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5.2.1.1. Primary structure of DNA

Primary structure of DNA

The primary structure of DNA is the sequence of nucleotides (linked by phosphodiester bonds) of a single chain or strand, which can appear as a simple extended strand or something folded in itself.

As we have seen, a DNA chain has two free ends: the 5', attached to the phosphate group, and the 3', attached to a hydroxyl.

To represent a DNA chain, usually only the sequence of nitrogenous bases of the deoxyribonucleotides that compose it is indicated. Thus, we will use A to refer to deoxyadenosin-5, -monophosphate (AMP), the same as for the rest of nitrogenous bases, such as T, C and G.  

By combining the four nitrogenous bases, an almost unlimited number of different DNA strands can be obtained. For example, supposing that human DNA has 5.6·109 pairs of nucleotides, there could be 45,600,000,000 different DNA strands in which the genetic information of the organism would be contained.

DNA, according to the order of nucleotides, contains the information necessary to synthesize a protein. If the nucleotide sequence changes, a different protein will be synthesized. The sequence GCATACTTG will generate a protein other than CTAGCGTAT, although they contain the same nucleotides.

gene is the part of the DNA chain that contains the information necessary for the amino acids that are part of a protein to be synthesized in an orderly fashion (primary structure of the proteins that it determines, also its structure in space and its function in the cell).

Individuals of the same species have the same percentage of guanine, cytosine, adenine and thymine, so their characteristics will be very similar .

Common mistake: ester and phosphodiester bond

It is common to see confusion between an ester bond and a phosphodiester bond in some tests.

The ester bond occurs between an alcohol group (-OH) and a carboxylic acid group (-COOH), releasing a water molecule (H2O).

The phosphodiester bond is a type of covalent bond that occurs between a hydroxyl group (OH-) at the 3' carbon and a phosphate group (PO43−) at the 5' carbon of the  incoming nucleotide, thus forming an ester double bond. In this reaction, a water molecule is released and a dinucleotide is formed. In the phosphodiester bond, two oxygen atoms of a phosphate group and the hydroxyl groups of two other different molecules are attached. Thus, the phosphate group acts as if it were a connecting bridge between these two molecules.

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