DNA and RNA - Overview of DNA and RNA 

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SCIENCE ANIMATION TRANSCRIPT: Today, we're going to be talking about the only two types of nucleic acids that exist, DNA and RNA. DNA stands for deoxyribonucleic acid, and RNA stands for ribonucleic acid. As nucleic acids, both DNA and RNA are long organic macromolecules, which means they are very large molecules that contain the element carbon. Both DNA and RNA are composed of nucleic acid monomers called nucleotides. Nucleotide monomers always contain a phosphate group, a five-carbon sugar, sometimes referred to as a pentose, and a nitrogenous base. But DNA nucleotides have the five-carbon sugar deoxyribose while RNA has a different five-carbon sugar called ribose. The pentose sugar in DNA has one less oxygen than ribose in RNA, which leads to its name, deoxyribose. Let's focus on the nitrogenous bases in DNA for a moment. Each DNA nucleotide contains only one of four possible nitrogenous bases, adenine, guanine, thymine, and cytosine. These four bases are usually abbreviated A, G, T, and C. When nucleotide bases connect through hydrogen bonds, it's important to know that the bases always pair up with the same partners. Adenine only pairs with thymine, and guanine only pairs with cytosine. This is called the base pair rule. Scientists classify nitrogenous bases according to the shape of their molecules. Adenine and guanine both have a double-ring structure containing a six-membered and a five-membered nitrogen-containing ring fused together. These types of ring structures are chemically classified as purines. But thymine and cytosine, both, only have a single six-membered nitrogen-containing ring shape, so they are chemically classified as pyrimidines. Here's a little hint to help you remember this. Pyrimidine has a Y in its name, and so do the bases classified as pyrimidines, thymine and cytosine. Like DNA, an RNA nucleotide may have the nitrogenous bases, adenine, guanine, or cytosine. But unlike DNA, which has the nitrogenous base thymine, RNA has the nitrogenous base uracil instead. This means that any nucleic acid that contains thymine must be a strand of DNA and that any nucleic acid that contains uracil must be a piece of RNA. Like thymine, uracil has a single nitrogen-containing ring. So it would also be classified as a pyrimidine. So what do the nitrogenous bases have to do with building a DNA molecule? Well, each strand of DNA begins to assemble when the phosphate group in one nucleotide bonds with the five-carbon sugar in another nucleotide. The two strands that make up a DNA molecule are linked through hydrogen bonds between their nitrogenous bases. When the spiral DNA molecule is untwisted, these base pairs look like rungs on a ladder. The spiral DNA molecule is described as having a double-helix shape. James Watson and Francis Crick first described DNA's double-stranded helical structure in 1953 after looking at x-ray images of a DNA molecule taken by Rosalind Franklin. Now, let's look at the structure of RNA and see how it compares to the structure of DNA. While DNA is a double-stranded molecule, RNA is always single stranded. In eukaryotic cells, DNA is found only in the nucleus. But RNA can be found in the cytoplasm as well as in the nucleus. DNA controls heredity by containing the code or set of instructions for building the functional and structural proteins that make up your body. And RNA uses these varied instructions to build all the proteins a living organism needs. There are three types of RNA, messenger RNA, referred to as mRNA, ribosomal RNA, referred to as rRNA, and transfer RNA, referred to as tRNA. Messenger RNA starts off in a cell's nucleus. Here, it transcribes or copies the instructions for assembling a protein from a portion of the cell's DNA. Next, the messenger RNA takes these instructions or protein recipe to a ribosome in the cytoplasm. The ribosome, which is made out of ribosomal RNA and proteins, is the site where this recipe is read and translated into a protein. Finally, transfer RNA in the cytoplasm brings specific amino acids to the ribosome so that the particular protein encoded by the recipe can be made. We'll cover more detail about the way DNA and RNA make proteins in another video. To sum up, DNA and RNA are the only two nucleic acids that exist. Both DNA and RNA are built from nucleic acid monomers called nucleotides. Nucleotides always contain a phosphate group, a five-carbon sugar, and a nitrogenous base. Deoxyribose is the sugar in DNA, and ribose is the sugar in RNA. The nitrogenous bases in DNA are adenine, thymine, cytosine, and guanine. The nitrogenous bases in RNA are adenine, uracil, cytosine, and guanine. In DNA, adenine always pairs together with thymine, and cytosine always pairs together with guanine...
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