Different types of RNAs and their functions
In the previous steps, we’ve learned about the central dogma of molecular biology describing the genetic information flow from DNA to RNA to proteins.
Those RNAs carrying the code for protein synthesis are called “coding RNAs” or “messenger RNAs (mRNAs)”. Surprisingly, recent evidence revealed that very little of our human genome sequences (less than 2%) could actually end up producing proteins. However, most of the rest genome sequences are actively transcribed to generate the so-called “non-coding RNAs (ncRNAs)”. These ncRNAs do not undergo translation to synthesize proteins, but may hold the key to broaden our understanding of gene regulation and human diseases. Many of them are reported to serve as various regulatory elements in the genome, whereas most are still of unknown importance to gene regulation.
We would like to briefly introduce several different types of RNAs:
Coding-RNA (messenger RNA; mRNA)
Messenger RNA (mRNA) carries the genetic code from DNA in a form that can be recognized to make proteins. The coding sequence of the mRNA determines the amino acid sequence in the protein produced. Once transcribed from DNA, eukaryotic mRNA briefly exists in a form called “precursor mRNA (pre-mRNA)” before it is fully processed into mature mRNA. This processing step, which is called “RNA splicing”, removes the introns—non-coding sections of the pre-mRNA. There are approximately 23,000 mRNAs encoded in human genome.
Non-coding RNA (ncRNA)
Ribosomal RNA (rRNA):
Ribosomal RNA is the catalytic component of the ribosomes. In the cytoplasm, rRNAs and protein components combine to form a nucleoprotein complex called the ribosome which binds mRNA and synthesizes proteins (also called translation).
Transfer RNA (tRNA):
Transfer RNA is a small RNA chain of about 80 nucleotides. During translation, tRNA transfers specific amino acids that correspond to the mRNA sequence into the growing polypeptide chain at the ribosome.
Small nuclear RNAs (snRNA; 150 nt):
Small nuclear RNAs are always associated with a group of specific proteins to form the complexes referred to as “small nuclear ribonucleoproteins (snRNP)” in the nucleus. Their primary function is to process the precursor mRNA (pre-mRNA).
Small nucleolar RNAs (snoRNA; 60-300 nt):
Small nucleolar RNAs are components of small nucleolar ribonucleoproteins (snoRNPs), which are complexes that are responsible for sequence-specific nucleotide modification.
Piwi-interacting RNAs (piRNA; 24-30 nt):
Piwi-interacting RNAs bind the PIWI subfamily proteins that are involved in maintaining genome stability in germline cells. Piwi-interacting RNAs also play a role in gametogenesis.
MicroRNAs (miRNA; 21-22 nt):
MicroRNAs are small ncRNAs of ~22 nucleotides (nt) and the most widely studied class of ncRNAs. These RNA species mediate post-transcriptional gene silencing through RNA interference (RNAi), where an effector complex of miRNA and enzymes can target complementary mRNA by blocking the mRNA from being translated or accelerating its degradation. In human, miRNAs are estimated to regulate the translation of >60% of protein-coding genes.
Long noncoding RNAs (lncRNA):
Long noncoding RNAs are a heterogeneous group of non-coding transcripts larger than 200 nt in size and make up the largest portion of the mammalian non-coding transcriptome. It is estimated that more than 8,000 lncRNAs encoded in the human genome. lncRNAs are essential in many physiological processes. To date, various mechanisms of gene regulation by some lncRNAs have been reported, whereas most are still of unknown function.
If you want to learn more about this topic, you may find the following links useful.