A quick introduction to microbiology and retrovirology

URL: https://barryhisblog.blogspot.com/p/a-quick-introduction-to-microbiology.html

Just about everyone has heard of DNA (DeoxyriboNucleic Acid), but what is it? It is a polymer (a long chain of sub-molecules strung together like beads on a necklace). RNA, proteins and plastics are polymers too. We'll come to that shortly.

The "beads" (called nucleotides or bases) that form DNA are named adenine (A), cytosine (C), guanine (G), and thymine (T). In the famous double helix, they form up in base pairs, where adenine pairs with thymine, and cytosine pairs with guanine.

Together, they "code" for the molecules (proteins) with which the rest of any living organism is mostly made.

Special molecules, called RNA polmerases "read" sections of the DNA base sequences and transcribe them into equivalent RNA (RiboNucleic Acid) molecules. RNA is very similar to DNA, with four bases, adenine, cytosine, uracil, and guanine.

The transcribed RNA is called "messenger RNA" or mRNA. It is this that goes on to be translated into proteins. Triplets of base pairs, called codons, which "code" for different amino acids are "read" by a ribosome that produces a string of amino acids that form a protein.

The "Central Dogma" that DNA makes RNA that makes proteins was a bit of a tongue-in-the-cheek joke. There is no dogma in science like there can be in political ideologies and religions, with ideas held to be absolutely true and never to be questioned. But at the time, the central dogma was thought to explain all of molecular biology. However, then came the discovery of reverse transcriptase that went against the flow of information from DNA to RNA to proteins. It is used by viruses that make infected cells read their viral RNA genomes and produce DNA versions, the reverse of the normal transcription process. This is why these viruses are called RETROviruses, the retro prefix meaning anything that goes in a backward direction like retro rockets in a spacecraft or backwards in time like retro fashion styles.

The reverse-transcribed viral DNA is then inserted into (integrated with, in the jargon) the DNA of the host infected cell. A special viral enzyme, integrase, does this. Snipping the host DNA and inserting the viral genome, before joining the bits up again, incidentally leaving a telltale bit of duplicated host DNA at either end of the integration site.

The host cell machinery then dutifully transcribes and translates the viral DNA, resulting in the production of more copies of the original virus which can then exit the cell and go on to infect more host cells, either within the same organism or in neighbouring organisms.

There are other forms of RNA molecules that do not code for proteins, some with well understood functions, some not yet understood and perhaps some that do nothing. There is something of a controversy over this, generated notably by the ENCODE project. But is understood that RNAs are involved in regulation - the promotion or inhibition of DNA transcription. Epigenetics is an important area of study, and it looks at such things as cellular differentiation. All cells in a creature have the same DNA. How is it that the same DNA can produce so many cell types - muscles, skin, nerves and so on? It's down to controlling which genes are transcribed, and it's regulatory genetics that determines that. Mutations that affect regulation can have profound effects.