DNA Repair Processes PDF

## Pyrimidine Dimers **(a)** - Two thymine monomers are shown, each with an -NH group, a double-bonded oxygen, and a single-bonded oxygen. The monomers are connected to an adjacent carbon ring via a single-bonded oxygen in each monomer. * **h** indicates the use of ultraviolet light to create a di...

## Pyrimidine Dimers **(a)** - Two thymine monomers are shown, each with an -NH group, a double-bonded oxygen, and a single-bonded oxygen. The monomers are connected to an adjacent carbon ring via a single-bonded oxygen in each monomer. * **h** indicates the use of ultraviolet light to create a dimer. **(b)** - **thymine dimer** is shown where both monomers are now connected to the carbon rings of the other. One carbon ring is connected to the other via two single bonds. **(c)** - The reaction of water with the ultraviolet-activated hydrogen radical, resulting in a hydroxyl radical. **(d)** - A diagram showing the formation of a thymine dimer in a DNA double-helix, as a result of ultraviolet light. - **Thymine dimer** is labeled, and the dimer is causing a kink in the double-helix. ## Repairing Thymine Dimers **(a) Nucleotide excision repair** - A single-strand of DNA is shown, displaying a thymine dimer. - **Damage recognized.** The thymine dimer is highlighted. - **Nuclease cuts strand.** The strand is cut on both sides of the dimer, leaving a section containing only the dimer. - **Helicase removes damaged section.** The dimer section is removed, leaving a gap in the strand. - **DNA pol I and DNA ligase make repairs.** The gap is filled with new nucleotides, and the strand is rejoined. - **(a)** indicates this is the nucleotide excision repair process. **(b) Photoreactivation** - A single strand of DNA is shown, displaying a thymine dimer. - **Photolyase binds.** Photolyase is shown binding to the thymine dimer. - **Visible light.** Photolyase absorbs visible light and splits the thymine dimer. - **Thymine dimer bond broken.** The dimer is no longer intact. - **(b)** indicates this is the photoreactivation process. ## Photoreactivation Repair - **(a) Photoreactivation repair** - A diagram showing the steps involved in this repair process. - **Dimer forms**. A thymine dimer is shown. - **Blue light**. Blue light is shown being absorbed by a **PRE enzyme**, which is the photolyase enzyme. - **Dimer repaired**. The PRE enzyme uses blue light energy to break apart the thymine dimer. - **Normal pairing restored**. The strand is now intact, with the thymine bases in their normal positions. - **(b) Excision repair** - A diagram showing the steps involved in this repair process. - **Dimer forms**. A thymine dimer is shown. - **Excision of dimer**. An enzyme, encoded by the **uvr gene**, removes the thymine dimer from the DNA strand. - **DNA polymerase I fills in gap**. The gap left by the removed dimer is filled with new nucleotides. - **DNA ligase seals gap**. This gap is sealed using DNA ligase. - **Normal pairing restored**. The strand is now intact, with thymine and adenine bases in their normal positions. ## Photoactivation for Repair of Thymine Dimer - **Visible light** is shown being absorbed by **photolyase.** - **Thymine dimer**. The damaged section of DNA is shown, containing the thymine dimer. - **Photolyase**. Photolyase is shown bound to the thymine dimer. - **Cleavage of cyclobutanering**. Photolyase breaks the cyclobutane ring that connects the dimer. - **Damage repaired**. The DNA strand is now intact. - **Photolyase released**. Photolyase is detached from the DNA. - **Fig 9.17** refers to the photoreactivation process. ## Cleavage of Thymine Dimer Crosslinks by Photoreactivation - **(a)** A single strand of DNA is shown with a thymine dimer. - **(b)** Thymine dimer formation caused by UV irradiation. - **(c)** The photoreactivating enzyme is bound to the thymine dimer. - **(d)** Absorption of blue light activates the photoreactivating enzyme which removes the dimer. - **(e)** The enzyme is released. ## Diagram of the Excision Repair Pathway - **(a)** A single-strand of DNA is shown with two thymine bases in the same strand. - **(b)** UV radiation causes thymine dimer formation. - **(c)** The endonuclease enzyme cuts the DNA strand on either side of the thymine dimer. - **(d)** An exonuclease enzyme removes several nucleotides, including the dimer. - **(e)** DNA polymerase fills in the gap. - **(f)** DNA ligase seals the gap. ## Simplified version of Postreplication Recombination Repair - **(a)** A single-strand of DNA is shown containing a thymine dimer. - **(b)** The same strand is shown with a thymine dimer present. - **(c)** The strand is replicated, but gaps are present due to the inability of the DNA polymerase to replicate past the thymine dimer. - **(d)** 'Sister chromosomes' are shown, with one containing a thymine dimer and one without. Recombination occurs, resulting in one chromosome with the dimer and one without. ## SOS Repair - **Figure 8-7** displays the steps involved in SOS repair. - **Irradiation, formation of dimers**. A single-strand of DNA is shown with a thymine dimer. - **Induction of SOS system, error-prone replication (red) bypasses dimers**. A replication event is shown, but the gap caused by the dimer is not filled. This description summarizes the major points illustrated in the attached image, excluding unnecessary details. Please note that this is not a comprehensive explanation of the processes described.