Science Fundamentals: RNA
The Science Fundamental Series is designed to give quick, brief introductions to fundamental concepts in science. Hopefully these quick explanations of science terms and processes are helpful, and are part of the groundwork for your scientific literacy. If there are topics that you would like me to cover, please let me know in the comments. I have a Ph.D. in microbiology/biochemistry, but I am happy to do some digging into topics that I don’t deal with every day.
This Science Fundamental is on RNA. It is less well known than the related molecule DNA, but just as important, and it may have even been the precursor to DNA.
RNA or ribonucleic acid is a nucleic acid similar in structure to DNA. RNA is usually single-stranded, as opposed to the double-stranded DNA structure, however, the single-stranded RNA often folds in on itself to mimic the double-stranded nature of DNA. In addition, RNA has alternating phosphate groups and ribose rather than the deoxyribose in DNA. These ribose sugars are attached to one of the four bases: Adenine, Uracil, Cytosine, and Guanine. Notice that the DNA base Thymidine was replaced with the RNA base Uracil, which is just an unmethylated form of thymidine.
Messenger RNA (mRNA)
One of the main functions of RNA is to carry messages from the DNA in the genome to the ribosome (protein factories) so that proteins can be made. mRNA is made by the protein: RNA polymerase. This protein binds specific sites in the promoter region of a gene, and uses DNA as a template. RNA polymerase copies the DNA sequence of the gene into RNA. We call this process Transcription. This primary RNA is then modified by other proteins that add a tail on the back end, called the poly(A) tail, and then a cap on the front end. In addition, even more proteins can alter the RNA sequence. Some genes in the DNA have ‘intergenic regions’ called introns. These introns are extra bits of DNA that are not necessary for the protein, and so are removed before the ribosome. In very complex cases, these introns allow one gene to encode for many different proteins depending on how the introns are cut out or left in the message.
Once the mRNA has been processed by the plethora of proteins, it can move onto the ribosomes, or protein factories. These ribosomes take the message from the mRNA, and translate it into protein. Once this is done, the mRNA is degraded. In fact, mRNA is inherently unstable. Logically this works out, as the cell doesn’t want to keep making proteins that it already has. Cells have limited resources and therefore want to conserve them.
Ribosomal RNA (rRNA)
Another function of RNA is ribosomal RNA (rRNA). Ribosomes are mixtures of RNA and proteins, where the rRNA is the catalytic portion. This means that the rRNA is the part of the ribosome that actually performs the translation from the mRNA. Interestingly, the rRNA is different for prokaryotes (bacteria) and eukaryotes (animals and plants). Even then, we can sequence the rRNA from soil and tell what kinds of bacteria are there, simply by the natural variation in rRNA.
Transfer RNA (tRNA)
Transfer RNA is a small RNA that is bound specifically to an amino acid, the building blocks of proteins. The tRNA moves to the ribosome to help build the growing chain of protein. Each tRNA has a region called the anticodon. The anticodon matches up with the specific DNA sequence called a codon that encodes for the amino acid carried by that tRNA.
While these are the productive aspects of RNA, it has many other functions. RNA can have direct impacts in gene expression. We call these regulatory RNA, as they can regulate the transcription of genes. Sometimes the RNA binds to sections of DNA that promote the transcription called enhancers. Sometimes RNA can prevent the expression of a single gene, interfering RNA, or lockdown entire sections of DNA with many genes, called long non-coding RNA
Further, the new gene-editing technique CRISPR is based on RNA in prokaryotes. And there are even viruses that use RNA instead of DNA.
Hopefully this brief introduction to RNA is helpful. If you find it useful, please let me know. Drop me a line. If you have any suggestions for future Science Fundamentals, then let me know. I’ll be happy to look at specific topics for you.
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Glossary
RNA- a single-stranded nucleic acid present in all living cells that has structural similarities to DNA
Ribosome- a small protein complex consisting of RNA and associated proteins that bind messenger RNA and transfer RNA to synthesize proteins.
Transcription-the process of translating a segment of DNA into RNA
Introns- non-coding regions of mRNA, or the encoding DNA, that interrupt the gene and are eliminated by splicing before translation
References
https://www.genome.gov/genetics-glossary/RNA-Ribonucleic-Acid
https://en.m.wikipedia.org/wiki/RNA
