What are the four kinds of nitrogen bases in DNA?

Nitrogen Bases of DNA – These are rather complex single or double ring structures containing nitrogen and are attached to the carbon of the sugar. Nitrogen bases are of two types based on the number of rings.

Purines
Purines are double ringed nitrogenous bases which are linked to sugar by using nitrogen present at 9' -position. Adenine and guanine come under this group.

Pyrimidines
Pyrimidines are single ringed nitrogen bases which are linked to sugar by using nitrogen present at 1 ' -position. Thymine and cytosine come under this group.

Adenine 6-aminopurine

Guanine 2-amino 6-oxypurine

Cytosine 4-amino 2-oxypyrimidine

Thymine 2,4-oxy S-methyl pyrimidine

DNA stands for deoxyribonucleic acid. Strands of DNA are long polymers built of millions of nucleotides that are linked together. Individually, nucleotides are quite simple, consisting of three distinct parts:
One of four nitrogen bases
Deoxyribose (a five-carbon sugar)
A phosphate group
The image below shows a simplified representation of a nucleotide. The P represents the phosphate molecule, the S represents the sugar (deoxyribose), and B represents one of the four nitrogen bases.

The structure of the phosphate group is shown below.

1. Nitrogen Bases
The four nitrogen bases are:
Adenine
Guanine
Cytosine
Thymine
Nucleotides are named after which of the four nitrogen bases it has. So the names of the four DNA nucleotides are adenine, guanine, cytosine, and thymine. These will be referred to as A, G, C, and T respectively. (Note: Throughout this site, the words nucleotide and base are often used to represent the same thing – a nucleotide).
Adenine and guanine are classified as purines since they are double-ringed molecules. Cytosine and thymine and pyrimidenes due to the fact that they are single-ringed molecules. Structural diagrams of the four bases are shown in the table below.

A pyrine binds with a pyrimidene in DNA to form a basepair. Adenine and thymine bind together to form the A-T basepair. Likewise, guanine and cytosine come together to form the G-C basepair. The bases are joined together by weak hydrogen bonds, and it is this hydrogen bonding that produces DNA's familiar double helix shape. An image illustrating the how two bases pair with hydrogen bonding is shown below (The blue lines are the hydrogen bonds.)

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RE:
what are the four kinds of nitrogen bases in DNA?

Adenine, guanine, cytosine, and thymine are the four nitrogen bases in DNA.

adenine and thymine are always together
and cytosine and guanine are always together

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