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Current time:0:00Total duration:15:24

DNA replication and RNA transcription and translation

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Video transcript

we've already talked about how DNA's structure as this double helix this twisted ladder makes it suitable for being the molecular basis of heredity and what we want to do in this video is get a better appreciation for why it is suitable and the mechanism by which it is the molecular basis for heredity 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 really just give you the conceptual idea of what happens so right over here this could be a fragment of DNA I have what I have this is 8 base pairs depicted and just to be clear we talked about this in the introductory video to DNA DNA is a is much more than you know a handful of base pairs the DNA molecule can be tens of millions of base pairs long so for example this might be a section of a much longer molecule so a much longer strand of DNA and I even there I'm probably not not giving justice to it but this might just be this very very small section right I'm just in a different color this little section right over here zoomed in so once again it might be part of a molecule that has not seven or eight base pairs but it might have seventy million base pairs so and just like that so just like that so let's understand what a molecular basis of heredity would need to do well first of all it would need to be replicable you're needed or something would need to be able to replicate it as a cell divides it's the the the two new cells would want to have the same genetic material so how does DNA replicate and this process is called replication so let me replication and we covered this in the introduction video as well but it's nice to see the different processes next to each other and replication well you can imagine taking either both splitting these two sides of the ladder and actually let's do that so let me copy and paste so if I take that side right over there and so let me copy and then paste it and then there we go a little bit of his dropping below the video but I think that serves the purpose and then and then let's copy and paste the other side so let me select that and then I copy and then I paste and it's just like that and so you can imagine if you were to split these these things you could call them two sides of the ladder then either side could be used to construct the other side and then you would have two strands two identical strands of the DNA and so let's let's see what that actually looks like so let me get my pen tool out now let me DES elect this I get the pen tool out have to be it's a new tool I'm using so let me make sure I'm doing it right all right so from this side from this left side or at least what we are looking at the left side you can then construct another right side based on this information a always always pairs with T if we're talking about DNA so so adenine pairs with thymine just like that dye mean pairs with adenine let me do that a little bit neater thymine pairs with adenine guanine pairs with cytosine that's cytosine cytosine pairs with guanine falling a little bit down here and just like that I was able to construct a new right hand side using that left hand side so maybe I'll do the the the new the new sugar phosphate backbone in yellow and we could do the same thing here using the original right hand side so using the original right hand side once again the t's pair with the A's time let me do that in that ad means color so we have an adenine and thymine adenine and thymine adenine and thymine thymine pairs with adenine so thymine adenine thymine adenine guanine pairs with cytosine guanine guanine and then cytosine pairs with guanine so cytosine just like that and so you can take half of each of this ladder and then you could use it to construct the other half and what you essentially done is you've you've wrapped located the actual DNA and this is actually a kind of conceptual level of how replication is done before a cell divides and replicates and then the entire cell duplicates itself so that's replication so the next thing you're probably thinking about okay well you know it's nice to be able to replicate yourself but that's kind of useless if that information can be used to to define the organism in some way to to express what's actually happening and so let's think about how how the genes in this this this DNA molecule are actually expressed so I'll write this as expression expression and actually that actually warrants a a little bit of a detour because you hear sometimes the words DNA and chromosome and gene used somewhat interchangeably and they are clearly related but it's worth knowing what what is what so when you're talking about DNA you're talking literally about this molecule here that has this sugar phosphate base and has these it's the sequence of base pairs it's got this double helix structure and so this whole thing this could be a DNA molecule now when you when you have a DNA molecule that's packaged together with other molecules and proteins and kind of given a broader structure then you're talking about then you're talking about a chromosome 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 this whole thing could be a strand of DNA but this part right over let's say an orange I'll do it this part in orange right over here could be one gene it might define it it could soar information for one gene it could define a protein this one might this section right over here could do 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 is 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 and likes you let me write this down RNA R and a so RNA stands for ribonucleic acid ribonucleic acid let me write that down rye Bo Rai Bou nucleic nucleic acid 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 it's not deoxyribonucleic acid its ribonucleic acid the R let me make it clear where the RNA comes from the R is right over there then you have the nucleic that's the end it's found or what well it's a nucleic and then it's a acids same reason why we called the DNA nucleic acid so you have this R and a so what 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 with nuclei the DNA sits there but it has to that information has to for the most part get outside of the nucleus in order to be expressed and one of the functions that are in a 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 so the step that you go from DNA to mRNA messenger RNA is called transcription let me write that down trans [ __ ] ssin trans description transcription and what happens in transcription let's go back to looking at one side of this 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 I'll actually I didn't want to 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 so let's say you have let's say that you have part of this DNA molecule or you have one half of it just like we did when we replicated it but now we're not just trying to 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 that section of for at least for that gene so this might be part of a gene that actually whoops let me make sure I'm using the right tool this might be part of a gene that is you know this section of our DNA molecule right over there and so transcription is a very similar conceptual idea where we're now going to construct a strand of RNA and specifically mRNA because 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 what 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 you you for uracil so you're a cell your cell that's the base we're talking about let me write it down you're a sill uracil thymine is still going to pair with adenine it's still going to pair with adenine it's like that guanine is gonna pair with cytosine guanine and cytosine and cytosines going to pair cytosine is going to pair with guanine and so when you do that now they these two characters can detached and now you have a single strand of RNA and in this case messenger RNA that can that can that that has all the information on that section of on that section of a DNA and so now that thing can leave the nucleus go attached 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 so translation 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 so let's take our mRNA or this little section of our mRNA and actually let me do it let me draw it like this let me draw it like this and let's see I have it is UAC so that's gonna be you AC then you you then a CG okay and then we have an a let me make sure I change to the right color we have an A there and then we have Suu a CG alright now let me put a C right over there I'm just taking this and I'm writing it horizontally I have a seat 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 each sequence of three in it 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 these I guess you could say this three-letter word or this thriller sequence that's called a codon codon and this is going to be the next codon the next codon and we actually haven't drawn the next codon after that because 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 code onwards I guess you could say and 4 times 4 times 4 is 64 so you have 64 possible possible codons which is good because you have 20 possible amino acids so this is overkill it allows codons to be used for other purposes as well and they also you know you might have more than one codon coding for the same amino acid so you have 64 possible codons I need to code for 20 amino acids and so this codon right over here it with the ribosome and we'll talk more about how that happens can code for can code for say could 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 it's they're actually matched together by another type of RNA this is mRNA we're talking about right over here this is M RNA but there's another type of RNA called tRNA that essentially that essentially brings these two characters together so the tRNA and I'm just gonna you know it's got some structure here I'm not drawing it completely right but it's going to match right over here where maybe it has it has an a a u and a G right over here and on this end it was attached to this amino acid and so it matches them together and then they're gonna have another another tRNA that might attach to amino acid too which way I will do in purple amino acid too and that just happens to coincide with so it can complement right over here so it attaches in the right places eh-eh you right over here this tRNA and so it'll construct the sequence of amino acids and as you put these amino acids together then you are actually constructing a protein so protein is a is a essentially a bunch of a sequence of these amino acids put together so a sequence of these amino acids put together these proteins are essentially the the molecules that do that run life for the most part obviously no if you eat an animal it's going to be made up of fat and sugars and proteins but the proteins are the things that actually do a lot of the weather their enzymes whether they're structural the muscle is formed from proteins these are the things and I'm just drawing a small segment of them they could be thousands or more of these of these amino acids long and they kind of form these incredibly complex shapes and they have all of these functions this is what's kind of doing the work of life and this for the most part and this is how this is kind of how the information for life is stored
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