You cannot force a puzzle piece into the wrong place! Four Letter Alphabet Think of all the words you can spell. I bet there are loads. But each word is made using the same selection of letters.
Yes, sometimes we leave letters out, sometimes we repeat letters, but we always have the same selection of letters.
Depending on how we arrange the letters of the alphabet we can make new words. The same is true in the four letter alphabet of DNA. These are called codons. These sentences are called genes. These proteins control everything in a cell. In this way, DNA is like the boss of a company, and not the brain of the cell. It issues instructions, but doesn't do very much of the actual work : These proteins help each cell do its job. Each gene makes one protein , and only one protein. Build a lego tower 10 blocks tall.
Use only 4 colours. How many combinations can you make? How can four letters make something as complicated as a human body? Let us take a trip back to my favourite childhood toy—Lego. Give a child 80 pieces of one colour and ask them to build a tower. No matter how they try, they can only make one possible combination of colours. Now give a child a box of Lego with 20 lots of 4 different colours and ask them to make a tower.
The size is still the same, but the combination and order of colours is different each time they build. The possibilities are endless Remember it is the sequence of letters order of the colours in this analogy that stores the information.
Each set of 3 letters is a word. With four different letters, there are 64 possible three-letter-words. Imagine how many combinations of these words there are in a sentence just letters long!
Whenever you are trying to convey complex ideas to young children, analogies are your friend. Just make sure they know what the analogy means, and they are not just saying "DNA is like Lego" or "cells are like buses.
Where Next? And when you're talking about a gene, you're talking about a section of DNA that's used to express a certain trait. Or actually used to code for a certain type of protein. So for example this could be, this whole thing could be a strand of DNA, but this part right over, let's say in orange I'll do it, this part in orange right over here could be one gene, it might define information for one gene, it could define a protein, this section right over here could be used to define another gene.
And genes could be anywhere from several thousand base pairs long, all the way up into the millions. And as we'll see, the way that a gene is expressed, the way we get from the information for that section of DNA into a protein which is really how it's expressed, is through a related molecule to DNA, and that is RNA. Actually let me write this down. So RNA stands for ribonucleic acid.
Ribonucleic acid, let me write that down. And so you might remember that DNA is deoxyribonucleic acid, so the sugar backbone in RNA is a very similar molecule, well now it's got its oxy, it's not deoxyribonucleic acid, it's ribonucleic acid. The R, let me make it clear where the RNA come from, the R is right over there, then you have the nucleic, that's the n, and then it's a, acid. Same reason why we call the DNA nucleic acid.
So you have this RNA. So what role does this play as we are trying to express the information in this DNA? Well the DNA, especially if we're talking about cells with nucleii, the DNA sits there but that information has to for the most part get outside of the nucleus in order to be expressed.
And one of the functions that RNA plays is to be that messenger, that messenger between a certain section of DNA and kind of what goes on outside of the nucleus, so that that can be translated into an actual protein.
Let me write that down. And what happens in transcription, let's go back to looking at one side of this DNA molecule.
So let's say you have that right over there, let me copy and paste it. So there we go, actually I didn't wanna do that. I wanted the other side. So actually I think I'm on the wrong, let me go back here. And so let me copy and then let me paste. There we go. But now we're not just trying to duplicate the DNA molecule, we're actually trying to create a corresponding mRNA molecule. At least for that section of, at least for that gene.
So this might be part of a gene Actually whoops, let me make sure I'm using the right tool. This might be part of a gene that is this section of our DNA molecule right over there.
And so transcription is a very similar conceputal idea, where we're now going to construct a strand of RNA and specifically mRNA 'cause it's going to take that information outside of the nucleus.
And so it's very similar except for when we're talking about RNA, adenine, instead of pairing with thymine, is now going to pair with uracil.
So let me write this down, so now you're gonna have adenine pairs not with thymine but uracil. DNA has uracil instead of the thymine. But you're still going to have cytosine and guanine pairing. So for the RNA and in this case the mRNA that's going to leave the nucleus A is going to pair with U, U for uracil, so uracil, that's the base we're talking about, let me write it down, uracil.
Thymine is still going to pair with adenine, just like that. Guanine is gonna pair with cytosine, and cytosine is going to pair with guanine. And so when you do that, now these two characters can detach, and now you have a single strand of RNA and in this case messenger RNA, that has all the information on that section of DNA. And so now that thing can leave the nucleus, go attach to a ribosome, and we'll talk more about that in future videos exactly how that's happened, and then this code can be used to actually code for proteins.
Now how does that happen? And that process is called translation. Which is really taking this base pair sequence and turning it into an amino acid sequence. Proteins are made up of sequences of amino acids. So translation. And then we have an A, let me make sure I change it to the right color. I have a C here, not a G, it's a C. And then finally I have a G. And of course it'll keep going on and on and on. And what happens is each sequence of three, and you have to be very careful where it starts, and so this is in some ways a delicate and surprising, but at the same time surprisingly robust process, every three of these bases code for a specific amino acid.
And so three bases together, so these bases right over here, these I guess you could say this three letter word or this three letter sequence, that's called a codon.
And this is going to be the next codon. And we actually haven't drawn the next codon after that 'cause we need three bases to get to the next codon. And how many possible codons do you have? Well you have one of four bases and you have them in three different places, so you have four times four times four, possible codon words I guess you could say.
And four times four times four is So you have 64 possible codons. Which is good because you have 20 possible amino acids. So this is overkill and allows codons to be used for other purposes as well.
And they also, you might have more than one codon coding for the same amino acid. So you have 64 possible codons that need to code for 20 amino acids. And so this codon right over here with the ribosome, and we'll talk more about how that happens, can code for amino acid 1. So let me just write it here, this is amino acid 1.
And actually this amino acid is brought to here, they're actually matched together by another type of RNA, this is mRNA we're talking about right over here.The cells, growing just under the scalp, eventually die, leaving the keratin behind. An example of the latter is a cell that has reached an end stage of development and will. So let's say you have that right over there, let me copy and paste it.
So, let us take a look at how you would explain DNA to a six-year-old. And we're gonna focus on a conceptual level, I'm not gonna go into all of the, I guess you could say biochemical details. So using the original right hand side, once again the T is paired with the A, let me do that in adenine's color.
But this might just be this very, very small section, let me do this in a different color, this little section right over here, zoomed in. And we covered this in the introduction video as well, but it's nice to see the different processes next to each other. The rungs are very special. At least for that section of, at least for that gene. Remember it is the sequence of letters order of the colours in this analogy that stores the information.
And they also, you might have more than one codon coding for the same amino acid. Now when you have a DNA molecule and it's packaged together with other molecules and proteins and kind of given a broader structure, then you're talking about a chromosome. And that process is called translation. So this might be part of a gene Actually whoops, let me make sure I'm using the right tool.
This keratin, combined with the keratin left by many other cells, emerge from your scalp as hair. And these proteins are essentially the molecules that run life for the most part. But they are very choosy about their friends: A and T are best friends and always hang out together G and C are best friends and always hang out together Another way of looking at it is that A, T, G and C are like jigsaw pieces. So translation. And genes could be anywhere from several thousand base pairs long, all the way up into the millions. Build a lego tower 10 blocks tall.