DNA and the Genes Chapter 14 1 of 37 What is the correct model for DNA replication 2 of 37 The polymerization of the DNA molecule is exergonic because the monomers are deoxyribonucleoside triphosphates 3 of 37 How 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 4 of 37 Replication always occurs in the 5 to 3 direction What 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 5 of 37 What are the players involved in the replication of DNA 6 of 37 Synthesis of the leading strand 7 of 37 Synthesis of the leading strand cont 8 of 37 What 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 9 of 37 What 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 10 of 37 How 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 11 of 37 12 of 37 How 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 13 of 37 14 of 37 Assertion Reason Because DNA polymerase III The region labeled with the synthesizes molecules from letter A shows the 5 to 3 direction where the lagging strand will be synthesized The Region labeled with the letter A shows where the lagging strand will be synthesized A B C D E Assertion is true Reason is true Reason is the correct explanation Assertion is true Reason true Reason is NOT the correct explanation Assertion is true Reason is false Assertion is false Reason is true Assertion is false Reason is false 36 15 of 37 How 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 16 of 37 17 of 37 What 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 18 of 37 19 of 37 Nucleotide 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 20 of 37 21 of 37 Assertion Reason DNA polymerase Because The sliding clamp has great can t replicate the affinity for single stranded end of the lagging DNA molecules Once it strand because binds to the ssDNA it helps the sliding clamp DNA polymerase to can t hold on to replicate the new strand by that end keeping it in place AR5 A B C D E Assertion is true Reason is true Reason is the correct explanation Assertion is true Reason true Reason is NOT the correct explanation Assertion is true Reason is false Assertion is false Reason is true Assertion is false Reason is false 22 of 37 36