Question

DNA和RNA有何不同,有甚么用,如何组成?

Answer 1

想像DNA是一部电影的胶卷，RNA是放映的设备，而蛋白质便是一场声色俱佳的电影！您是否能由这个比喻中，体会到RNA在细胞内所扮演的角色？DNA与RNA的差异到底到哪儿？德国的细胞学家福尔根解开了这个谜题

DNA与RNA的结构差异： DNA是双链，RNA是单链

两者在五碳糖的第二个碳原子上，DNA连接的是氢原子，而RNA连接的是羟基

DNA所含的硷基种类为ATCG，而RNA为AU（尿嘧啶）CG

遗传学是研究基因对个体表现的特徵，甚么是叫基因呢(Gene)，基因是每个人从父母得下来的遗传基本单元，它决定每个个体表现的特徵，基因十分细小，无法从肉眼可以看见却藏著生物化学DNA的讯息。排列在人体细胞内的46条染色体

(Chromosome)上-46条染色体可分为23对，包括22对常染色体或者管理身体各种特徵的染色体和另外一对是性染色体，决定个人的性别。当基因运作上出现错误，便会导至三种遗传病例。

Ribonucleic acid (RNA) is a nucleic acid polymer consisting of nucleotide monomers. RNA nucleotides contain ribose rings and uracil unlike deoxyribonucleic acid (DNA), which contains deoxyribose and thymine. It is transcribed from DNA by enzymes called RNA polymerases and further processed by other enzymes. RNA serves as the template for translation of genes into proteins, transferring amino acids to the ribosome to form proteins, and also translating the transcript into proteins.

Nucleic acids were discovered in 1869 by Johann Friedrich Miescher (1844-1895), who called the material 'nuclein' since it was found in the nucleus. It was later discovered that prokaryotic cells, which do not have a nucleus, also contain nucleic acids.

The role of RNA in protein synthesis had been suspected since 1939, based on experiments carried out by Torbjörn Caspersson, Jean Brachet and Jack Schultz.

The sequence of the 77 nucleotides of a yeast RNA was found by Robert W. Holley in 1964, winning Holley the 1968 Nobel Prize for Medicine.

RNA is primarily made up of four different bases: adenine, guanine, cytosine, and uracil. The first three are the same as those found in DNA, but in DNA thymine replaces uracil as the base complementary to adenine. This base is also a pyrimidine and is very similar to thymine. Uracil is energetically less expensive to produce than thymine, which may account for its use in RNA. In DNA, however, uracil is readily produced by chemical degradation of cytosine, so having thymine as the normal base makes detection and repair of such incipient mutations more efficient. Thus, uracil is appropriate for RNA, where quantity is important but lifespan is not, whereas thymine is appropriate for DNA where maintaining sequence with high fidelity is more critical.

There are also numerous modified bases found in RNA that serve many different roles. Pseudouridine (Ψ) and the DNA nucleoside thymidine are found in various places (most notably in the TΨC loop of every tRNA). There are nearly 100 other naturally occurring modified bases, many of which are not fully understood.

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Comparison with DNA

Unlike DNA, RNA is almost always a single-stranded molecule and has a much shorter chain of nucleotides. RNA contains ribose, rather than the deoxyribose found in DNA (there is a hydroxyl group attached to the pentose ring in the 2' position whereas RNA has two hydroxyl groups). These hydroxyl groups make RNA less stable than DNA because it is more prone to hydrolysis. Several types of RNA (tRNA, rRNA) contain a great deal of secondary structure, which help promote stability.

Like DNA, most biologically active RNAs including tRNA, rRNA, snRNAs and other non-coding RNAs (such as the SRP RNAs) are extensively base paired to form double stranded helices. Structural *** ysis of these RNAs have revealed that they are not, "single-stranded" but rather highly structured. Unlike DNA, this structure is not just limited to long double-stranded helices but rather collections of short helices packed together into structures akin to proteins. In this fashion, RNAs can achieve chemical catalysis, like enzymes. For instance, determination of the structure of the ribosome in 2000 revealed that the active site of this enzyme that catalyzes peptide bond formation is composed entirely of RNA.