DNA Repair Mechanisms Part-02

# DNA Repair Mechanisms:

Damage to DNA caused by replication errors or mutations may have serious consequences. The multiplicity of repair mechanisms that have evolved in organisms ranging from bacteria to humans emphatically documents the importance of keeping mutation at a tolerable level. Some types of DNA repair mechanisms are –

1)      light-dependent repair or photoreactivation

2)       excision repair

                     I.            Base Excision Repair

                   II.            Nucleotide Excision Repair

3)      Mismatch repair

4)      postreplication repair

5)      the error-prone repair system (SOS response).

6)      Double-strand break repair.

DNA Repair Mechanisms part 01

 

 # What is DNA Damage?

DNA damage is an alteration in the chemical structure of DNA, such as a break in a strand of DNA, a base missing from the backbone of DNA, or a chemically changed base such as 8-OHdG. Damage to DNA that occurs naturally can result from metabolic and hydrolytic process.

# Cause & Consequences of DNA damage:

Cause:

¨       Despite an efficient repair system for the damaged DNA, replication errors do accumulate that ultimately result in mutation. The human body possesses 10¹⁴ nucleated cells, each with 3 x 10⁹ base pairs of DNA. It is estimated that about 10¹⁶ cell divisions occur in a lifetime. If 10^{-1 0} mutations per base pair per cell generation escape repair, this results in about one mutation per 10⁶ base pairs in genome.

¨       Besides the possible errors in replication, the DNA is constantly subjected to attack by both physical and chemical agents. These include radiation, free radicals, chemicals etc., which also result in mutations.

Consequences:

       It is fortunate that a great majority of the mutations probably occur in the DNA that does not encode proteins, and consequently will not have any serious impact on the organism. This is not, however, all the time true, since mutations do occur in the coding regions of DNA also. There are situations in which the change in a single base pair in the human genome can cause a serious disease e.g. sickle cell anemia.