DNA Test Review
May 20, 2018
Hi, welcome students. I am going to cover the topics of DNA replication transcription and translation. After watching the recording please let me know if you have any questions or want me to touch a little bit further on the topic that you might want to have more information about. So the first thing that we're going to recap is the structure of DNA. Remember that DNA is twisted like a twisted ladder, and this rungs or the steps on the ladder are known as complementary base pairs. The ones that go together are a and T and G and C the size of the latter are what may be your hands would hold on to as you climb a ladder would make up the phosphate backbone which is the dot of deoxyribose, which is the sugar, that you see, I'll show you in just a second. And then the phosphate group. So what you're going to do over here is let's look, so this is the phosphate group, and this is that 5 carbon sugar, so that's the deoxyribose, so this is making the side of the ladder. Whereas this is the stairs of the ladder. And what this picture is showing you is the not the other pair. So let's see if we can't match the T's go with the a's. The a goes of a T, the C goes with the G and the G goes with a C and then in between these you would have another 5 carbon sugar and phosphate group. So make up the other side of the latter. DNA structure the big picture, DNA strands are wrapped around proteins called histones. They form structures called chromosomes, and chromosomes make up your genetic, and it tells you what color eyes you have, what color hair. That is known as your genes. And they can be found in the nucleus of a cell. Here is a snapshot of it's basically a karyotype of your chromosomes and a human, so there are actually 23 pairs of chromosomes, so that would give you a total of 46 chromosomes of every human, and a lot of students think that there's 46 chromosomes floating around somewhere in your body that has the entire genetic makeup. Keep in mind that there are 46 or your copies of chromosomes are in every single cell in your body. They're found in all the nucleus of the cells, the genetic makeup of who you are, so just keep that in mind a lot of students kind of have that misconception of there's just these chromosomes floating one place in my body. But it's not, it is in every single one of your cells that make up you. So the first DNA process that we're going to be talking about is DNA replication. In which you're looking at on the board right now is that there is actual replicating DNA strands as occurring and this is an actual microscopic picture of that happening. So this is where the DNA is wound up. And this is known as the DNA replication bubble. So the central dogma of molecular biology is basically three processes. DNA replication transcription and translation. Replication is making new DNA by copying using those complementary base pairings. Transcription is converting DNA into RNA and then translation is converting the RNA message into amino acids which will eventually make up proteins. So if you take DNA, you're going to have to do it into two process to make proteins. Why do you have to go through these processes while replication you can't make a new cell without making a copy of the DNA first, so replication has to occur to have that copy of DNA to put into the new cell we just left the topic of mitosis in meiosis, so in order for creating new cells via mitosis, you have to have DNA replication which is part of that interface that we were talking about last unit. Transcription is the conversion of DNA into RNA, so basically transcription is going to write a message from DNA and you're going to take it to the protein makers which is known as your ribosomes, and your cells to create those proteins that are necessity for making a new cell as well. So we're going to break down these processes. So here is a visual representation of what I was trying to tell you. I'm not previous slide. You have DNA, replication, DNA goes from DNA to RNA or to transcription goes from DNA to RNA, and then translation goes to RNA to a protein. So that's the overview of what these processes are going to do and we're going to break it down and talk about more details on how these happen. So what is DNA replication? DNA replication is the process by which DNA is copied during the cell cycle. The original DNA strands equals the templates to make up the new DNA strands. So the first step is that the two DNA strands are going to separate, and the structure that, excuse me, the protein that unzips these base pairs, which would go right through those hydrogen bonds. It is called the DNA helicase, so helicase is going to unzip the DNA strand exposing these base pairs, so these little red blue and green things are the a's the T's the Jesus sees that we are talking about. During the second phase, you have another enzyme called DNA polymerase, which is going to connect those free floating nucleotides in the nucleus, and pair them up based on their base pairings on the old DNA strand. So it will continue to do that and create this new DNA strand which you see in the green on both sides of the replication fork. So after this entire strand of DNA is replicated, you're going to end up with two new strands of DNA, which you see over here in this picture. Half of the D, the strain is going to be the old DNA, and the other half is the new DNA. So these are going to be identical because of those complementary base pairing rules. All right, next you have how does DNA replication occur? You have to have those enzyme the protein catalyst helicase, like I was showing you in the picture that will unzip the DNA. The DNA polymerase, which is another protein catalyst or enzyme, binds the new free floating nucleotides with their base pairs to make the new strand. When finished the two complete molecules of DNA, each like the original strand, so you're going to create two. New strands of DNA. So having the result in one half being the old strand one half being the new strand, it's something called