1 MB 207 – Molecular Cell Biology DNA Damage and RepairDNA recombination
2 DNA Damage and Repair Maintaining genetic stability is very important - accurate mechanism for replicating DNA.- mechanism for repairing DNA alterations that arise both spontaneously and from exposure to DNA-damaging environmental agents.Nearly all DNA damage is harmful but occasionally beneficial because mutations provide genetic variability.How important is DNA repair?DNA is the only biomolecule that is specifically repaired. All others are replaced.DNA damage is repaired shortly after it occurs and hence it does not affect future generations>100 genes participate in various aspects of DNA repair, even in organisms with very small genomes.Cancer is caused by mutations as well as many other diseases.
4 Spontaneous mutations Hydrolysis reactions caused by random interactions between DNA and the molecules around it.Two types of spontaneous mutations:DepurinationDeaminationDepurinationthe loss of a purine base by spontaneous hydrolysis of glycosidic bond that links it to deoxyribose.this glycosidic bond is labile under physiological conditions. Therefore, susceptible to hydrolysis that DNA loss thousands of purine bases in the human cell everyday.
5 DeaminationPrimary amino groups of nucleic acid bases are unstable. They can be converted to keto groups in the hydrolysis reactions and become deaminated.Involve cytosine, adenine and guanine, changes the base pairing properties of the affected base.Cytosine is more susceptible to deamination, giving rise to uracil. Others: Adenine to Hypoxanthine, Guanine to Xanthine, and 5-methyl cytosine to Thymine.Usually caused by random collision of a water molecule with the bond that links the amino group of the base to the pyrimidine or purine ring.Rate is about 100 deaminations per day.If not repaired, the error base sequence may be propagated when the strand serves as a template in the next round of replication.
8 A. Deamination of cytosine B. DepurinationA. Deamination of cytosineproduces uracilMissing purineResults in the substitution of one base for another when the DNA is replicatedIf uncorrected, can lead to either the substitution or the loss of a nucleotide pair.
9 Mutagens (mutation-causing agents) Two major categoriesChemicalsRadiations- alter DNA structure by a variety of mechanisms.Base analogs- resemble nitrogenous bases in structure and are incorporated into DNA.Base modifying agents- reacts chemically eith DNA bases to alter their structures.Intercalating agents- insert themselves between adjacent bases of the double helix.
10 Radiations Sunlight (ultraviolet radiation) - alters DNA by triggering pyrimidine dimer formation (formation of covalent bonds between adjacent pyrimidine bases).- blocked replication and transcription.X-rays and related form of radiation emitted by radioactive substances- ionizing radiation because it removes electrons from biological molecules.- generating highly reactive intermediates that cause various types of DNA damage.
11 DNA damages Distortion of double helix structure Strand breaks The thymine dimerDistortion of double helix structurePhotodamageUV light absorbed by the nucleic acid bases can induce bond formation between adjacent pyrimidines (C or T) within one strand.The two adjacent pyrimidines are pulled closer to each other than in normal DNAStrand breaksSingle-strand and double-strand breaks are produced at low frequency during normal DNA metabolism by topoisomerases, nucleases and repair processes as well as by ionizing radiation.This type of damage is introduced into DNA in cells that are exposed to ultraviolet irradiation
12 Types of DNA damages hydrolytic attack (blue arrows) Spontaneous oxidative damage (red arrows)hydrolytic attack (blue arrows)Uncontrolled methylation (green arrows))
14 Repairing Damaged Bases Damaged or inappropriate bases can be repaired by several mechanisms:Direct chemical reversal of the damageExcision Repair, in which the damaged base or bases are removed and then replaced with the correct ones in a localized burst of DNA synthesis. There are three modes of excision repair, each of which employs specialized sets of enzymes.Base Excision Repair (BER)Nucleotide Excision Repair (NER)Mismatch Repair (MMR)3. Double strand breaksNon-homologous end joiningHomologous recombination4. Translesion synthesis- DNA damage tolerance process that allows the DNA replication machinery to replicate past DNA lesions such as thymine dimers or AP sites
15 Direct Reversal of Base Damage The most frequent cause of point mutations in humans is the spontaneous addition of a methyl group (CH3-) (an example of alkylation) to Cs followed by deamination to a T. Fortunately, most of these changes are repaired by enzymes, called glycosylases, that remove the mismatched T restoring the correct C. This is done without the need to break the DNA backbone (in contrast to the mechanisms of excision repair described below).Some of the drugs used in cancer chemotherapy ("chemo") also damage DNA by alkylation. Some of the methyl groups can be removed by a protein encoded by our MGMT gene. However, the protein can only do it once, so the removal of each methyl group requires another molecule of protein.This illustrates a problem with direct reversal mechanisms of DNA repair: they are quite wasteful.
