why can DNA transcription and translation processes be ‘wrong’ and make mistakes?
In: Biology
DNA/RNA are built out of smaller units called nucleotides, which are a bit like the links in a chain – a strand of DNA/RNA is made up of many individual nucleotides stuck together. To produce a new strand, you need to get a bunch of nucleotides and put them in the right order, so your main job as a protein designed to do this is to figure out what the right order is.
There are four DNA nucleotides – T, A, C and G. There’s also a fifth nucleotide, U, which is the RNA version of T for an interesting but irrelevant reason. These nucleotides are the links you see in graphics of a DNA helix, like [this one](https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Fd2noibkeom3qqq.cloudfront.net%2Fwp-content%2Fuploads%2F2017%2F02%2FDNA-helix-molecule-genetic.jpg&f=1&nofb=1). Notice how each of these links is split in the middle into two different colours. These are a visual representation of the DNA structure. See, DNA has two different strands each with its own nucleotides, that stick together a bit like a zip – the nucleotides on one strand of the DNA stick to the nucleotides on the other strand to form this stable helix structure.
The fact they stick together like this is how proteins know what the right order of nucleotides is when replicating the DNA. Nucleotides exist in matching pairs that preferentially stick together. T and A are a pair, and C and G are a pair. To produce more DNA/RNA, proteins unzip the DNA to expose the nucleotides on one of the strands, and then free floating nucleotides come into the protein complex and if they stick to the next exposed nucleotide in the line, the protein glues them onto the end of the emerging strand. So if the exposed DNA strand says TTCAGACGA, then the proteins will produce an RNA strand of AAGUCUGCU, because those are the RNA nucleotides that stick to the DNA strand (noting that U here is substituting in for T).
However, this stickiness is not particularly strong. It’s done using interactions called hydrogen bonds, and the number of these bonds formed between a pair determines its stickiness. T and A pairs form 2 hydrogen bonds, while C and G pairs form 3. These have positive and negative ends, a bit like magnets, so TG and AC pairings aren’t going to happen, but you can get TC and AG pairings, they’ll just be uncomfortable and not fit properly. There are other proteins that check for these uncomfortable pairings and replace them with the right nucleotide, but they’re not super reliable and there’s a good chance they replace the wrong one and cause a mutation.
Transcription errors occur then when the wrong nucleotides stick together. This isn’t a big deal for RNA cos RNA lasts for such a short time, but the fixing proteins can permanently establish these mistakes as mutations in DNA, occasionally.
Translation errors follow a similar process, but due to a mis-stickying between the nucleotides of the mRNA and the nucleotides of the already built tRNA molecules.
Latest Answers