the semiconservative model, so each parent strand remains intact in every DNA molecule is half old and half new. So if you see the word semi conservative, that's what that means that the end of DNA replication you're going to have two identical strands of DNA, half of its made up of that parent strand or the old strand, and then half of it is going to be made up with that new strand. All right, the accuracy and speed of replication this is just for information. At DNA replication happens over and over and over again on the same DNA molecules. It can occur at a hundreds of different places on a single DNA molecule. So it occurs with amazing speed and is extremely accurate. DNA polymerase also works as a proofreader. So a few eras are being made during replication and it has a spot checker to make sure all those base pairings are correct. Called another DNA polymerase. So basically, when you have this long strand of DNA, that you see down here in the picture, it's going to occur in multiple of those helicases are going to unzip and create these replication bubbles all along, and they're going to be moving in both directions, replicating the new strand. So this is why it happens so quick that it's happening on these long, long, long, long, long, strands of DNA, but at multiple places, so once they're all connected, you're going to have that semi conservative two strands of DNA. All right, so that ends the process of DNA replication so now we're going to move on to transcription, which is the first part of protein synthesis. Are the first step on how proteins are made. So transcription is going to be copying a sequence of DNA to produce a complementary strand of RNA, so remember that DNA and RNA are different because of the structure of. Those two molecules, DNA is going to be a double helix so that means it's going to be two size, where RNA is just basically one half of that ladder. So you're just going to have the phosphate sugar backbone and then one of those nitrogen bases. Another difference between DNA and RNA is that RNA has uracil, which is represented by you, and they do not have ts or thymines, for those nitrogen bases, said thymines are only located in DNA, and uracils are only located in RNA. There are three types of RNA, and remember that in all three types of these RNA, you're going to have that ribose sugar. The nitrogen bases but remember the uracil or places thymine, and then it is a single stranded helix, which is just a fancy way of saying its one half of a ladder. The three types of RNA are mRNA, which stands for messenger RNA, rRNA, which is ribosomal RNA, and then tRNA, which is transfer RNA, and we're going to be talking about all three of these during the processes of transcription and translation. The main goal of messenger RNA is it's going to take DNA described it into that messenger RNA during the process of transcription, so transcription again is going to convert DNA into mRNA. The ribosomal RNA basically all you're going to have to know is that it is found within the ribosome which is where the proteins are being made, and in that transfer RNA reads the mRNA or that message that's been brought to the ribosome from the nucleus and translate it into the amino acid sequence. So how does transcription occur? First you have to have the enzyme called RNA polymerase, which holds the DNA strands apart in pieces together nucleotides to make a new RNA strand from one side of the DNA molecule. Once the gene has been transcribed, the RNA strand detaches from DNA molecule and leaves the nucleus. So what's the point of this is that during translation of protein synthesis the cell uses information from the messenger RNA to produce proteins, so there's a huge problem here. So we know because of a previous unit that DNA is only found within a nucleus of the cell. So that DNA contains the description or the blue print on how to make proteins, but we know that proteins are made within the ribosome which is found in another location in the cell, so if the DNA can not leave the nucleus. But the ribosomes which are not in the nucleus, just another part of the cell needs the information to make or needs the blueprint. To make the proteins something has to get the information from the nucleus to the ribosomes, and that's exactly what the mRNA does. It is the messenger and it's going to take that message or the blueprint from that DNA out of the nucleus to the ribosomes, so those ribosomes can put those proteins together. So transcription, this is. A diagram of what's happening here. So again, you're going to unzip that DNA right along those hydrogen bonds between the nitrogen base pairings, and you have this RNA polymerase, which is what you see in red here. That's coming along and connecting this new strand, which is again a half a ladder of the mRNA. To take to the ribosome. So it looks for these codons called stop and start and stop codons that shows you that this is the start of the blueprint and the end of the blueprint of the protein that needs to be made. So this is how the RNA polymerase knows where to start copying the DNA strand and the stop the DNA strand. All right, so just a review, let's we've already talked about it briefly, but some of you are visual learners. So let's put it in a chart form. Of the difference between DNA and RNA. So again, DNA, the number of strands you're going to have two strands, RNA is going to be one strand. Again, the two strands is going to form a full ladder and RNA is going to be one half of the ladder. The sugar is deoxyribose. And an RNA, remember that Lincoln room is just ribose. And the nucleotides in DNA are going to see a's, T's, C's and G's, and an RNA you're going to see a's used C's and G's. All right, so now after that mRNA is made, the process of transcription is over, that mRNA is going to leave the nucleus and travel to the ribosome, which is another part of the cell. So at that ribosome, there's a process called translation that's going to hurt. So now since this messenger, the mRNA has left the nucleus, it gets the ribosomes