Where there are humans, there are stories, whether it’s the ones we write down on a page, the ones we project on a screen, or the ones we live.
Although not well-known and often misunderstood, the story of genetics is, in many ways, the story of life itself. I hope to begin to scratch the surface of this immense body of biological literature over the next few blogs, and maybe explain how humans merely existing as we do now is an underdog tale worth telling.
The great author Sir Terry Pratchett, in one of his stories, created a vast library filled with nothing but individual autobiographies. Each book, dedicated to a single person, was continuously adding new words as it sat on its shelf, narrating the story of the individual as they went about their lives.
If you picked up one of these books, you would be able to read the events of a person’s past, see their present happening, and maybe predict something that could happen in their future. Clearly in the realm of fantasy, right?
Well, yes, in the sense of a physical book. However, there is another way to look at our own story, and it is hidden deep within our cells. Hidden there a genetic tale of a human being who grows to be a certain height, with a certain hair colour, who grows up with certain quirks and certain irks. A biological book describing why you’ve become both what and who you are. An origin story.
So what does this book look like? Where can you find it? And most importantly, can you read the bit from two days ago that says where you put your keys down? (Don’t worry, everyone does it.)
Part 1: The Book of Genes
Almost every book shares some basic characteristics, from Charles Darwin’s Origin of Species to that dodgy romance novel your Aunt wrote but you refuse to acknowledge exists. Books tend to be made up of letters, words and sentences, and are most often split into chapters, which leaves us with a hierarchy of structure that looks like this…
Seems simple enough right? So a perfect starting point would be to find our genetic book, and see if we can apply this structure to it.
Finding where the genetic book is is relatively simple, as it exists in almost every single cell in your body. You could either think of it as a must-have bestseller, or one of those bibles you find in every hotel that most people don’t even realise exists.
The problem is, as you can see below, most cells kind of leave the book scattered around until they actually need it, equivalent to just having the pages strewn around your room. Cells like to make you think they are really organised but it’s chaos in there, trust me.
It’s fine though, we’ll just let the cell know we would like a gander and it’ll be cleaned up quicker than a student’s room when they need their deposit. We’ll check back on this cell in a minute, but for now, we can definitely say that this ‘mess’ is our genetic book, or the ‘Genome’. Our little bundle of information floating in the nucleus of our cells.
Do not judge this book by its cover (or lack thereof) however, once the cell decides to organise itself we have something that looks both a lot easier to break down and perhaps rather familiar:
These fairly uniform bundles of stuff are chromosomes. Each 11-shape is actually two rod-shaped copies. In humans, these pairs of bundles divide our genome into twenty-three bitesize chunks. This is why they are the perfect candidates for our book’s chapters.
This is where we finally get to the term we are most familiar with: each chromosome contains upon it several genes.
A ‘gene’ means different things depending on who you ask. Most refer to genes as traits that are transferred from parent to offspring, which is why if you look like one of your parents it’s described as “having their genes” or that certain characteristics have been “passed down” (Hair colour, dimples and the ability to roll your tongue are well-known ones).
Gene’s biological structure is incredibly complex, but for now we only need to know two things:
- Each gene is a single unit of inheritable information
- These genes are passed down from parent to child
These are the units that “genetics” takes its name from, and the single units of information that have to make sense by themselves, with a clear beginning and end. This self-contained nature of similar units means these fulfil the role of “sentences” in our genetic book.
So now we can see our genome as a book, and our genetic book as split into chapters of chromosomes, with each sentence of text equaling a gene. So what’s next? Well, the best way forward would be to read the words in one of these sentences…
Ah. Oops. This is my bad. I probably should’ve mentioned before now that your genetic book is not written in English. It’s actually written in a code, a genetic code.
The word ‘code’, even though it is the correct scientific term, may be unfair. The idea is not to hide meaning from us, in the same way everyone speaking a foreign language isn’t doing it so they can talk about you behind your back.
To read any further, we have to teach ourselves a new language, the language of DNA. You won’t find this language on an app (unlike Klingon, amazingly). It is difficult for many reasons, but one of the main ones is that this language doesn’t really have words in the same way we do, which tends to be a good starting point for learning.
So if we cannot start with words, where do we begin? Well, there is an even smaller denominator that must be looked at, and that is letters. There are 26 letters in our alphabet, and for reasons I’ll never understand I learned to recite them backwards as a child (although it may have been that deadly combination of having too much free time and someone telling me that I couldn’t do it.)
In the English language we can form words out of any of those 26 letters, with varying lengths of combinations with spaces in between. So how does the genetic alphabet compare?
Ah, well that looks a bit easier, and it uses some of the same letters. I think anyone could say that one backwards.
The genetic language has some other rules, including that all your words have to be exactly three letters long, meaning words in this new alphabet would look as follows…
So what effect does this have (beyond making spelling tests way easier)? Well, the main one is that it means it will take a lot more letters to get across even basic information, as many as eighty thousand per sentence (that would be a lot of commas).
