders in the cell and direct cellular proteins how to respond to them.
rDNA
The largest category of RNA is called ribosomal DNA, or rDNA. This category includes all of the RNA that codes for proteins, the building blocks of life.
All proteins are created from sequences of up to twenty amino acids. These molecules are brought into the cell by digesting food and are then assembled into a protein via a process called transcription.
Transcription is when a molecule of RNA is copied from a gene in the DNA. This RNA then travels to a part of the cell called the nucleus, where it associates with other molecules to help create a protein.
Ribosomes are tiny structures in the cell that aid in this process by carrying amino acids and linking them together to create a protein. All cells have ribosomes, which is why they need RNA that codes for them.