but guess what. The ribosome doesn't speak the same language as the mRNA. So that's why you have to go through translation. So what you're going to do is the mRNA and fo translated to construct the amino acid sequence of the protein or that polypeptide. So translation is going to go from the mRNA to the amino acid chain and then eventually to the protein. So in a ribosome the mRNA is red, the tRNA delivers the amino acids coated for by the mRNA, and they're going to be groups of three. So if you see here is the structure of the tRNA, so you and it looks like a T, you have this structure that looks like a T and on one end of the T, you have this amino acid. And then on the other end of the T, you have something called an anticodon, which is going to be a group of three nitrogen base pairs, so this example is a GA and a so this tRNA that it's going to connect because on this mRNA, there is a codon that matches it based on those pairs, so cease, you use going to match up with GAA. So that is how they translate during the process of translation to put string together these amino acids. Which will eventually make a polypeptide, which will eventually make a protein which you see down here. All right, so these are those codons that we are talking about in this chart is basically the translation process. You're going to have to know how to read this chart. So codons are words for of mRNA in three letters. So here is a strand of mRNA. AUG is always a start codon. So this is going to show where to start transcribing it on that DNA. I always remember it as a start code on because we start school in August. So AUG is the abbreviation for August, so that's just another little way to remember it. So you're going to read in groups of three, which you see here on the triangles. And those TNR molecules that have those anti codons are going to code for the bases. So how you do it is let's take this codon. Let's say ggc, you're going to find the first base pair, so it's G so that's we know that it's going to be in this row. The second base pair, which is G, so we know it's going to be in this column. So that means that we have now focused in down on this box, and then we have C so ggc, which is the codon that we're looking for, codes for this specific amino acid. Glycine. So that checks out. So this is how you read the third base pair. For those three codon charts to code for those amino acids. So take some time you can pause this to. See if you are on the right concept. I'll do a couple here, but go ahead and pause it. So you have time to work ahead of me. All right, so let's check your answers. So we're going to look at G, gg, again, is. Glycine. You, Gigi, is. Tryptophan. CUA is. Leucine. CUG. Again, same. And UGA should be a stuck going on. I remember UGA, the stop codon because I'm a Georgia Tech fan, so I like to stop EGA and it's tracks. So that codes for the stop. So this would be the amino acid chain here. So again, translation one, the mRNA leaves the nucleus and attaches to the ribosome. mRNA is red with the beginning of AUG, which is the start codon we start school in August, and then the anticodon UAC will bind to it with the help of that few RNA molecule. The amino acid chain gets longer and longer and the stop codon is reached. There are no anti codons for these codons, so polypeptide falls off the ribosome. So here is a way to kind of look at the overall process of these three DNA. Sequences. The first one is replication, it happens in the nucleus, and it makes new DNA from old DNA. This has to happen before a new cell splits during the process of mitosis. Transcription happens in the nucleus and the definition is making mRNA that messenger RNA from the DNA. After transcription that mRNA leaves nucleus goes to the cytoplasm and finds a ribosome where translation will occur. During translation you're going to make those amino acids and from the mRNA using the help of tRNA will link those proteins together to make a polypeptide which eventually will make up an entire protein. So here is all the stages happening. This is transcription and side the nucleus, the mRNA leaving, going into the ribosome, the ribosome with the help of the tRNA is going to grow this polypeptide chain and eventually make the protein. All right, now I'm just going to shift the topic here to talk about mutations, so this is going to be any change in organisms DNA and it can be caused by mutagens or substances that increases the chance of mutations to occur. There are three types of mutations you have a point mutation a frame shift mutation, chromosomal mutation. A point shift mutation is when one nucleotide is substituted for another, so the base pairing is wrong. A frame shift mutation is when there is an extra insertion or deletion of a nucleotide in the DNA sequence which shifts the reading frame and affects the final polypeptide because those codon charts can code for a wrong polypeptides if one of the nucleotides is wrong. And then you also have chromosomal mutations which involve the entire chromosome. Gene duplication which copies an entire gene and versions which are genes can get flip flopped or translocation which whole parts of a chromosome move to a non homologous partner. One of the most common chromosome mutations that are taught is down syndrome, which means that there is a trisomy 21, which is another fancy way of just saying that people who have down syndrome have an extra chromosome, so they have 47 total chromosomes in every single one of themselves, and is because they have an extra chromosome on the 21st pair. Here are the different types of mutations, deletion, duplication, inversion, insertion, and translocation. And that is it for those processes again guys. This is not to use to replace the activities that you have to work through through the modules. This is just a great review that you can watch right before the test, and make sure you're kind of following along and on the same page as me. Thanks for watching and please let me know if you have any questions tonight at the help session.