tailieunhanh - Environmental Life Cycle Costing - Chapter 14

Tham khảo tài liệu 'environmental life cycle costing - chapter 14', khoa học tự nhiên, công nghệ môi trường phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | CHAPTER 14 Mutagenic Pollutants INTRODUCTION A mutation is a process by which the hereditary constitution of a cell is altered ultimately resulting in a genetically altered population of cells or organisms. Although mutations can occur in the RNA of viruses and the DNA of cytoplasmic organelles the mutations of greatest interest occur within genes in the nucleus of the cell. The human body is estimated to contain more than 10 trillion cells and at some stage in its life cycle each contains a full complement of the genes needed by the entire organism. Genes composed of DNA in the nucleus are clustered together in chromosomes. In the chromosomes of all but the most primitive organisms DNA is combined with protein. DNA is the molecular basis of heredity in higher organisms and is made up of a double helix held together by hydrogen bonds between purine and pyrimidine bases . between adenine A and thymine T and between guanine G and cytosine C . Figure shows the chemical structures of the five bases in DNA and RNA. The pairing of bases in DNA is presented in Figure . The highly specific complementarity of these bases enables DNA to act as a template to direct its replication by DNA polymerases as well as the synthesis of RNA transcripts by RNA polymerases. For the information contained in DNA to be biologically expressed the sequence of the nucleotides of a gene is converted to the sequence of amino acids in a protein. It is the amino acid sequence that determines the enzymatic and structural properties of the protein thus formed. Clearly DNA plays a pivotal role in the expression and perpetuation of life. However it is also a critical target for the action of many mutagenic environmental chemicals . lesions in DNA may occur through the action of physical or chemical agents found in the environment. Occurrence of mutations however depends on the nature of the initial lesion and the responses of cells to DNA damage. If the damage 209 2001 by CRC Press