DNA Repair Lecture 4

DNA Repair Lecture 4 At the end of this lecture, students should be able to: • Understand different types of DNA repair. • Recognize the effect of DNA repair impairment and diseases development. mm In multicellular organisms, the response to DNA damage can result in two major physiological c...

DNA Repair Lecture 4 At the end of this lecture, students should be able to: • Understand different types of DNA repair. • Recognize the effect of DNA repair impairment and diseases development. mm In multicellular organisms, the response to DNA damage can result in two major physiological consequences: • Cells can undergo cell cycle arrest, repair the damage and re -enter the cell cycle, • cells can be targeted for cell death (apoptosis) and removed from the population. i cxgokugtanbreaka.n u ypramidine dinner Photo reactivation is Is Prokaryotes pyramiding Most DNA damage is repaired by removal of the damaged bases followed by resynthesis of the excised region. Direct repair is seen in repairing UV -induced pyrimidine dimers . The process is called photoreactivation because energy derived from visible light is utilized to break the cyclobutane ring structure . It is common in prokaryotic and eukaryotic cells, including E . coli , yeasts, and some species of plants and animals . Photoreactivation is not universal ; placental mammals (including humans) lack this mechanism of DNA repair Ils dinner FU d IF I É Excision repair • Excision repair is a more general means of repairing a wide variety of chemical alterations to DNA. • Consequently, the various types of excision repair are the most important DNA repair mechanisms in both prokaryotic and eukaryotic cells. • In excision repair, the damaged DNA is recognized and removed, either as free bases or as nucleotides. • The resulting gap is then filled in by synthesis of a new DNA strand, using the undamaged complementary strand as a template wisdom excision repair Types of excision repair • Base excision, • Nucleotide -excision • Mismatch repair It 2 A DNA 11 53 IT 3 I DNA 11 2M Ils ji a replication BASE -EXCISION REPAIR The repair of uracil -containing DNA is a good example of base -excision repair, in which single damaged bases are recognized and removed from the DNA molecule. 8 -oxoG excision is another example. Uracil can arise in DNA by two mechanisms: (1) Uracil (as dUTP ) is occasionally incorporated in place of thymine during DNA synthesis, (2) Uracil can be formed in DNA by the deamination of cytosine. The excision of uracil in DNA is catalyzed by DNA glycosylase , an enzyme that cleaves the bond linking the base (uracil) to the deoxyribose of the DNA backbone. This reaction yields free uracil and an apyrimidinic site. The remaining deoxyribose moiety is then removed, and the resulting single -base gap is filled by DNA polymerase and ligase. o O 2 E o damage to single base A T G E by Deamination C I in DNA 10 DNA Glycosylase remove damaged base 2 0 AP endonuclease AP site am bad base iww phosphodiester bond 03.5 9 s base DNA ligase 53.01s NUCLEOTIDE -EXCISION REPAIR Excision repair systems recognize a wide variety of damaged bases that distort the DNA molecule, including UV -induced pyrimidine dimers and bulky adduct groups added to DNA bases. Bu 1 CFM o Helicase O b Isps Nuclease DNA Polymerase In DNA ligase In E .coli and yeast group of genes are involved in producing repair proteins . In humans, DNA repair genes have been identified largely by studies of individuals suffering from inherited diseases resulting from deficiencies in the ability to repair DNA damage . The most extensively studied of these diseases is xeroderma pigmentosum (XP) e proplems in DNA repair photosensitivity Cancer Scientest discover enzymes genes because of inherited diseases map 1st gene recognise tpf MISMATCH REPAIR (MMR) During DNA replication, many such mismatched bases are removed by the proofreading activity of DNA polymerase . If the mismatch is missed, MMR enzymes corrects the newly synthesized strand . E.coli Human cells E I r Double -strand breaks Repair Double -strand breaks could occur naturally during DNA replication, when DNA polymerase at the replication fork encounters a nick in the template strand of DNA. In addition, double -strand breaks are induced by ionizing radiation (such as X -rays) and some chemicals that damage DNA by introducing breaks in opposite strands. Because double -strand breaks affect both strands of DNA, they cannot be repaired by mechanisms that require DNA synthesis across a site of damage. Instead, double -strand breaks are repaired by a distinct mechanism , recombinational repair , which rejoins the broken strands. Cells utilize two major pathways of recombinational repair. In the simplest case, double -strand breaks can be repaired simply by rejoining the broken ends of a single DNA molecule. However, this leads to a high frequency of errors resulting from deletion of bases around the site of damage. The non -homologous end - joining (NHE J) pathway , double strand breaks repair to them I Double -strand breaks can be repaired by homologous recombination with DNA sequences on an undamaged chromosome. homologous DNA sequence to serve as a template for DNA -synthesis -dependent repair and involves extensive DNA -end processing. HR is extremely accurate, as it leads to precise repair of the damaged locus using DNA sequences homologous to the broken ends Homologous recombination repair E K K i s depend on sister Chromatide as atemplate __ Translesion DNA synthesis The direct reversal and excision repair systems act to correct DNA damage before replication, so that replicative DNA synthesis can proceed using an undamaged DNA strand as a template . If these systems fail, the cell has alternative mechanisms for dealing with damaged DNA at the replication fork . Pyrimidine dimers and many other types of lesions cannot be copied by the normal action of DNA polymerases, so replication is blocked at the sites of such damage . However, cells also possess several specialized DNA polymerases that are capable of replicating across a site of DNA damage . The replication of damaged DNA by these specialized polymerases, called translesion DNA synthesis, provides a mechanism by which the cell can bypass DNA damage at the replication fork, which can then be corrected after replication is complete . O ÉÉ DNA repair and cancer The hereditary nonpolyposis colorectal cancer, or (HNPCC) is accounts for approximately 3 % of all colon cancer cases . Approximately 50 % of HNPCC cases are caused by mutations in the human homolog of the E . coli MutS gene (hMSH 2 ) and most of the remaining cases have mutations in the other genes involved in mismatch repair . E About 10 % of breast cancer cases are associated with inherited defects in two genes, BRCA 1 and BRCA 2 . Human BRCAI and BRCA 2 are large proteins (1 ,834 and 3 ,418 amino acid residues, respectively) that interact with a wide range of other proteins involved in transcription, chromosome maintenance, DNA repair, and control of the cell cycle . BRCA 2 has been implicated in the recombinational DNA repair of double - strand breaks . However, the precise molecular function of BRCA 1 and BRCA 2 in these various cellular processes is not yet clear . Women with defects in either the BRCA 1 or BRCA 2 gene have a greater than 80 % chance of developing breast cancer 5 j E Ea