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gmlyn金虫 (小有名气)
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The information encoded in genetic material (DNA) is responsible for establishing and maintaining the celluar and biochemical functions of an organism. In most organisms, the DNA is an extended double-stranded polymer. The sequence of units (deoxyribonucleotides) of one DNA strand is complementary to the deoxyribonucleotides of the other strand. This complementarity enables new DNA molecules to be synthesized with the same linear array of deoxyribonucleotides in each strand as an original DNA molecule. The process of DNA synthesis is called replication. A specific order of deoxyribonucleotides determines the information content of an individual genetic element (gene). Some genes encode proteins ,and others encode only ribonucleic acid (RNA) molecules. The protein-coding genes (structural genes) are decoded by two successive major cellular processes: RNA synthesis (transcription) and protein synthesis (translation). First, a messenger RNA (mRNA) molecule is synthesized from a structural gene. Second, an individual mRNA molecule interacts with other components, including ribosomes, transfer RNAs (tRNAs) , and enzymes, to produce a protein molecule. A protein consists of a precise sequence of aminoacids which is essential for its activity. Molecular biotechnologists use genes from a variety of organisms to create beneficial products. To understand how this is accomplished, it is helpful to know about the structure of DNA, replication, transcription, and translation. The chemistry of DNA has been studied since 1868. By the 1940s, it was known that DNA was made up of individual units called nucleotides that are linked to each other to form long chains. A nucleotide consists of an organic base (base), a five-carbon sugar (pentose), and a phosphate group. The phosphate group and organic base are attached to the 5’ carbon and 1’carbon atoms of the sugar moiety, respectively. In DNA, there are four kinds of organic bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The sugar of DNA is 2’-deoxyribose because it has a hydroxyl (-OH) group only on the 3’carbon of the sugar moiety. By contrast, ribose, which is another five-carbon sugar, has hydroxyl groups at both the 2’ and 3’ carbons of the pentose ring. The nuleotides of DNA are joined by phosphodiester bonds, with the phosphate group of the 5’ carbon of one nucleotide linked to the 3’ –OH group of the deoxyribose of adjacent nucleotide. A polynuleotide strand has a 3’-OH group at one end (3’ end) and a 5’ phosphate group at the other (5’ end). In 1953, James Waston and Francis Crick, using X-ray diffraction analysis of crystallized DNA, discovered that native DNA consists of two long chains (strands) that form a double-stranded helix. The coiled polynucleotide chains of DNA are held together by hydrogen bonds between the bases of the opposite strands. The bases occur as specific sets of complementary pairs. A pairs only with T, and G pairs only with C. The A· T base pairs are held together by two hydrogen bonds, and the G· C base pairs are held together by three. The number of complementary base pairs (bp) is often used to characterize the length of a double-stranded DNA molecule. For DNA molecules with thousands or millions of base pairs, the designations are kilobase pairs (kb) or megabase pairs (Mb) , respectively. For example, the DNA of human chromosome 1 is one double-stranded helix that has about 263 Mb. The A· T and G· C base pairs lie within the interior of the molecule, and the 5’ - 3’ – linked phosphorus and deoxyribose components form the backbone of each strand. The two strands of a duplex DNA molecule run in opposite directions to each other (antiparallel chains). One chain is oriented in a 3’- to 5’- direction and the other is oriented in a 5’- to 3’- direction. Because of the base pairing requirements, when one strand of DNA has, for example the base sequence 5’-TAGGCAT-3’ the comlementary strand must be 3’-ATCCGTA-5’ . By convention, when DNA is drawn on a horizontal plane, the 5’ end of the upper strand is on the left. In theory, genetic material has two major functions. It must be reproduced (replicated) with a high degree of accuracy and encode information for the production of proteins. The Waston-Crick model of DNA fully meets these important requirements. First, with base complementary, each preexisting DNA strand acts as a template for the production of a new complementary strand. Consequently, after one round of replication, two daughter molecules are produced, with each having the same sequence of nucleotide pairs as the original DNA molecule. Second, the sequence of nucleotides of a gene provides the code for the production of a protein. @hc-material @gyesang 发自小木虫Android客户端 |
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