DNA Structure
25/08/2013 21:48
The Composition and Structure of DNA
What is the molecular structure of DNA?
DNA and RNA are polymers—large molecules that consist of many similar smaller molecules, called monomers, linked together. The monomers that make up DNA and RNA are nucleotides.
Each nucleotide consists of a pentose (five-carbon) sugar, a nitrogenous (nitrogen-containing) base (usually just called a base), and a phosphate group.
In DNA, the pentose sugar is deoxyribose, and in RNA it is ribose.
The two sugars differ by the chemical groups attached to the carbon: a hydrogen atom (H) in deoxyribose and a hydroxyl group (OH) in ribose.
There are two classes of nitrogenous bases: the purines, which are nine-membered, double-ringed structures, and the pyrimidines, which are six-membered, single-ringed structures. There are two purines—adenine (A) and guanine (G)—and three different pyrimidines— thymine (T), cytosine (C), and uracil (U) in DNA and RNA.
There are two classes of nitrogenous bases: the purines, which are nine-membered, double-ringed structures, and the pyrimidines, which are six-membered, single-ringed structures. There are two purines—adenine (A) and guanine (G)—and three different pyrimidines— thymine (T), cytosine (C), and uracil (U) in DNA and RNA.
Both DNA and RNA contain adenine, guanine, and cytosine; however, thymine is found only in DNA, and uracil is found only in RNA.
In DNA and RNA, bases are covalently attached to the carbon of the pentose sugar. The purine bases are bonded at the 9 nitrogen, and the pyrimidines bond at the 1 nitrogen.
The combination of a sugar and a base is called a nucleoside. Addition of a phosphate group (PO 2- to anucleoside yields a nucleoside phosphate, which is one kind of nucleotide.
The phosphate group is attached to the carbon of the sugar in both DNA and RNA.
To form polynucleotides of either DNA or RNA, nucleotides are linked together by a covalent bond between the phosphate group of one nucleotide and the carbon of the sugar of another nucleotide. Polynucleotide chains have polarity, meaning that the two ends are different: there is a carbon (with a phosphate group on it) at one end, and a carbon (with a hydroxyl group on it) at the other end. The ends of a polynucleotide are routinely referred to as the 5'end and the 3'end.
Sources:
Content:
iGenetics, A Molecular Approach, third edition, Peter J. Russel
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