16 2. Excision RepairBase excision repair (BER), which repairs damage to a single nucleotide caused by oxidation, alkylation, hydrolysis, or deamination. The base is removed with glycosylase and ultimately replaced by repair synthesis with DNA ligase.B. Nucleotide excision repair (NER), which repairs damage affecting longer strands of 2–30 bases. This process recognizes bulky, helix-distorting changes such as thymine dimers as well as single-strand breaks (repaired with enzymes such UvrABC endonuclease). A specialized form of NER known as Transcription-Coupled Repair (TCR) deploys high-priority NER repair enzymes to genes that are being actively transcribed.C. Mismatch repair (MMR), which corrects errors of DNA replication and recombination that result in mispaired (but normal, that is non- damaged) nucleotides following DNA replication
17 DNA repair mechanisms 1) Base excision repair (BER) Removal of the incorrect base by an appropriate DNA glycosylase to create a deoxyribose sugar lacking it’s base (AP site - apurinic / apyrimidinic)Nicking of the damaged DNA strand by AP endonuclease upstream of the AP site, thus creating a 3'-OH terminus adjacent to the AP site, removal of sugar phosphate.Extension of the 3'-OH terminus by a DNA polymerase, DNA ligase seals nick.e.g. removal of uracil from DNA
18 DNA repair mechanisms 2) Nucleotide excision repair (NER) (bulky lesion)2) Nucleotide excision repair (NER)Removes a whole oligonucleotide that contain the damage.Steps:Multienzyme complex recognizes damaged regions based on their abnormal structure as well as on their abnormal chemistry (eg. pyrimidine dimer)Double incision of the damaged strand several nucleotides away from the damaged site, on both the 5' and 3' sidesAn associated DNA helicase removes the entire damaged strand, in-between the nicks.Bacteria multienzyme complex leaves a 12nt gap; doubles the size in human DNAFilling in of the resulting gap by a DNA polymeraseLigation by DNA ligase.
19 Repairing Strand Breaks Ionizing radiation and certain chemicals can produce double-strand breaks (DSBs) in the DNA backbone.Double-Strand Breaks (DSBs)There are two mechanisms by which the cell attempts to repair a complete break in a DNA molecule:i. Direct joining of the broken ends.-This requires proteins that recognize and bind to the exposed ends and bring them together for ligasing. They would prefer to see some complementary nucleotides but can proceed without them so this type of joining is also called Nonhomologous End-Joining (NHEJ).-Errors in direct joining may be a cause of the various translocations that are associated with cancers.(Translocation: Type of mutation in which a portion of 1 chromosome is broken off and attached to another)
20 ii. Homologous Recombination ii. Homologous Recombination. Here the broken ends are repaired using the information on the intact-sister chromatid (available in G2 after chromosome duplication), or on the-homologous chromosome (in G1; that is, before each chromosome has been duplicated). This requires searching around in the nucleus for the homolog — a task sufficiently uncertain that G1 cells usually prefer to mend their DSBs by NHEJ. or on the-same chromosome if there are duplicate copies of the gene on the chromosome oriented in opposite directions (head-to-head or back-to-back).-Two of the proteins used in homologous recombination are encoded by the genes BRCA1 and BRCA2. Inherited mutations in these genes predispose women to breast and ovarian cancers.
