guanine-containing = guanosine (attached to ribose) / deoxyguanosine (attached to deoxyribose).cytosine-containing = cytidine (attached to ribose - Figure 2.129) / deoxycytidine (attached to deoxyribose).thymine-containing = ribothymidine (attached to ribose) / thymidine (attached to deoxyribose).uracil-containing = uridine (attached to ribose) / deoxyuridine (attached to deoxyribose).When a base is attached to a sugar, the product, a nucleoside, gains a new name. Both DNA and RNA contain nucleotides with adenine, guanine, and cytosine, but with very minor exceptions, RNA contains uracil nucleotides, whereas DNA contains thymine nucleotides. Nucleotides containing ribose are called ribonucleotides and are found in RNA. Nucleotides containing deoxyribose are called deoxyribonucleotides and are the forms found in DNA. Figure 2.128 - Nucleotides, nucleosides, and bases (This is to distinguish the carbons on the sugars from those on the bases, which have their carbons simply labeled as 1, 2, 3, etc.) Deoxyribose differs from ribose at the 2’ position, with ribose having an OH group, where deoxyribose has H. By convention, the carbons on these sugars are labeled 1’ to 5’. There are two sugars found in nucleotides - deoxyribose and ribose (Figure 2.128). The nitrogenous bases found in nucleic acids include adenine and guanine (called purines) and cytosine, uracil, or thymine (called pyrimidines). Molecules that contain only a sugar and a nitrogenous base (no phosphate) are called nucleosides. These are a nitrogenous base, a pentose sugar, and at least one phosphate. Nucleotides contain three primary structural components. In order to discuss this important group of molecules, it is necessary to define some terms. The term nucleotide refers to the building blocks of both DNA (deoxyribonucleoside triphosphates, dNTPs) and RNA (ribonucleoside triphosphates, NTPs). Read in groups of three, the sequence of the bases directly specifies the sequence of the amino acids in the encoded protein.
Crucially, the sequence of the bases in DNA carry the information for making proteins. In addition to the hydrogen bonds between bases of each strand, the double helix is held together by hydrophobic interactions of the stacked, non-polar bases. The complementary structure immediately suggested to Watson and Crick how DNA might be (and in fact, is) replicated and it further explains how information is DNA is transmitted to RNA for the synthesis of proteins. Figure 2.127 - A DNA duplex with base pairs, a closeup of base pairing, and a closeup of a nucleotide Wikipedia Two hydrogen bonds are formed between adenine and thymine, but three hydrogen bonds hold together guanine and cytosine (Figure 2.127). The double helix, made up of a pair of DNA strands, has at its core, bases joined by hydrogen bonds to form base pairs - adenine always paired with thymine, and guanine invariably paired with cytosine. Today, every high school biology student is familiar with the double helical structure of DNA and knows that G pairs with C and A with T. Arguably, that one-page paper has had more scientific impact per word than any other research article ever published. Their famous paper, in the Apissue of Nature, opened the modern era of molecular biology. X-ray diffraction work of Rosalind Franklin and the observations of Erwin Chargaff were combined by James Watson and Francis Crick to form a model of DNA that we are familiar with today. Discovered in 1869 by Friedrich Miescher, DNA was identified as the genetic material in experiments in the 1940s led by Oswald Avery, Colin MacLeod, and Maclyn McCarty. The race to elucidate the structure of DNA was one of the greatest stories of 20th century science. We will begin with DNA, which is the hereditary information in every cell, that is copied and passed on from generation to generation. In this section, we will examine the structures of DNA and RNA, and how these structures are related to the functions these molecules perform. The nucleic acids, DNA and RNA, may be thought of as the information molecules of the cell.
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