I am often surprised about the real dimensions of biological entities versus how they are shown in textbooks and scientific illustrations and this is very striking for ribonucleic acid (RNA). Ribonucleic acids themselves are not photogenic as they move and wiggle, and in textbooks are shown as short strands bound by 1-2 proteins. Not really – ribonucleic acids are bundled up, associate with hundreds of proteins, cations, and other small molecules, and have a higher spherical dimension than proteins.
Quizz time! What is your guess for the physical length of a “typical human ribonucleic acid*” (let’s say 2-5kilobases)? Don’t look it up! Draw it on the image below in relation to a human egg, a skin cell, a yeast, bacterium, or viral capsid & send to [hjambor – at – gmail.com], I’ll include it in the collection below. Or just post your guess in micro-, nano, or picometers in the comments!
A single nucleotide, which is the smallest building block, spans 3.4 Angstrom, or 340 picometers, or 0.3 nanometers. Three nucleotides encode one amino acid in a protein, therefore ribonucleic acids* three times longer than the respective protein. In addition, ribonucleic acids have many nucleotides that only serve regulatory purposes, they help with or block protein translation, or they influence stability and degradation.
The average yeast ribonucleic acid is ~1500 nucleotides (Miura, BMC Genomics, 2008), which adds up to a whopping 510 nanometers, or 0.5 micrometers, spanning a good portion of the length of the entire budding yeast itself!
The average human ribonucleic acid molecule is 2000 to 6000 nucleotides, resulting in a physical length of 0.7 to 2 micrometers (Strachan and Read, 1999, Human molecular genetics). This is after a process called splicing, which removes about 60-80% of the nucleotides before a protein is even made from it. Before splicing, right when they are transcribed from the DNA template, human ribonucleic acids are 3-5 micrometers long – that is longer than a virus capsid, a bacterial cell, a yeast cell, and even larger than the diameter of the nucleus it is transcribed in! These are just averages, the longest human ribonucleic acids measure 100 (Titin) and even 600 micrometers (caspr2). To fit inside a cells, and the nucleus of a cell, ribonucleic acids curl up and are compacted. And even in the cytoplasm, where they are shorter, ribonucleic acids take up a lot of space – on average about half of the genes are transcribed at any given time point, and typically each ribonucleic acid is present in multiple copies.
Now compare your guess to the answers I got from molecular biologists – their replies varied from 10 nanometers to 100 micrometers! Mind you, my own guess was far off as well, and that after having worked with localized ribonucleic acids for over 10 years!
What do we learn? Biological entities cover 10 magnitudes of scales, therefore faithful representations of size is neither possible nor expected in illustrations that merely symbolize information. On the other hand, our visual memory is pretty good – once we saw information as a picture, we tend to believe it. By memorizing false relative scales, we may thus loose an important information that may help us interpret research data.
* For the enthusiast: I mean messenger ribonucleic acids (mRNAs), the class that encodes proteins. These are generally longer than other categories of RNAs that do not encode proteins, such as rRNAs and tRNAs, miRNAs, and piRNAs.