This is the first video over chapter 4. We need to understand a little bit about DNA and genetics in order to understand genetic disorders. These particular genetic disorders you will be covering in A and P 2. In this video, we're going to be looking at the structure of DNA and RNA, and go over some of the terminology. The second video will look at protein synthesis, and the third video is going to be looking at mitosis or cell division. Deoxyribonucleic acids are long thread-like molecules. We have 46 DNA molecules in the nucleus of most of our cells. It will be the exact same 46 DNA molecules. DNA and RNA are polymers of nucleotides. A nucleotide is a sugar, a phosphate group, and a nitrogenous base. If we were to look at a basic nucleotide, our monomer or our building block here, we have a sugar. Deoxyribose is what we use in DNA. We have a phosphate group, and then we have a nitrogenous base. When we look at our nitrogenous bases, we have two categories, the purines and the pyrimidines. The purines have a double ring. So they are a wide molecule. Our purines is adenine and guanine . We symbolize it with an A and the G. The pyrimidines only have a single ring, so they are a smaller molecule. This is cytosine, thymine, and uracil. When we look at DNA, we have adenine, thymine, cytosine, and guanine. RNA replaces thymine with uracil. When we look at DNA, it looks like a spiral staircase. It's a double helix. On the side of our staircase is sugar and phosphate; we call it the sugar phosphate backbone. In the middle of this is going to be our nitrogenous bases, and they are joined together by hydrogen bonds. You will always have a double ring hydrogen bonded to a single ring. So a purine is always bound to a pyrimidine on the other side. Between adenine and thymine are two hydrogen bonds. Between cytosine and guanine are three hydrogen bonds. So obviously since we have three hydrogen bonds, this between cytosine and guanine, is much stronger, much more stable, than between adenine and thymine. This law of complementary base pairing is where we see one strand determines the base sequence of the other. Let's look at an example. What I have here is half of my DNA strand. This line here is my sugar phosphate backbone, and then I have my sequence of nitrogenous bases coming down. The other side of this is determined by the first side. Wherever I have a T, I must have an A on the other side. A you must have a T; C you must have a G. This is this law of complementary base pairing. Some definitions here, the word chromatin, this is our DNA, and it is complex with histone proteins. So our histone proteins are right here. You can see that that DNA is coiled around it. The reason why we do this, is that our DNA is very, very thin. If we don't wrap it around some proteins, it becomes very fragile, and it's easy to break apart. So wrapping it around these histones, not only organizes it, it prevents it from being broken too easy. A gene is a part of this DNA that codes for a specific protein. When we look at the word genome, those are all the genes of a person. Humans have about 20,000 genes. That's actually only about two percent of the total DNA. The rest of the DNA is what we call non-coding DNA. Now when we look at these chromatin, this is what it would look like. Thin threads and basically, if we took all 46 of our chromosomes and we lined them up, would be about six feet long. That is a lot to cram in a tiny little nucleus in each one of our cells. When we're dividing, we're going to double this. We have to make a copy of our DNA. So what we do is, we coil this up. This is only in a dividing cell, and what we'll have is this. This is the only time we can actually take a good photograph of our chromosome. So let's look at this a little closer. Remember, this is only seen in cells about to divide. And when we look at the whole thing, this is called a chromosome. And in reality it is a DNA strand with an exact copy of it, and they're held together by this little piece of protein. When that cell divides, one of these DNA strands goes in one cell, and the exact copy goes into the other cell. And remember, you are going to have 46 of these. Now to keep this straight, that we have a DNA and exact copy, we have a particular name we call those two, sister chromatids. Going on with RNA, so ribonucleic acids. RNA is what we use to help make our proteins. DNA is only found in our nucleus. It stays in the nucleus, but where we make our proteins is in the cytoplasm. You remember that it's at a ribosome. Our go-between, between getting the code from the DNA, and going to the ribosomes in the cytoplasm, is going to be the RNA. We got three types of RNA for protein synthesis. We have messenger RNA. We put a little m in front of that RNA. Ribosomal RNA is an RNA that is part of the ribosome. We put an r in front of it, and transfer RNA is what brings in our amino acids. Remember we're making proteins; and the building blocks of proteins are amino acids. Now let's look at how RNA is different from DNA. RNA is single-stranded; it only has one nucleotide chain. It is not a double helix like DNA. So we look at this example, this is a transfer RNA. It is one strand, but you can see it can be folded up, and held in this particular shape by bonds, internal bonds and hydrogen bonding. What else is different? We use a different sugar. We use ribose, instead of deoxyribose. We use uracil instead of thymine, and the other big difference is that RNA functions mostly in the cytoplasm; whereas DNA stays in the nucleus. Here's a nice chart that looks at the difference between DNA and RNA. It's a nice review, especially before your test. This is the end of the first video. The next video is going to be looking at protein synthesis.