DNA

Deoxyribonucleic acid (DNA) in a living cell contains the master blueprint for the production of proteins and for the replication of itself. It is the repository for the hereditary information. DNA accomplishes protein production by first providing a pattern for the production of mRNA, a process called transcription. The RNA then contains the information from the DNA to manufacture a protein, a process called translation. Some proteins are structural, but some are control proteins called enzymes. These enzymes are employed in the production of proteins, even copies of themselves, so enzymes are used to make other enzymes. Each protein, including enzymes, is made according to a pattern of nucleotides along a segment of the DNA called a "gene". A single living cell contains thousands of enzymes.

Another way to organized these ideas is in terms of the "central dogma" of molecular biology.

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Though a useful organizing structure, the "central dogma" has numerous exceptions. For example, retroviruses use "reverse transcription" to construct DNA from RNA. In general, not every gene gets expressed all the way to the construction of proteins. Some RNAs have other tasks to do, such as the ribosomal RNA and other specifically tasked RNAs with specific tasks in the cell.

DNA Structure

The basic structure of DNA was discovered by James Watson, Francis Crick and Maurice Wilkins, in 1953 using x-ray diffraction data produced by Rosalind Franklin. The discovery is chronicled by Watson's "The Double Helix".

This is a depiction of the famous double helix patterned after Karp's presentation. That of Audesirk & Audesirk is similar.The color coding emphasizes the role of the four distinct nitrogenous bases adenine, thymine, guanine and cytosine. 这是一幅根据Karp的展示所绘制的著名双螺旋结构图。Audesirk与Audesirk的版本也类似。颜色编码强调了四种不同的氮碱基——腺嘌呤、胸腺嘧啶、鸟嘌呤和胞嘧啶的作用。

The P and S on the backbone of the helix represent the phosphate and the pentose sugar which together with the base make up the nucleotides. 螺旋的骨干上的P和S代表磷酸和核糖，它们与碱基一起构成核苷酸。

At right above is a depiction of a section of the sugar-phosphate backbone of DNA which conforms into one strand of the double helix. The bases may be attached in any order, and this gives the vast number of possibilities of arrangements which are possible in the genetic code. In the double helix, bases are only attached by hydrogen bonds to their complementary base, that is A-T and G-C. This arrangement makes possible the separation of the strands and the replication of the DNA double helix.

A remarkable feature of DNA structure is that the message written in the alphabet of the four bases is written in a true code - that is, the order of the bases is not determined by the chemistry and physics any more than the order of words in a written book is determined by the physics and chemistry of the ink and paper.

One thing that these sketches do not show is that the planes of the bases are essentially perpendicular to the axis of the helix.