DNA Structure: A Brief Review
المؤلف:
Cohn, R. D., Scherer, S. W., & Hamosh, A.
المصدر:
Thompson & Thompson Genetics and Genomics in Medicine
الجزء والصفحة:
9th E, P7-8
2025-10-26
50
Before the organization of the human genome and its chromosomes are considered in detail, it is necessary to review the nature of the DNA that makes up the genome. DNA is a polymeric nucleic acid macromolecule com posed of three types of units: a five-carbon sugar, deoxy ribose; a nitrogen-containing base; and a phosphate group (Fig. 1). The bases are of two types, purines and pyrimidines. In DNA, there are two purine bases, adenine (A) and guanine (G), and two pyrimidine bases, thymine (T) and cytosine (C). Nucleotides, each composed of a base, a phosphate, and a sugar moiety, polymerize into long polynucleotide chains held together by 5′-3′ phosphodiester bonds formed between adjacent deoxyribose units (Fig. 2A). In the human genome, these polynucleotide chains exist in the form of a double helix (see Fig. 2B) that can be hundreds of millions of nucleotides long in the case of the largest human chromosomes.
Fig1. The four bases of DNA and the general structure of a nucleotide in DNA. Each of the four bases bonds with deoxyribose (through the nitrogen shown in magenta) forming a nucleoside which bonds with a phosphate group to form the corresponding nucleotides.
Fig2. The structure of DNA. (A) A portion of a DNA polynucleotide chain, showing the 5′-3′ phosphodiester bonds that link adjacent nucleotides. (B) The double-helix model of DNA, as proposed by Watson and Crick. The horizontal “rungs” represent the paired bases. The helix is said to be right-handed because the strand going from lower left to upper right crosses over the opposite strand. The detailed portion of the figure illustrates the two complementary strands of DNA, showing the AT and GC base pairs. Note that the orientation of the two strands is antiparallel. (Based on Watson JD, Crick FHC: Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid, Nature 171:737–738, 1953.)
The anatomic structure of DNA carries the chemical information that allows the exact transmission of genetic information from one cell to its daughter cells and from one generation to the next. At the same time, the primary structure of DNA specifies the amino acid sequences of the polypeptide chains of proteins, as described in the next chapter. DNA has elegant features that give it these proper ties. The native state of DNA, as elucidated by James Watson and Francis Crick in 1953, is a double helix (see Fig. 2B). The helical structure resembles a right-handed spiral stair case in which its two polynucleotide chains run in opposite directions, held together by hydrogen bonds between pairs of bases: T of one chain paired with A of the other, and G with C. The specific nature of the genetic information encoded in the human genome lies in the sequence of Cs, As, Gs, and Ts on the two strands of the double helix along each of the chromosomes, both in the nucleus and in mitochondria (see Fig. 2.1). Because of the complementary nature of the two strands of DNA, knowledge of the sequence of nucleotide bases on one strand automatically allows one to determine the sequence of bases on the other strand. The double-stranded structure of DNA molecules allows them to replicate precisely by separation of the two strands, followed by synthesis of two new complementary strands, in accordance with the sequence of the original template strands (Fig. 3). Similarly, when necessary, the base complementarity allows efficient and correct repair of damaged DNA molecules.
Fig3. Replication of a DNA double helix, resulting in two identical daughter molecules, each composed of one parental strand and one newly synthesized strand.
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