The ending amino acid, Trp, is no longer present. AZT is a triphosphate derivative of thymine. When this compound is incorporated into a lengthening DNA strand, DNA synthesis is terminated. AZT can be utilized by reverse transcriptase, but is not a substrate for the DNA polymerase in the host cell. Therefore, AZT is not incorporated during normal DNA synthesis, and should not affect normal cell function.
Thymine dimers can form between thymine bases on the same DNA strand, and are repaired in two ways.
This enzyme splits the dimers apart when exposed to the proper wavelength of light. An enzyme with many subunits, called UvrABC endonuclease, binds to the DNA strand at the point of the thymine dimer, and removes a stretch of eight nucleotides around the thymine dimer. I and ligase then put the proper sequence in, based upon the nucleotides on the other strand. This is the SOS response. This response, in which a number of repair enzymes are produced in response to DNA damaged, is controlled by the LexA and RecA proteins.