This complementary base-pairing enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix. 120–121) is paired with a single-ring base (a pyrimidine) A always pairs with T, and G with C ( Figure 4-4). In each case, a bulkier two-ring base (a purine see Panel 2-6, pp. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar-phosphate backbones are on the outside (see Figure 4-3). The three-dimensional structure of DNA- the double helix-arises from the chemical and structural features of its two polynucleotide chains. This polarity in a DNA chain is indicated by referring to one end as the 3′ end and the other as the 5′ end. Moreover, the two ends of the chain will be easily distinguishable, as one has a hole (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. If we think of each sugar as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′ hydroxyl) on the other (see Figure 4-3), each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. A DNA molecule is composed of two (more.) DNA is made of four types of nucleotides, which are linked covalently into a polynucleotide chain (a DNA strand) with a sugar-phosphate backbone from which the bases (A, C, G, and T) extend. These same symbols (A, C, G, and T) are also commonly used to denote the four different nucleotides-that is, the bases with their attached sugar and phosphate groups.ĭNA and its building blocks. Because only the base differs in each of the four types of subunits, each polynucleotide chain in DNA is analogous to a necklace (the backbone) strung with four types of beads (the four bases A, C, G, and T). The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate (see Figure 4-3). In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name deoxyribonucleic acid), and the base may be either adenine (A), cytosine (C), guanine ( G), or thymine (T). 120-121), nucleotides are composed of a five-carbon sugar to which are attached one or more phosphate groups and a nitrogen-containing base. Hydrogen bonds between the base portions of the nucleotides hold the two chains together ( Figure 4-3). Each of these chains is known as a DNA chain, or a DNA strand. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.A DNA Molecule Consists of Two Complementary Chains of NucleotidesĪ DNA molecule consists of two long polynucleotide chains composed of four types of nucleotide subunits. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.Īn important property of DNA is that it can replicate, or make copies of itself. Nucleotides are arranged in two long strands that form a spiral called a double helix. Together, a base, sugar, and phosphate are called a nucleotide. Each base is also attached to a sugar molecule and a phosphate molecule. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.ĭNA bases pair up with each other, A with T and C with G, to form units called base pairs. Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Mitochondria are structures within cells that convert the energy from food into a form that cells can use. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). Nearly every cell in a person’s body has the same DNA. DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms.
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