The past two weeks we took a brief overview of mitosis and meiosis, the processes of cell division. This week we will return to examining one of the three major macromolecules in the nucleus and cell that are indispensable for all life forms that we have identified to date. Of DNA, RNA and proteins, most people know less about RNA than the other two. Therefore, RNA will be the topic of our discussion today.
RNA is the abbreviation for ribonucleic acid. RNA is not composed of a long chain of nucleotides as is DNA, but are much shorter in length. The nucleotides are composed of a nitrogenous base (nucleobase), a phosphate group and a ribose sugar. Four of the key differences between RNA and DNA is the sugar used, one of the nucleobases, number of strands and length of strands. RNA uses the sugar ribose and DNA uses deoxyribose which is a sugar with one less oxygen atom. Where DNA used the nucleobase thymine, RNA uses uracil. Thymine and uracil have similar properties. DNA is famous for its double helical strand structure, but RNA has only one strand (with a few exceptions) which takes many different three dimensional shapes depending on its function at the time. DNA strands are quite long consisting of many nucleobases while RNA strands are shorter and do not contain nearly as many nucleobases.
RNA is perhaps the most multi-functional structure in the cell. Various sources list the number of different types of RNA as running from 5 to 18 and categorized into as many as 4 functional groups. Although we will not go into detail on all 18 listed forms of RNA or all 4 functional groups, I will list them so you can see the broad range of diversity found in RNA.
Protein Synthesis RNA
- Messenger RNA (mRNA) – mRNA codes for the synthesis of proteins by the ribosomes.
- Transfer RNA (tRNA) – During translation, tRNA transfers particular amino acids to the ribosome where a polypeptide chain is being constructed.
- Transfer-messenger RNA (tmRNA) – tmRNA is found in bacteria and functions as both mRNA and tRNA.
- Ribosomal RNA (rRNA) – rRNA is found in the ribosome where it decodes tRNA for the amino acids that are in turn transported by the tRNA during translation.
- Signal Recognition Particle RNA (SRP RNA) – SRP RNA is involved with the co-translational translocation and the post-translational transport of the signal recognition particle. The signal recognition particle is involved with the recognition and targeting of particular proteins in relation to the endoplasmic reticulum.
- MicroRNA (miRNA) – miRNA is a small strand that is involved in the silencing of genes and can be found targeting over half of the human genes.
- Long Noncoding RNA (Long ncRNA) – Long ncRNA are defined as having 200 or more nucleotides. It is involved in the regulation of gene transcription, regulating basal transcription machinery, post-transcriptional regulation which involves splicing and epigenetic regulation.
- Antisense RNA (aRNA) – aRNA is a complementary strand to mRNA and will bind to the mRNA which results in the obstruction of the mRNA function. aRNA is usually introduced into a cell by pathogens such as E. coli.
- Small Interfering RNA (siRNA) – siRNA is also known as Silencing RNA due to its roll in the silencing of a specific gene. Unlike most RNA strands, siRNA is a double stranded RNA molecule.
- Trans-acting siRNA (tasiRNA) – tasiRNA is found in terrestrial plants and is involved in the repression of certain gene expressions.
- Repeat Associated siRNA (rasiRNA) – rasiRNA are 24-29 nucleotide strands that are involved in the regulation of chromatin structure along with the suppression of certain mRNA cleavage functions. It is also associated with Piwi proteins.
- Clustered Regulatory Interspaced Short Palindronic Repeat RNA (crRNA)
- 7SK RNA (7SK)
- Cis-natural antisense transcript
DNA Replication RNA
- Guide RNA (gRNA) – gRNA are found in protists and are involved in the insertion or deletion of the uridine residues into the mRNA in mitochondria.
- Small Nuclear RNA (snRNA) – snRNA are short RNA molecules that are found in the nucleus and are involved in the maintenance of the telomeres along with several other splicing and regulatory functions within the nucleus.
- Small Nucleolar RNA (snoRNA) – snoRNA are short RNA molecules that help control any chemical changes to rRNA, tRNA and snoRNA.
- Ribonuclease P (RNaseP) – RnasP is a ribonucleic acid that cleaves off a specific sequence from a tRNA.
- Ribonuclease MRP (RNaseMRP) – RNaseMRP functions in both the mitochondria and the nucleus. In the nucleus, it cleaves a specific section of the rRNA. In the mitochondria, it is involved in the beginning of the mtDNA replication process.
- SmY RNA – SmY RNA is known to occur in some nematodes and associated with mRNA.
- Small Cajal Body Specific RNA (scaRNA) – scaRNA are associated with the nuclear organelle known as the Cajal body. It is involved in the methylation of RNA polymerase II and the biogenesis of snRNA.
- Y RNA – Y RNA is a non-coding RNA associated with the repression of the Ro ribonucleoprotein particle.
- Telomerase RNA (TERC) – TERC is involved with the extension of telomeres.
Virus and Parasitic RNA
- Viral Genome
- Satellite RNA
Many of the functions listed above are brief overviews of the different forms of RNA. Most of the RNAs listed above have additional functions that are not listed here. The main purpose for the list above is to demonstrate that RNA is extremely diverse in size, shape and function.
Considering all of the diversity found within the RNA family of molecules, the only possible explanation for their origin has to be that of an all knowing Creator God who designed everything to work so intricately and precisely with everything else.