Nhớ lại từ chương 3 rằng các hướng dẫn cho sinh sản và quá trình trao đổi chất trong các sinh vật được chứa trong các axit nucleic, đó là các phân tử lớn biopolymeric bao gồm các đơn vị nucleotide gồm một đường, một cơ sở đạm, và một nhóm phosphate. Có hai loại axit nucleic. Đầu tiên trong số này là deoxyribonucleic acid (DNA), trong đó đường 2deoxyribose và các căn cứ có thể là thymine, adenine, guanine, và cytosine. Loại thứ hai của axit nucleic là axit ribonucleic (RNA), trong đó đường ribose và các căn cứ. | Chapter 8 Genetic Aspects of Toxicology introduction Recall from Chapter 3 that the directions for reproduction and metabolic processes in organisms are contained in nucleic acids which are huge biopolymeric molecules consisting of nucleotide units each composed of a sugar a nitrogenous base and a phosphate group. There are two kinds of nucleic acids. The first of these is deoxyribonucleic acid DNA in which the sugar is 2-deoxyribose and the bases may be thymine adenine guanine and cytosine. The second kind of nucleic acid is ribonucleic acid RNA in which the sugar is ribose and the bases may be adenine guanine cytosine and uracil. The monomeric units of nucleic acids are summarized in Figure and an example nucleotide is shown. A nucleic acid molecule which typically has a molecular mass of billions consists of many nucleotides joined together. Alternate sugar and phosphate groups compose the chain skeleton and the nitrogenous base in each nucleotide gives it its unique identity. Since there are four possible bases for each kind of nucleic acid the nucleic acid chain functions like a four-letter alphabet that carries a message for cell metabolism and reproduction. As discussed in Chapter 3 the structure of DNA is that of a double helix in which there are two complementary strands of DNA counterwound around each other. In this structure guanine G is opposite cytosine C and adenine A is opposite thymine T in the opposing strand. The structures of these nitrogenous bases are such that hydrogen bonds form between them on the two strands bonding the strands together. During cell division the strands of DNA unwind and each generates a complementary copy of itself so that each new cell has an exact duplicate of the DNA in the parent cell. Chromosomes The nuclei of eukaryotic cells contain multiply coiled DNA bound with proteins in bodies called chromosomes. The number of chromosomes varies with the organism. Humans have 46 chromosomes in their body cells .
