Integrity and accuracy of genetic information

Chapter 6: Part Chapter of the textbook: Genetics: From Genes to Genomes, 4th edition (2011), Hartwell H et al CHAPTER OUTLINE: 6.4 DNA Replication (cont) 6.5 Recombination at the DNA Level VNU-University of Science - DNThai Integrity and accuracy of genetic information must be preserved Each organism ensures the informational fidelity of its DNA in three important ways: • Redundancy Either strand of the double helix can specify the sequence of the other • Precision of cellular replication machinery – DNA polymerase I and III have proofreading ability (more about this in Chapter 7) • Enzymes that repair chemical damage to DNA The cell has an array of enzymes devoted to the repair of nearly every imaginable type of chemical damage VNU-University of Science - DNThai 6.5 Recombination at the DNA Level New combinations of alleles are created by two types of events in meiosis:  Independent assortment – each pair of homologous chromosomes segregates freely from the other (Chapter 4) Creates new allele combinations for unlinked genes  Crossing over – two homologous chromosomes exchange portions of DNA (Chapter 5) Creates new allele combinations for linked genes Ensures proper chromosome segregation during meiosis • Mistakes can result in nondisjunction VNU-University of Science - DNThai DNA molecules break and rejoin during recombination: The experimental evidence M Meselson and J Weigle, co-infected E coli with radio-labeled phage Bacteriophage lambda with genetic markers grown on E coli in media with heavy (13C and 15N) or light (12C and 14N) isotopes Separated phage DNA on CsCl density gradient Genetic recombinants had DNA with hybrid densities VNU-University of Science - DNThai Fig 6.22 Heteroduplex regions occur at sites of genetic exchange Two strands of DNA don't break and rejoin at the same location • Breakpoints on each strand can be 100s-1000s bp apart Heteroduplex – region of DNA between breakpoints • One strand is maternal and other is paternal • Strands can have mismatches VNU-University of Science - DNThai Fig 6.23 Mismatches in heteroduplexes can be repaired • DNA repair enzymes eliminate mismatches • Either allele can be converted • Gene conversion – deviations from expected 2:2 segregation, e.g 3:1 or 1:3 • In yeast, gene conversion occurs 50:50 with and without crossing over of flanking markers VNU-University of Science - DNThai Fig 6.23c Crossing-over at the molecular level: A model • Experimental observations that led to development of a model of recombination • Tetrad analysis in yeast showed that only two of the four chromatids are recombinant • Recombination occurs only between homologous regions and is highly accurate • Crossover sites often associated with heteroduplex regions • Gene conversion can occur in absence of crossing over – Not all recombination leads to crossovers VNU-University of Science - DNThai Double-strand-break repair model of meiotic recombination • Homologous chromosomes break, exchange DNA, and rejoin • Breakage and repair creates reciprocal products of recombination • Recombination events can occur anywhere along the DNA • Precision in the exchange (no gain or loss of nucleotide pairs) prevents mutations from occurring • Gene conversion can give rise to an unequal yield of two different alleles VNU-University of Science - DNThai A Model of Recombination at the Molecular Level with steps • Step 1: Double-strand break formation • Step 2: Resection • Step 3: First strand invasion • Step 4: Formation of a double Holliday junction • Step 5: Branch migration • Step 6: The Holliday intermediate • Step 7: Alternative resolutions • Step 8: Probability of crossover occurring VNU-University of Science - DNThai Step 1: Double-strand break formation Dmc1 breaks phosphodiester bonds of both strands of one chromatid  Spo11 in yeast is homologous to Dmc1 of multicellular eukaryotes VNU-University of Science - DNThai Part of Fig 6.24 10 Step 2: Resection 5' ends of each broken strand are degraded to create 3’ singlestranded tails VNU-University of Science - DNThai Part of Fig 6.24 11 Step 3: First strand invasion One single-strand tail invades a non-sister chromatid and forms stable heteroduplex Displacement loop (D-loop) from invaded chromatid is stabilized by single-strand binding protein VNU-University of Science - DNThai Part of Fig 6.24 12 Step 4: Formation of double Holliday junctions • D-loop enlarged by new DNA synthesis at 3'-end of invading strand • New DNA synthesis fills in gap in bottom strand using displaced strand as template VNU-University of Science - DNThai Part of Fig 6.24 13 Step 5: Branch migration Heteroduplex region of both DNA molecules is lengthened Part of Fig 6.24 VNU-University of Science - DNThai 14 Step 6: The Holliday intermediate VNU-University of Science - DNThai Part of Fig 6.24 15 Step 7: Alternative resolutions Cutting of Holliday junctions by endonucleases in either vertical or horizontal plane is equally likely VNU-University of Science - DNThai 16 Step 8: Probability of crossover occurring • Non-crossover occurs when both junctions are resolved in same plane • Crossover occurs with the two junctions are resolved in different planes VNU-University of Science - DNThai Part of Fig 6.24 17 Essential concepts DNA is the nearly universal genetic material This fact was demonstrated by experiments showing that DNA causes the transformation of bacteria and is the agent of virus production in phage-infected bacteria According to the Watson-Crick model, proposed in 1953 and confirmed in the succeeding decades, the DNA molecule is a double helix composed of two antiparallel strands of nucleotides; each nucleotide consists of one of four nitrogenous bases (A, T, G, or C), a deoxyribose sugar, and a phosphate An A on one strand can only pair with a T on the other, and a G can only pair with a C VNU-University of Science - DNThai 18 Essential concepts DNA carries digital information in the sequence of its bases, which may follow one another in any order Because of the restriction on base pairing, the information in either strand of a double helix defines the information that must exist in the opposite strand The two strands are considered complementary The DNA molecule reproduces by semiconservative replication In this type of replication, the two DNA strands separate, and the cellular machinery then synthesizes a complementary strand orf each By producing exact copies of the base sequence information in DNA, semiconservative replication allows life to reproduce itself VNU-University of Science - DNThai 19 Essential concepts • Recombination arises from a highly accurate cellular mechanism that includes the base pairing of homologous strands of nonsister chromatids Recombination generates new combinations of alleles in sexually reproducing organisms VNU-University of Science - DNThai 20 .. .Integrity and accuracy of genetic information must be preserved Each organism ensures the informational fidelity of its DNA in three important ways: • Redundancy Either strand of the double... occur at sites of genetic exchange Two strands of DNA don't break and rejoin at the same location • Breakpoints on each strand can be 100s-1000s bp apart Heteroduplex – region of DNA between... helix composed of two antiparallel strands of nucleotides; each nucleotide consists of one of four nitrogenous bases (A, T, G, or C), a deoxyribose sugar, and a phosphate An A on one strand can only