These three letter ‘blocks’, known as ‘codons’ (literally ‘part of a code’), are formed by a string of molecules of DNA, and within its tiny alphabet lies all the information your body needs to function. It is normally presented in a “double helix” structure (or as I call it “two twisty things”) as seen here. The four different letters in the alphabet correspond to the four different types of ‘rung’ you can have on this twisting ladder, which are known as ‘bases’ (shown in four different colours below).
As you would expect, with a limit of three letter words, and only four letters to work with, it is feasible to write out every word that could possibly exist in the genetic language (64). This is known as the ‘triplet code’ in typical exciting scientist vocabulary, but feel free to call it a ‘genetic dictionary’ if you’re feeling a bit adventurous.
This leaves us with these triplets as the ‘words’ in our book, and the ‘bases’ as letters.
And with these examples, the diagram from book to genome is completed. I have hopefully begun to show how this information can be found within ourselves.
Yet I’m sure you feel it, that empty sensation of something being missing. You’re saying “Yes, so now I know what’s IN the book, but what does it mean? What do I do with this book? What does my body do with this information?”
Well, this is where things get interesting. As I discussed before, this book is written in genetic code. This code took scientists years to decipher the language of, and it is also found within almost every cell in your body.
These two facts are not a coincidence, and point to the true purpose of this genetic book. To be read by our cells themselves.
Part 2: A best-celler.
So why do so many of our cells have a copy of this book? Is it the thrilling narrative? The cool settings? The character drama?
Well, not quite, cells actually use the book an instruction manual on how to make everything they need.
“Hold on.” you may say now, “‘Instructions’? You told me this book would be an epic tale, a narrative of how my life has come to be what it is, and now you’re telling me this is more like reading an instruction manual?”
I must admit, at a base level, these are a set of blueprints. Our book would have no room for narrative flair or dramatic irony upon first glance. But let’s be honest, for all most of us know, every instruction manual could be an undiscovered Shakespeare play. I mean someone has to write instructions, and jobs for writers are hard to come by these days. For all we know J.R.R Tolkien began his career writing the text of those little tags that tell you how to wash your clothes.
This isn’t any old instruction manual either, within it is contained the information required to make every single protein your body possibly needs.
Proteins are commonly referred to as the “building blocks” of the cell, and this is a fairly accurate comparison, almost all the important structures in the cell are made up of one or a combination of these building blocks. With our newly understood book, we should be able to follow our cell’s process from instructions to final product.
Here is our cell, he has just realised he needs a protein, let’s call it PROTEIN Z, because that sound interesting and mysterious.
So the first thing we have to do is find the correct gene, and thus sentence, in the book of life. We know this gene is on chromosome 5, so let’s flick to that section and see…
So here we have a stretch of DNA on chromosome 5, we know that the sentence is in amongst these words and letters somewhere, but where?
Well, in a book a sentence isn’t hard to pick out, you just look for the capital letter and full stop, these indicate where each idea starts and finishes. Is there an equivalent in our gene?
As a matter of fact, there is, and they are appropriately referred to as ‘stop’ and ‘start’ codons. These are two specific words that tell our cell both where to start and where to finish reading, as you can see below.
Once our cell knows it is reading the correct sentence, it produces a copy form the chromosome. This means that the original sentence does not get lost if it needs to use it again (back up your stuff people!) The original instructions are kept in the nucleus, a secure library that makes sure nothing damages the instructions (although this isn’t always possible, as we will explore in the next blog).
The small copied sentence is transported out of the nucleus to a protein production site in the cell…
At this new site, the sentence is run through a little structure called a ‘ribosome’. This nifty little unit takes our instruction copy and then ‘translates’ the sentence. But how does it do this? Well, this is where having a genetic dictionary would come in handy for us humans.
Each ‘word’ in the code that we saw earlier, is an instruction for one “amino acid”, if proteins are the building blocks of the cell, then amino acids are the building blocks of the building blocks.
For instance, when a ribosome reads the letters A T and G, it translates this word as the amino acid Methionine: Our capital letter, oddly, is a word. No gene begins without this 3 letter combination, as it shows the ribosome where to begin.
After beginning with the “Met” amino acid, the ribosome has another 22 amino acids to choose from. These all have specific traits, but will be covered in another blog, all that is important now is that these are chained together in the order they are read.
Once the ribosome reads a full stop in the sentence, of which there are three types (TAA, TAG, or TGA), it knows to stop reading, and lets the protein loose.
…and there we have it. Protein Z!
As I mentioned before, these proteins can be made up of up to 20,000 of these little amino acid building blocks, and it would take over 3 hours to read out the full name of it. This protein thankfully has a shorter name: ‘Titin’, and is part of what keeps your muscles springy. Another protein, MCR1, has a key role in producing red hair.
But then, if we all have DNA that is 99.9% identical, why does everyone not have red hair? Why do some people’s books have some sentences whilst others do not? What happens when our sentences have spelling errors, or we miss a full stop or capital letter? I mean, we’ve all done it…
These are questions that will be tackled in my next blog. The book of life 2: The ghost of genetics past. There, we’ll look at how our book came to be the way it is. After all, where do you think your story came from?