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gmlyn金虫 (小有名气)
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[求助]
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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. 发自小木虫Android客户端 |
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2楼2017-04-17 18:28:43
ak47aug
木虫 (小有名气)
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gmlyn(fjtony163代发): 金币+50, 代发 2017-05-18 03:10:59
fjtony163: 翻译EPI+1, 代发 2017-05-18 03:11:09
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遗传物质（DNA）编码的信息负责建立和维持生物体的细胞膜和生物化学功能。在大多数生物体中，DNA是延伸的双链聚合物。一条DNA链的单元（脱氧核糖核苷酸）的序列与另一条链的脱氧核糖核苷酸互补。这种互补性使得新DNA分子可以用每条链中相同的脱氧核糖核苷酸线性阵列作为原始DNA分子合成。 DNA合成的过程称为复制。脱氧核糖核苷酸的特定顺序决定了个体遗传元件（基因）的信息含量。一些基因编码蛋白质，其他基因只能编码核糖核酸（RNA）分子。蛋白质编码基因（结构基因）通过两个连续的主要细胞过程进行解码：RNA合成（转录）和蛋白质合成（翻译）。首先，从结构基因合成信使RNA（mRNA）分子。第二，单个mRNA分子与其他成分（包括核糖体，转移RNA（tRNA）和酶）相互作用以产生蛋白质分子。蛋白质由准确的氨基酸序列组成，对其活性至关重要。 分子生物技术学家使用来自各种生物体的基因来产生有益的产品。要了解这是如何实现的，了解DNA，复制，转录和翻译的结构是有帮助的。 人们自1868年以来就开始研究DNA的化学问题。到20世纪40年代，已知DNA由称为核苷酸的单个单元组成，它们彼此连接形成长链。核苷酸由有机碱（碱），五碳糖（戊糖）和磷酸酯组成。磷酸基和有机碱分别连接到糖部分的5'碳和1碳原子上。在DNA中，有四种有机碱：腺嘌呤（A），鸟嘌呤（G），胞嘧啶（C）和胸腺嘧啶（T）。 DNA的糖是2'-脱氧核糖，因为它仅在糖部分的3'碳上具有羟基（-OH）基团。相比之下，作为另一种五碳糖的核糖在戊糖环的2'和3'碳原子上具有羟基。 DNA的核苷酸通过磷酸二酯键连接，一个核苷酸的5'碳的磷酸基与相邻核苷酸的脱氧核糖的3'-OH基团连接。多核苷酸链在一端（3'末端）具有3'-OH基，另一端具有5'磷酸基（5'末端）。 1953年，詹姆斯·瓦斯顿（James Waston）和弗朗西斯·克里克（Francis Crick）用结晶DNA的X-射线衍射分析发现天然DNA由形成双链螺旋的两条长链（链）组成。 DNA的螺旋多核苷酸链通过相对链的碱基之间的氢键保持在一起。碱基作为特定的互补对发生。 A对只有T，G对只有C。A·T碱基对通过两个氢键保持在一起，G·C碱基对保持在三个。互补碱基对（bp）的数目通常用于表征双链DNA分子的长度。对于具有数千或数百万碱基对的DNA分子，分别为千碱基对（kb）或兆基位对（Mb）。例如，人染色体1的DNA是一个具有约263Mb的双链螺旋。 A·Tαβ和G·C11碱基对位于分子的内部，并且5' - 3'连接的磷和脱氧核糖组分形成每条链的主链。 双链DNA分子的两条线彼此相反（反向平行链）相反方向。一条链在3'至5'方向取向，另一条以5'至3'方向取向。由于碱基配对要求，当DNA的一条链具有例如碱基序列5'-TAGGCAT-3'时，链段必须为3'-ATCCGTA-5'。按照惯例，当在水平面上绘制DNA时，上链的5'端位于左侧。 在理论上，遗传材料有两个主要功能。必须以高精确度复制（复制）并编码蛋白质生产信息。 DNA的Waston-Crick模型完全符合这些重要要求。首先，与碱基互补，每个先前存在的DNA链作为生产新的互补链的模板。因此，在一轮复制后，产生两个子分子，每个分子具有与原始DNA分子相同的核苷酸对序列。第二，基因的核苷酸序列提供了生产蛋白质的代码。 |
3楼2017-04-19 18:21:35
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