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208 Biotechnology UV radiation and it may lead to the formation of the pyrimidine dimers (T-T dimers), which distorts the DNA helix. There is a mechanism in the cell by which these dimers are removed. It is facilitated by a protein complex called UVr. (Fig.7.36b). Initially a component of UVr complex scans the DNA for the presence of T-T dimers that distort the DNA helix. After identification, other components of UVr cleaves the phosphodiester bond four nucleotides downstream and eight nucleotides upstream to the DNA damage site. In the next step, the other component or UVr complex which has DNA helicase property unwinds the DNA and removes the cleaved nucleotide fragment. Finally, DNA polymerase I adds the nucleotide in 5’-3’ region and the gap gets sealed by DNA ligase. Mismatch repair (MMR)— During DNA replication, sometimes an incorrect nucleotide get incorporated. There exists a mechanism by which such a mismatch is corrected through different types of proteins namely MutH, MutL, MutS and MutT. These four proteins recognise and cleave the replicated DNA segment about 1,000 nucleotides from the point of mismatch. Gap thus created is filled by DNA polymerase followed by joining of nick by DNA ligase. This process is also important for proof-reading during replication. A few other DNA repair mechanism have also been observed and studied both in prokaryotes and eukaryotes. You will study them in your higher classes. However, based on the account of mutation and repair mechanism the stable nature of genetic information can be appreciated vis-à-vis its ability to undergo changes. 7.9 R egulation of G ene E xpression Do you know in a multicellular organism there are many types of cells differing in structure and function, nevertheless their genes are identical? This is because all cells are derived from zygote by mitotic divisions. All activities of an organism are controlled by genes. Most of the genes of an organism express themselves by producing proteins. All genes are not expressed in all cells as their products are not needed at one time. Only those genes whose products are required in a cell are expressed while other genes are not expressed as its products are not Reprint 2026-27