什麼是去氧核糖核酸

deoxyribonucleic acid的縮寫。

核酸的兩種形式之一（另一種是核糖核酸）；分子結構複雜的有機化合物，見於所有的原核細胞、真核細胞及多種病毒中。DNA編碼帶有遺傳訊息，決定着遺傳性狀的傳遞。1953年華生和克里克確定其分子結構為雙條長鏈相互盤旋形成雙螺旋的聚合物，每條鏈由一長串的單體核酸組成。而核酸是由一種帶有磷酸的去氧核糖（戊糖）分子和一種含氮礆基構成。含氮礆基有四種︰腺嘌呤（A）、鳥嘌呤（G）、胞嘧啶（C）、胸腺嘧啶（T）。兩條核酸鏈則靠礆基間的氫鍵相連；這種鍵合的次序是特定的，也就是腺嘌呤只與胸腺嘧啶結合，胞嘧啶只與鳥嘌呤結合。DNA複製時，雙鏈分開，每條單鏈作為模板，按礆基間氫鍵配對的規則，將一個新礆基結合到原有的礆基上，構成一條新鏈。最後，產生了兩個新的雙鏈DNA分子，每個分子包含一條原有的DNA鏈和一條新鏈，這種形式的複製是遺傳性狀得以穩定繼承的關鍵。所謂「基因」，指的就是DNA的一個片段，它編排了細胞內某種特定蛋白質的合成密碼。DNA是以染色體（稠密的蛋白質-DNA複合體）形式存在於細胞中。真核細胞的染色體位於細胞核內，但在粒線體和葉綠體中也可發現DNA。有些原核細胞（如細菌）和一些真核細胞中有一種稱為「質粒」的染色體外DNA結構，這是一種能獨立自我複製的遺傳物質。質粒現已廣泛用於DNA重組技術中，以研究基因的表現方式。亦請參閲Franklin, Rosalind (Elsie)、genetic engineering、mutation、Wilkins, Maurice (Hugh Frederick)。

DNA

One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. Its structure, with two strands wound around each other in a double helix to resemble a twisted ladder, was first described (1953) by Francis Crick and James D. Watson. Each strand is a long chain (polymer) of repeating nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). The two strands contain complementary information: A forms hydrogen bonds (see hydrogen bonding) only with T; C only with G. When DNA is copied in the cell, the strands separate and each serves as a template for assembling a new complementary strand; this is the key to stable heredity. DNA in cells is organized into dense protein-DNA complexes (see nucleoprotein) called chromosomes. In eukaryotes these are in the nucleus, and DNA also occurs in mitochondria and chloroplasts (if any). Prokaryotes have a single circular chromosome in the cytoplasm. Some prokaryotes and a few eukaryotes have DNA outside the chromosomes in plasmids. See also Rosalind Franklin, genetic engineering, mutation, Maurice Wilkins.