When two adjacent bases in the same strand of dna dimerize (form a covalent bond between them), what happens to the dna?

The DNA of every higher organism consists of two strands which are joined together through bonding between nitrogenous bases. We know well that the base of one strand forms hydrogen bond between the base of the other strand to hold the DNA intact. Thymine always pairs with Adenine and Guanine always pairs with Cytosine, this way an intact and normal DNA structure is formed.

However, sometimes the Ultra violet (UV) rays cause serious damage to DNA. When UV rays are absorbed by DNA, where the bases are thymine or cytosine they add energy at that point. This energy causes the formation of covalent bond between adjacent cytosine- cytosine or thymine-thymine bases of same DNA strand (Please see attached figure). This abnormal process is called the formation of Pyrimidine dimers.  

What harm they can incur to us?

Our body cells have excellent DNA proofreading mechanisms that correct the errors in DNA. But sometimes when DNA has to replicate, the dimers don’t let the DNA polymerase enzyme to correctly read DNA strand because there are abnormal structures. The enzyme incorporates wrong base, at the position of dimers for example if C-C dimer is present, adenine instead of guanine will be added at that position. This causes the presence of incorrect base in newly synthesized DNA.  

This process leads to mutation in the cells which is very common cause of increased risk of cancers nowadays.