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We talk a lot about genetics, here on The Pulse, because genetics is increasingly important throughout medicine, including in pregnancy health, and also because the process leading to pregnancy and development within the womb and after is all based on things happening at the molecular level within cells. This means the level of genes, which are sequences of building blocks of the molecule DNA analogous to how the letters of the alphabet are building blocks, strung together into the words and sentences that you’re reading on this page. DNA, which stands for deoxyribonucleic acid, is a chemical that was known mostly only to biochemists prior to the 1940s, when scientist began realizing that DNA —rather than proteins— was the genetic material, the material within cells that stores hereditary information that is passed down to new generations. Since 1953, when James Watson and Francis Crick revealed the actual physical structure of the DNA molecule that allows it to hold information, the term DNA has worked its way into the everyday lexicon to an extent that it is actually misused. Often, for instance, you will hear public figures speaking of how some societal pastime, belief, or policy is “part of our DNA”.
Despite all of this, in public discourse, you don’t hear as much, if anything about the cousin of DNA, namely RNA. Standing for ribonucleic acid, RNA is just as important to living things as DNA; in fact, RNA is more diverse than DNA in terms of its capabilities and the number of jobs that it performs. On top of that, knowing about RNA is important in connection both with pregnancy and COVID-19, so let’s spend today’s blog discussing RNA, the unsung hero of the cells that comprise your own body and that of the future human that may be developing within you..
What RNA and DNA have in common is that both are long molecules consisting of building blocks called nucleosides, each of which consists of a sugar molecule that contains five carbon atoms and an entity called a nucleobase, which for short is often just called a “base”. In DNA the specific type of sugar is called deoxyribose, whereas the sugar in RNA is called ribose, which is very similar to deoxyribose. There are just four different kinds of bases for RNA and four different kinds of bases for DNA, resulting in four different kinds of nucleoside for each molecule.
Now, the nucleosides containing the the bases are the building blocks of the molecules, just like letters are building blocks for words. As building blocks, they form DNA or RNA by connecting one after the other in series. Since DNA and RNA can be extremely long, there are numerous possible sequences of building blocks and the sequences of the building blocks carry information, just like a sequence of zeroes and ones carries information of a computer.
The nucleoside building blocks have names relating to the bases that they contain, but for simplicity biologists typically refer to just the the first letter of each base as the building block. Doing things this way, for DNA, the four building blocks are A, T, G, and C, whereas RNA uses a building block called U instead of T, so the RNA building blocks are A, U, G, and C. Whereas DNA contains two strands of building blocks that tend to form a helical shape, RNA can exist as either a single or double strand. Typically, unless an RNA molecule is very short, it will be mostly single stranded, forming various shapes, but with some double stranded regions resulting from different parts of the RNA strand paring up into a double strand, causing a fold or loop in the unpaired portion of the strand. The double stranding throughout DNA and in some segments of RNA occurs because certain bases tend to pair up with certain other bases. Specifically, A pairs with T or with U, whereas G pairs with C. Although beyond the scope of this blog, the pairing between A-T, A-U, and G-C happens because of a beautiful phenomenon in organic chemistry called hydrogen bonding that you can look up and explore if you’re interested in learning more.
Whereas DNA stores your genetic information for the long-term and is reproduced only when cells themselves reproduce, RNA molecules must be made frequently, as they are short-lived and play a variety of different roles. Additionally, there are some viruses —including SARS-CoV2, the virus that causes COVID-19 (viral RNA is what the main tests for COVID-19 detect)— that do not have DNA and use RNA as their genetic storage material. While there are no full-fledged life forms known on Earth that contain only RNA and use it for genetic storage, scientists hypothesize that RNA life forms could have existed billions of years ago and that DNA evolved in an RNA world of sorts. Consequently, the discovery of a microorganism, such as a bacterium, that uses RNA and not DNA would be a major milestone in the history of biology.
Now there are many subtypes of RNA, but in your cells there are just three main subtypes. There is messenger RNA (mRNA). Each molecule of mRNA is the RNA version of a set of instructions encoded in DNA. When a gene within DNA is active, strands of mRNA are put together based on the sequence of that DNA gene. This process is called transcription, because the language of DNA is simply copied into a slightly altered form, the language of RNA. This would be analogous to transcribing between British and American English. The other two types of RNA in your cells are transfer RNA (tRNA) and ribosomal RNA (rRNA). There are various types of tRNA and rRNA, they are transcribed from DNA genes just like mRNA, but their job is to help to get messages contained within mRNA translated, that is written into a totally different language, the language of proteins. It is a different language, because proteins are made of building blocks completely different from the building blocks of RNA and DNA. As the embryo-fetus develops within you, cells divide increasingly into daughter cells and the new cells differentiate, meaning that they become more specialized. They turn into bone cells, nervous cells, heart muscle cells, and so forth. For this to happen, various genes within the embryonic or fetal DNA are turned on and off when needed. This, in turn, leads to the tuning up or down the production of various mRNA molecules.
As for COVID-19, RNA has been on center stage, because one of the vaccines showing promise, the Moderna vaccine, is what’s called an mRNA vaccine. This means that the vaccine consists of a strand of mRNA, contained within a carrying vehicle that gets it inside a person’s cells, where its sequence is translated into a protein that the SARS-CoV2 virus has on its surface. In the case of vaccinated people, however, it would be their own cells carrying that protein on their surfaces, resulting in the immune system learning how to recognize that viral protein.