What is a Nucleotide?

A nucleotide is the basic unit that makes up DNA.

A singular nucleotide is made up of x3 components: 

1. A base (1 of 4 molecules = A, T, C or G)
   
2. 5 Carbon sugar (deoxyribose = DNA or ribose = RNA)
   
3. Phosphate group (forms a bond between the sugar molecules in a DNA chain = phosphodiester bond)

Bases

There are x4 different nitrogenous bases that comprise DNA.
1. Adenosine
2. Thymine
3. Cytosine
4. Guanine
 

They come under x2 groups depending on their shape and chemical structure. 

1. x1 rings = Pyrimidines
   
2. x2 rings = Purines

Pyrimidines
Thymine and Cytosine are the smallest of the 4 bases. With only 1 ring in their structure.

Point to Remember!
*** In RNA, Thymine is replaced by Uracil ***

Purines
Adenosine and Guanine are the biggest of the 4 bases. Their structure contains a pyrimidine ring + a smaller 5 sided ring. Therefore making a 2 ringed structure.

Key points
A nucleotide (base + sugar + phosphate) is the building blocks for nucleic acid (DNA & RNA)

A nucleoside (base + sugar) 

Nucleotides can serve as energy carriers in the form of ATP (Adenosine TriPhosphate)

Nucleotides can aid in cell signalling e.g. cyclicAMP (cAMP)