Structure of Nucleic Acids Deoxyribonucleic acid (DNA) encodes all the information needed to create life's diversity. Principles of Biology

Nucleic acids are polymers composed of nucleotide monomers. Nucleic acids are one of the four classes of large biological molecules that are essential to cellular structure and function. Ribnonucleic acid (RNA) is a macromolecule that has several roles, the most basic of which is to deliver information from DNA to sites of protein synthesis. For both DNA and RNA, the repeating subcomponents are the nucleotides that are repeated over and over to make up polynucleotides. Principles of Biology

Nucleotide structure includes three main components. Nucleic Acids Nucleotide structure includes three main components. Both DNA and RNA are made up of nucleotide subunits, each of which contains three parts: a phosphate group, a sugar, and a nitrogenous base (often referred to simply as the "base"). The nitrogenous base identifies the nucleotide as guanine, thymine, adenine, cytosine, or uracil, depending on its structure. When a sugar is linked to a nitrogenous base but the phosphate group is absent, this is called a nucleoside. Five kinds of nucleotide monomers form polynucleotides. In DNA, the four kinds of nucleotides are guanine, thymine, adenine, and cytosine. RNA contains uracil in place of thymine. RNA includes ribose (C5H10O5) in its structure, a monosaccharide pentose sugar with five carbon atoms in a ring shape. DNA includes the structural component deoxyribose, which differs from ribose in that it has one less oxygen atom. Principles of Biology

Nucleotide bases are classified into purines and pyrimidines. Nucleic Acids Nucleotide bases are classified into purines and pyrimidines. A pyrimidine is a four-carbon ring connected by nitrogen atoms at the 1 and 3 positions. The pyrimidines cytosine (C) and thymine (T) are found in DNA, and cytosine and uracil (U) are found in RNA. A purine molecule is a pyrimidine ring connected to an imidazole ring; the overall structure is a double ring. Adenine (A) and guanine (G) are the purines in both DNA and RNA. Principles of Biology

Figure 1 Two types of nitrogenous bases are found in DNA and RNA. Nucleic Acids Figure 1 Two types of nitrogenous bases are found in DNA and RNA. a) Pyrimidines consist of a single ring of four carbon atoms and two nitrogen atoms. The three molecules differ in the presence of amine (–NH2) and methyl (–CH3) functional groups. b) Purines consist of a double ring of a pyrimidine and an imidazole. They differ in the position of their amine groups and the presence of a carbonyl functional group. Principles of Biology

Nucleic Acids Polynucleotides form when phosphodiester bonds connect nucleotide monomers along a sugar-phosphate backbone. Both DNA and RNA contain long molecular chains formed by links between the phosphate and sugar units of repeating nucleotide monomers. This phosphodiester bond connects the 5 position carbon on one sugar with the 3 position carbon on the next sugar. Phosphodiesterase is an enzyme that breaks apart phosphate groups to catalyze the formation of bonds between neighboring nucleotides. This reaction gives off energy; thus, it is exergonic. Principles of Biology

Nucleic Acids Figure 2 Polynucleotides form when nucleotides are covalently linked by phosphodiester bonds forming a pentose sugar-phosphate backbone. a) The three components of a nucleotide include the phosphate and sugar groups and a nitrogenous base. b) Phosphate units link to sugar units, forming phosphodiester bonds that make up the backbone of a polynucleotide molecule. Principles of Biology

Nucleic Acids Figure 3 British scientist Rosalind Franklin provided data that were key to discovering the structure of DNA. Rosalind Franklin provided exceptional-quality data from X-ray diffraction patterns of purified DNA. Her data allowed Crick and Watson to derive the molecule’s double helix structure. Principles of Biology

DNA and RNA form complex secondary structures. Nucleic Acids DNA and RNA form complex secondary structures. The secondary structure of DNA is a double helix: two intertwined strands of DNA. The double helix structure of DNA is made up of a phosphate-sugar backbone with paired nucleotide bases on the interior of the molecule. The only pairs of nucleotide bases in DNA are between purines and pyrimidines. Principles of Biology

Nucleic Acids Figure 4 Hydrogen bonds form between purine and pyrimidine bases in DNA. Guanine can pair only with cytosine, using three hydrogen bonds, and adenine can pair only with thymine, using two hydrogen bonds. These are the base pairing rules of the DNA double helix. (Note that thymine is replaced by uracil in RNA). Principles of Biology

Figure 5 A molecular representation of DNA. Nucleic Acids Figure 5 A molecular representation of DNA. The DNA double helix features two polynucleotide strands with sugar-phosphate backbones linked by purine-pyrimidine pairs. Principles of Biology

Other structural characteristics of DNA. Nucleic Acids Other structural characteristics of DNA. The Z-DNA conformation resembles B-DNA except the helix is left-handed in directionality DNA in the helix can be twisted in one of several forms, or conformations. The most common shape of the DNA helix in living cells is called the B-DNA form. The A-DNA conformation of DNA is similar to B-DNA but is shorter and more compact. Both the A-DNA and B-DNA conformations are right-handed double helices, which twist to the right-hand side. Principles of Biology

Figure 6 Complementary strands of DNA are antiparallel. Nucleic Acids Figure 6 Complementary strands of DNA are antiparallel. The directionality of the sugar-phosphate backbone results in one strand running opposite to the other. Principles of Biology

Nucleic Acids Figure 7 A stem-loop hairpin occurs in RNA when a sequence can fold back on itself. When a strand of RNA folds over itself, the two folded strands may become joined due to hydrogen bonding between complementary base pairs. An unpaired loop may arise in an area where base pairing between the folded strands cannot occur. Principles of Biology

Nucleic Acids Figure 8 Two identical DNA molecules can be produced from a single original molecule. Three steps result in the formation of two new DNA molecules from a single original molecule. First, the parent strands separate. Second, nucleotides attach to the separated parent strands according to Chargaff's base-pairing rule. The complementary base pairing ensures that the two new daughter molecules are identical to the original parent molecule. When the sugar-phosphate bonds form, the complementary daughter strands polymerize. Notice that the directionality of the complementary strands is opposite to those of the parent template strands. Principles of Biology

The ordering of nucleotide bases encodes hereditary information. Nucleic Acids The ordering of nucleotide bases encodes hereditary information. A gene is the smallest unit of heredity in organisms. A sequence of nucleotide bases in DNA make up a gene. With its ordered sequence of bases, a gene contains the instructions to make proteins, which confer a property or feature on a cell. It is through the incredible range of proteins that DNA, a relatively simple molecule, directs the cell's complex structures and functions. Principles of Biology

RNA has many roles. Nucleic Acids Principles of Biology RNA has multiple roles that vary depending on RNA type: mRNA, tRNA, and rRNA. Messenger RNA (mRNA) is produced through transcription, the process by which one strand of the DNA double helix is exposed and bound to free bases. Ribosomal RNA (rRNA), along with more than 50 proteins, is a component of ribosomes, which "read" the mRNA sequence and synthesize the appropriate protein by linking amino acids using another specialized type of RNA, the transfer RNA (tRNA). Principles of Biology