Slide 1What is the correct model for DNA replication?Slide 3How does replication start?What about eukaryotic DNA?What are the players involved in the replication of DNA?Synthesis of the leading strandSynthesis of the leading strand cont…What does DNA polymerase III require to create the new strand?What is the leading strand?How does DNA polymerase III work in the lagging strand?Slide 12How is the end of the linear chromosome replicated?Slide 14Slide 15Slide 16Slide 17What will happen when a mistake is produced during replication?Slide 19Nucleotide Excision RepairSlide 21AR51 of 37DNA and the GenesChapter 142 of 37What is the correct model for DNA replication?3 of 37The polymerization of the DNA molecule is exergonic because the monomers are deoxyribonucleoside triphosphates.4 of 37How does replication start?•Bacteria have a location where the replication process begins.•This replication bubble forms at a specific sequence of bases called the origin of replication. •The replication bubble grows as the DNA is replicated.•The replication occurs in both directions.•Replication always occurs in the 5’ to 3’ direction.5 of 37What about eukaryotic DNA?•The replication of the eukaryotic organism is also bidirectional.•They have multiple replication sites along the chromosome.•The replication fork is a Y-shaped region where the parent DNA double helix is split into single strands, which are then copied.6 of 37What are the players involved in the replication of DNA?7 of 37Synthesis of the leading strand8 of 37Synthesis of the leading strand cont…9 of 37What does DNA polymerase III require to create the new strand?•This enzyme works only in the 5’ to 3’ direction.•It requires a free 3’-hydroxyl (-OH) group.•It also requires a single stranded DNA molecule.10 of 37What is the leading strand?•The leading strand is the 5’ to 3’ strand of DNA from the double helix.•The lagging strand is the 3’ to 5’ strand of DNA from the double helix.11 of 37How does DNA polymerase III work in the lagging strand?•The synthesis of the lagging strand starts when RNA primase synthesizes a small stretch of RNA that acts as a primer.•DNA polymerase acts by adding bases to the 3’ end of the lagging strand.•However, DNA polymerase III moves away from the replication fork.–A new segment of single stranded DNA is exposed.12 of 3713 of 37How is the end of the linear chromosome replicated?•The region at the end of the chromosome is called a telomere (“end part”).•The single stranded DNA telomere remaining on the lagging strand after replication is eventually degraded, resulting in the shortening of linear chromosomes.14 of 3715 of 37The Region labeled with the letter A shows where the lagging strand will be synthesized.A. Assertion is true; Reason is true; Reason is the correct explanationB. Assertion is true; Reason true; Reason is NOT the correct explanationC. Assertion is true; Reason is falseD. Assertion is false; Reason is trueE. Assertion is false; Reason is falseAssertion ReasonThe region labeled with the letter A shows where the lagging strand will be synthesized.BecauseDNA polymerase III synthesizes molecules from the 5’ to 3’ direction.3616 of 37How do eukaryotes maintain the integrity of the linear chromosomes? •The ends of the chromosomes don’t have any genes.•The telomeres consist of short stretches of bases that are repeated over and over.–TTAGGG in humans is repeated 100 times.•An enzyme called telomerase is involved in replicating telomeres.•Telomerase catalyzes the synthesis of DNA from an RNA template.–The enzyme carries the RNA molecule that acts as a built-in template.17 of 3718 of 37What will happen when a mistake is produced during replication?•DNA polymerase III also proof-reads the new strand.–This is done by a part of the DNA pol. III called the epsilon subunit.–This subunit acts as a exonuclease.–It only removes the wrong nucleotide if the -OH group is free.•If by any chance the mistake is left behind (because DNA Pol. III did not recognize it), another group of enzymes will correct this mistake.19 of 3720 of 37Nucleotide Excision Repair•Genes are under constant assault.•Ultraviolet light (for example) can cause covalent bonds to form between adjacent pyrimidine bases, as illustrated in the example below. –Thymine dymer.–The kink caused by this causes the enzymes responsible for transcription and DNA synthesis to stop.–This defect is corrected by another set of enzymes.21 of 3722 of 37AR5A. Assertion is true; Reason is true; Reason is the correct explanationB. Assertion is true; Reason true; Reason is NOT the correct explanationC. Assertion is true; Reason is falseD. Assertion is false; Reason is trueE. Assertion is false; Reason is falseAssertion ReasonDNA polymerase can’t replicate the end of the lagging strand because the sliding clamp can’t hold on to that end.BecauseThe sliding clamp has great affinity for single stranded DNA molecules. Once it binds to the ssDNA, it helps DNA polymerase to replicate the new strand by keeping it in