21 DNA repair mechanismsTwo different types of end-joining for repairing double-strand breaks1. Nonhomologous end-joiningpermits joining of double-strand breaks even if there is no sequence similarity between themBroken ends are rejoined by DNA ligation with the loss of one or more nucleotides at the joining siteAlters the original DNA sequence either by deletions or short insertions.2. Homologous end-joiningMore difficult to accomplish but is more precisecells are diploid – contain 2 copies of each double helixRecombination mechanisms used to transfer nucleotide sequence information from the homologous intact DNA double helix to the site of the double-strand breakBoth system involve a lot of different proteins and the processes are much more complicated
22 DNA end-joining for repairing ds breaks Accidental break (ionizing radiation, oxidizing agents, replication errors)Loss of nucleotides due todegradation from endsDNA ligationCopying process involvinghomologous recombinationComplete sequence restored by copyingfrom second chromosome(replication process uses the undamaged chromosome as the template for transferring genetic information to the broken chromosome, repairing it with no change in the DNA sequences)Region with altered segmentdue to missing nucleotidesNonhomologous end-joining- Common in mammalian cellsHomologous end-joining
23 Summary of DNA repair systems TypeDamageEnzymeMismatch repairReplication errorsMutS, MutL, and MutH in E. coliMSH, MLH and PMS in humansPhotoreactionPyrimidine dimersDNA photolyaseBase excision repairDamaged baseDNA glycosylaseNucleotide excision repairPyrimidine dimerBulky adduct on baseUvrA, UvrB, UvrC and UvrD in E. coliXPC, XPA, XPD, ERCI-XPF and XPG in humansDouble strand break repairDouble strand breaksRecA and RecBCD in E.coliTranslesion DNA synthesisPyrimidine dimer or apurinic siteY-family DNA polymerase, such as UmuC in E. coli
24 DNA RecombinationA process that a DNA segment moves from one DNA molecule to another DNA moleculeDNA molecules recombine by breaking and rejoiningPhosphodiester bonds are broken and rejoined.Importance of DNA recombination:the process of introducing genetic variation: Genetic variation is crucial to allow organisms to evolve in response to a changing environment. E.g., genetic recombination results in the exchange of genes between paired homologous chromosomes during meiosis.an important mechanism for repairing damaged DNA.involved in rearrangements of specific DNA sequences that alter the expression and function of some genes during development and differentiation.Two broad classes are commonly recognized -general recombination & site-specific recombination.
25 General recombination in meiosis A heteroduplex jointGeneral recombination in meiosis
26 General Recombination Allow large section of the DNA double helix to move from one chromosome to anotherResponsible for the crossing-over of chromosomes during meiosisChromosome must synapse (pair) in order for chiasmata to form where crossing-over occursDNA synapsis: base pairing between complementary strands from 2 DNA moleculesChiasmata: regions where paired homologous chromosomes exchange genetic material during meiosis, a cross-shaped structureOnly occurs between homologous DNA molecules
27 General Recombination Two homologous DNA molecules line up.Nicks (single or double??) are introduced.Each nicked strand then invades the other DNA molecule by complementary base pairing.The cut strands cross and join homologous strands, forming the Holliday structure (or Holliday junction) (R. Holliday (1964).Once a Holliday junction is formed, it can be resolved 2 ways by nicking and rejoining of the crossed strands to yield 2 different heteroduplexes:recombinant heteroduplexes: resulting DNA molecules are a combination of both parental DNA molecules.non-recombinant heteroduplexes: resulting DNA molecules contain only DNA from one parent molecule with a small portion of heteroduplex.
28 Holliday junction cleavage Paternal chromosome AMaternal chromosome BDNA clearage‘splice’ or crossover products reassortment or flanking genes‘patch’ or non- crossover products no reassortmentDSB repair model for homologous recombination. The figure shows the step leading to generation of recombination intermediate with 2 Holliday junctions.RecombinantNon-recombinant
30 General Recombination: example Various enzymes (homologues) are involved in the recombination process:Rec A: catalyze the exchange of strands between homologous DNAs that causes heteroduplexes to formRecB, C, & D: complex of three proteinsacts as a helicase and transiently unwinds the double-stranded DNAWhen it encounters the specific nucleotide sequence GCTGGTGG (the chi site), the enzyme acts as a nuclease to introduce a single-stranded nickContinue to unwind the double helix, forming a displaced single strand to which RecA can bind to initiate strand exchange.Ruv A, B: catalyze the movement of the crossed-strand site in Holliday junctionsRuvC: resolves the Holliday junction by cleaving the crossed strands, which are then joined by ligase
