What is thymine?

Thymine is a nitrogen base seen in DNAs.

Nuclear acids are made of Nucleotides. These nucleotides have three major parts:

• A pentose monosaccharide
• A phosphate group
• 
  A nitrogen base group
  

There are four types of nitrogen bases:

1. Adenine
2. 
   Thymine
   
3. Cytosine
4. Guanine

From these four nitrogen base groups, Adenine and Guanine are called Purines and Thymine and Cytosine are called Pyrimidine.

There are two polynucleotide chains in a DNA and they are connected by the Hydrogen Bonds between these four Nitrogen Bases. Here Adenine and Thymine create 2 H bonds between them( and cytosine and guanine makes three H bonds between them).

In RNA. the other nuclear acid form, there are no Thymine and instead of that there is a nitrogen base called Uracil.