separate peptide chains. An example is hemoglobin, which is actually a combined 4peptide chain structure, held together by connections between the different amino acids.
NUCLEIC ACIDS Nucleic acids are unique molecules that make up the genetic code of a cell and of an organism. There are two types of nucleic acids. These are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). They differ in a couple of ways as will be described. DNA is a polymer (meaning it is a chain of monomers), consisting of three different parts. The first is a nitrogenous base, which is a nitrogen-containing ring, of which there are five types (but only four types in DNA, making this one difference between RNA and DNA). The different nitrogenous bases are adenine, guanine, thymine, and cytosine. Any one of these can be attached to a pentose sugar which, in the case of DNA is deoxyribose. Unlike the typical hexose sugar with six carbon atoms, a pentose sugar has five carbon atoms. Together, the combination of a nitrogenous base and a pentose sugar is called a nucleoside. When these molecules are added to a phosphate group, it is called a nucleotide. These nucleotides are chained together to make DNA. As you probably have learned, DNA has a unique structure, which is called a “double helix”. Basically, it looks like a spiral staircase. Two chains of nucleotides are connected to each other in a spiral, which are connected to each other through the different nitrogenous bases. The adenine (A) molecule always connects to a thymine (T) molecule, while the cytosine (C) molecule always connects to a guanine (G) molecule. Figure 6 shows what these molecules look like as they bind:
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