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Structure of DNA


The most common figure associated with DNA is the double helix, the structure that sort of looks like a twisted ladder. Almost everyone knows about the structure but not many know what it is composed of.

A strand of DNA contains chemicals called nucleotides and a DNA molecule is made up of 2 polynucleotide chains arranged on the double helix (the backbone). These nucleotides are composed of three parts: a phosphate, a sugar (deoxyribose), and a type of compound base.

The deoxyribose and phosphate form the backbone of nucleic acid (the side of the ladder) while the base connect the two polynucleotide chains (like the rungs of the ladder).

There are four main types of bases: Adenine, Guanine, Thymine, and Cytosine but they are just referred to by the first letter in their name:

A, G, T, and C.

The amount of each nucleotide and the how they are arranged is different for all living things. The ways to organize the nucleotides are endless, and that's why everybody in the world looks different.

On the right is a picture of the double helix to show you how it is all organized:

On the sides of the double helix, the two strands of polynucleotide, (think of this as the sides of the ladder) are where the phosphates and sugars, arranged in different orders, are located.

The bases connect the two sides (think of this as rungs on the ladder) with a weak chemical bond.


The bases are connected together to form a base pair, and the bases can only be paired off in a specific way. An adenine base on one side of the chain only bonds with a thymine base on the opposite side, and a guanine base can only bond with a cytosine base.

Or you can look at it this way:

A=T and G=C

This is also known as Chargaff's rule and is very important for the replication of DNA, which will be discussed next.

DNA Replication >>




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