31 The different resolutions of a general recombination intermediate in mitotic and meiotic cells.
32 Site-Specific Recombination Occurs between sequences with a limited stretch of similarity; involves specific sitesMediated by proteins that recognize the specific DNA target sequences rather than by complementary base pairingTransposons/transposable elements/ “Jumping genes“: mobile genetic elements that can move throughout the genomeTwo distinct mechanisms:1. Transpositional site-specific recombination: insertion of mobile genetic elements into any DNA sequence, no formation of heteroduplex2. Conservative site-specific recombination: site specific recombination that requires a short DNA sequence that is the same on both donor and recipient, involve formation of heteroduplex
33 Transposon (cont’ next) Transposase / Integrase: act on the specific sequence at the end of transposon and disconnecting it from the flanking sequence and then inserting it to a new target site
34 Cut and Paste transposition (DNA-only transposons ) Steps of cut and paste transposition:Binding of transposase subunits to the terminal inverted repeatsTranspososome formation (synaptic complex)Excision of the transposon (contrast to replicative mechanism)DNA strand transferGap repair – DNA polymerase
35 Replicative transposition (DNA-only transposons) Steps of Replicative transposition mechanism:Binding of transposase protein to transposon sequence.Transposon DNA is replicated and a copy is inserted at a new chromosomal site, leaving the original chromosome intact.nick
37 Retrovirus-like transposition LTR on the two ends of the elementTranscription to generate RNA copiyRNA template to synthesize DNA using reverse transcriptasecDNA is recognized by integraseGap repair
38 Non-retroviral transposition -poly-A retrotransposons move by a ‘Reverse Splicing’ mechanism called target site primed reverse transcriptionA significant fraction of vertebrate chromosomes is made up of repeated DNA sequencesIn human chromosomes, these repeats are mostly mutated/truncated versions of a retrotransposon called L1 element (LINE= lone interspersed nuclear element)L1 element are mostly immobileTranslocation result in human diseaseeg. Hemophilia – L1 insertion into a gene for blood clotting factor VIII.Mechanism: require a complex of endonuclease and a reverse transcriptase
39 Non-retroviral transposition Generates a ssDNA elementdirectly linked to target DNAProcessing of ssDNA toproduce dsDNA of L1
40 Conservative site-specific recombination Breaking and joining occur at two special sites, one on each participatingDNA moleculesenzymes involve can break and rejoin two DNA helix, often reversible,ie. DNA integration, DNA excision or DNA inversion can occureg. Bacteriophage lambda/ bacterial viruses – mobile DNA element,moving in and out of host chromosomeseg. Bacteriophageeg. Salmonella typhimuriumInversion of DNA segment changesthe type of flagellum produced
41 Conservative site-specific recombination Insertion of a circular bacteriophagelambda DNA chromosome intobacteria chromosome:Integrase binds to specific ‘attachment site’ on each chromosome2. Cuts and switch the partner strands3. Re-seals forming heteroduplex joint (7nt bp long)4. Phosphodiester bond breakagerelease energy used for strandjoining5. Intergrase dissociates.
42 a specific gene in a group of cells in a transgenic animal: How a conservative site-specific recombination enzyme is used to turn ona specific gene in a group of cells in a transgenic animal:(used in mice or Drosophila to study the effect of expressing a gene of interestin the animal, using Cre recombination enzyme & loxP recognition sites)
43 Major types of transposable elements Structural featuresMechanism of movementDNA-mediated transpositionBacterial replicative transposonsTerminal inverted repeats that flank antibiotic resistance and transposase genesCopying of element DNA accompanying each round of insertion into a new target siteBacterial cut and paste transposonsExcision of DNA from old target site and insertion into new siteEukaryotic transposonsInverted repeats that flank coding region with intronsRNA-mediated transpositionViral-like retrotransposons~250 to 600bp direct terminal repeats (LTRs) flanking genes for reverse, transcriptase, integrase and retroviral-like Gag proteinTranscription into RNA from promoter in left LTR by RNA polymerase II followed by reverse transcription and insertion at target sitePoly-A retrotransposons3’ A-T rich sequence and 5’ UTR flank genes encoding an RNA-binding protein and reverse transcriptaseTranscription into RNA from internal promoter; target primed reverse transcription initiated by endonuclease cleavage