What is the Most Important Defect in Xeroderma Pigmentosum?

The Biochemistry Online – Free Online Lectures for Medical, Dental and Allied Health Sciences students by the Biochemistry Club.

Xeroderma Pigmentosum

  • This disease is an example of formation of Thymine dimers.
  • There are Thymine bases that are adjacent to each other.
  • Due to ultra-violet radiation sometimes two Thymine bases form a dimer with each other by forming a covalent bond.
  • There is an enzyme that is called Excision endonuclease. 
  • This enzyme recognizes the lesion and excises the Thymine dimer that is formed due to UV light.
  • Other pyrimidines can also form dimers like cytosine-cytosine dimer and uracil-uracil dimer. But they are not clinically relevant.
  • This disease occurs when an individual inherits a defective copy of the enzyme Excision endonuclease.
  • This is a autosomal recessive disease that means that the individual must have copies of mutant genes from both parents.
  • The excised part is then replaced by DNA polymerase.
  • DNA ligase then comes in to repair the remaining part.


Patient will have the following symptoms:

  • Patient will be very pale because he will be sensitive to UV light.
  • There will be a lot of freckles on skin due to sensitivity to UV light.
  • They will have multiple skin cancers and
  • They will also suffer from corneal ulcerations.
  • These patients have to live a vampire like life and cannot expose themselves to sun.
All these lectures are organized and listed in detail in their proper sections. To view these lessons please visit the Genetics Section in Biochemistry II. This can be approached by directly clicking the Biochemistry II button or it can also be approached from the drop down menu in Biochemistry II tab. Once you reach the section click the curriculum tab to view the full list in the section.
Any further questions or inquiries are welcome.

NOTE: Private coaching / tutoring services are also available. Students can also get these online.

Leave a Comment

Your email address will not be published. Required fields are marked *