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Structure of DNA (deoxyribose nucleic acid)

Key Concepts

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Structure of DNA

Deoxyribose is a 5 carbon sugar molecule and forms the backbone of the DNA polymer chain.

One of the four principal bases shown below can attach to this deoxyribose sugar molecule.

adenine (A)
cytosine (C)
guanine (G)
thymine (T)

The principal base attaches through its nitrogen to the carbon in the ring that is bonded to both the oxygen atom of the ring and an OH (hydroxy) group.

A phosphate group can link 2 deoxyribose sugar molecules by attaching through the other 2 available OH (hydroxy) groups.

These three components:

make up a DNA monomer or nucleotide.
These nucleotides link up to form the DNA polymer.

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Representing a DNA Polymer Strand

The diagram below shows a piece of a strand of DNA:

The sequence of bases shown from top to bottom is:

The sequence of phosphate groups and deoxyribose sugars is always the same, only the sequence of bases changes.

It is common to therefore represent a strand of DNA using lines to represent the sugar-phosphate backbone, and the letters A (adenine), C (cytosine), G (guanine), and T (thymine) to represent the bases, as shown in the example below:

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Double Stranded DNA

In a double strand of DNA hydrogen bonds occur between pairs of bases:

Since the sequences of bases on one strand determine the sequence of bases on the other strand, the two strands are referred to as complementary.

The diagram below shows how a piece of uncoiled double stranded DNA could be represented.

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