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Thymine Dimers Formation and Repair
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Why can exposure to the sun cause skin cancer and other types of cellular damage? Xeroderma pigmentosa is a rare genetic disease in which affected people rapidly develop skin cancer and other types of skin damage in response to sunlight. Persons with xeroderma pigmentosa lack the ability to repair DNA damage caused by the ultraviolet radiation in sunlight. DNA is subject to damage from the environment. Changes to DNA structure may be copied with the rest of the molecule. Damage may also prevent the DNA from replicating itself. If the damage is major or if it affects nucleotides coding for a crucial part of a protein the function of the protein may be seriously affected or even completely destroyed. Cells contain a number of mechanisms to repair damaged DNA to prevent errors from being passed from one generation to the next. DNA repair mechanisms are known in both prokaryotic and eukaryotic cells. There are a number of different mechanisms for repairing DNA most of which either involve either directly reversing the error or by cutting out (excising) the damaged piece of DNA and replacing it.

View the animation below, then complete the quiz to test your knowledge of the concept.





1UV light damages DNA by causing
A)covalent bonds to form between two cytosines next to each other on the same DNA strand
B)covalent bonds to form between thymine nucleotides next to each other on the same DNA strand
C)covalent bonds to form between two thymine nucleotides on opposing DNA strands
D)covalent bonds to form between a thymine and an adenine on the same DNA strand


2When DNA is damaged by UV light and is not repaired
A)DNA replication can proceed as normal but transcription is prevented
B)transcription can proceed as normal but replication is prevented
C)neither DNA replication nor transcription can occur and the organism will probably die
D)the damaged part will be excised and allow DNA replication to proceed


3In light repair
A)the covalent bonds between the thymine dimers are broken
B)a single thymine is excised
C)the damaged segment of DNA is excised by DNA polymerase
D)the damaged segment of DNA is excised by DNA ligase


4Excision repair takes place only in the presence of light.
A)True
B)False


5In dark repair, the final phosphodiester bond is formed by DNA ligase.
A)True
B)False





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