if a dna sequence is altered from tagctga to tagtga, what kind of mutation has occurred? This is a topic that many people are looking for. amritsang.org is a channel providing useful information about learning, life, digital marketing and online courses …. it will help you have an overview and solid multi-faceted knowledge . Today, amritsang.org would like to introduce to you Impact of mutations on translation into amino acids | High school biology | Khan Academy. Following along are instructions in the video below:
So lets start looking at a short sequence of dna and the letters. I im going to use these are the shorthands for the various nucleotide bases that up a sequence of dna. So lets say that i have some thymine thymine cytosine guanine cytosine thymine adenine thymine.
Thymine and lets throw another thymine in there so that would be our sequence of dna. And what would be the corresponding sequence of rna that it would be transcribed into if you remember this from previous videos. Pause this video and try to figure that out well the key thing to appreciate is if were talking base pairs in dna adenine pairs with thymine cytosine pairs with guanine.
But if were talking about pairing into rna well then instead of thymine in the rna. You would have uracil. So the rna here is well the thymine in the dna would correspond to an adenine in the rna.
Adenine guanine. Cytosine. Guanine.
Adenine and now since. This is an rna. Strand.
Instead of having a thymine right over. Here. This would be a uracil adenine adenine adenine.
So this process that we just did this is transcription transcription transcription from dna dna to rna. Now the next step. If were talking about the whole process of how does this information actually have an effect on the body is were gonna go from the rna and translate that into a protein and the way.
We do that weve seen this in previous videos is every three of these bases. Thats a codon and it codes for a particular. Amino acid.
Now to figure out what amino acid. It codes for we look at an amino acid translation table and theres different types that you might see this is the most typical type. So.
The first base is a second base is a third base is g. First base a second base a were in this cell. Third.
Base is g. And so that will code for the amino acid lysine. So we could write l.
Y. S. Short for lysine here and we couldve also gotten that from a different type of translation table for example you might see a circular one that looks like that but we wouldve gotten the same result aag start at the center aag codes for lysine.
Then the next codon and if youre getting as excited about this as i am i encourage you to pause this video and try to keep translating this the next codon is cga cga arginine arginine and then the next. One is uaa uaa well here they have this little black circular dot. What does that mean well that means stop codon and sometimes theyll just write the word stop there.
So this is stop. There is not an amino acid called stop this actually signals to and this is happening at a ribosome. This is signaling for the translation process to stop.
This is the end of our amino acid chain of our polypeptide chain. And so we will stop right over there. But now lets do some interesting things lets think about situations.
Where there are mutations. Where some of these bases maybe something gets inserted maybe something gets deleted maybe something gets swapped out and so lets start with whats known as a point mutation. So lets say this c gets swapped.
Out for an a well if that happened then on the rna strand. All of a sudden. This would be a uracil and if that is a uracil this aag would still be there coding for lysine.
But this second codon is now different.
What would it now code for well cua cua itll now code for leucine instead of arginine leucine l e u. This is fairly typical for a substitution mutation it might change a particular amino acid. But sometimes it could be more significant for example.
If this g. Was swapped out for an a then this c on the rna. Would then be a u.
And then what would happen well this first codon would still code for lysine. But the second one would be uga uga now all of a sudden it codes for a stop codon and so the actual translation process would stop. Which could be a very very big deal if this dna sequence.
If the normal non mutated polypeptide had to keep going on and on and on over here. It just happened to have a stop codon next. But you could imagine if they had just you know another thousand codons before the end.
But all of a sudden you had a point mutation to stop early that would significantly affect the protein that its coding for now another type of mutation that typically has a fairly significant affect is a frameshift mutation and thats where something gets inserted or deleted. And shifts everything so for example instead of the a being swapped in for the g. What if the a got inserted here so then our sequence would look like this t t c.
And then we have a and then you have g c t g c. T a t t t so what just happened here this was our original sequence. But the a got inserted here it didnt replace the g and so everything got shifted to the right now.
What are we coding for well when we transcribe to rna. This will be a a g u. C.
G. A. U.
A a a and now this first codon still codes for lysine. Weve seen that multiple times. But what about this second codon this second codon over here ucg ucg.
Thats serine. We got a different amino acid. And whats interesting is its not just that one amino acid is changing.
Were gonna see that keeps happening. So now we have aua aua here we have isoleucine so isoleucine right over here. Which is different than what we had before we dont have a stop codon anymore and we would keep going on and on and so you could imagine a frameshift mutation.
Where you either insert something or you take it out so that the whole frame gets shifted can have a dramatic impact on what it will transcribe and then translate for now lucky for us even though mutations are always going on there are many proofreading mechanisms in biological systems to make them less frequent than they otherwise would be and people are still understanding how these proofreading mechanisms fully happen. Another thing to appreciate is we often associate a mutation as being equal to a bad thing and often times. It is a bad thing.
What used to be a functional protein may no longer be a functional protein. Because the amino acids. The coding got stopped short or there was a frameshift mutation thats just coding for completely different things so sometimes it could be very bad and some diseases actually are caused by strange mutations.
Like that that show up often times the mutation might not be a big deal. Maybe something gets swapped out maybe. Only one amino acid changes.
And it doesnt really change the ability of the protein to do its job in which case. It doesnt matter. But every now and then a mutation can actually be a good thing in fact.
We need the mutation in order to have variation in a population and variation is what natural selection and evolution run off of if you dont have variation. Then youre not going to have different things that get selected in different environments. And youre not going to have that gradual change over time so a big picture.
Hopefully you got a better appreciation for how transcription and then translation. Let me write that down and then so thats transcription from dna to rna. And then this is translation translation from rna to protein to protein.
We have appreciation of how that happens we got appreciation of how to use these translation tables. But also how either a point mutation or a frameshift mutation can eventually affect the protein that gets coded for .
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