DNA REPLICATION Copyright © 2009 Pearson Education, Inc.

DNA replication depends on base pairing BASICALLY… The two strands separate Each strand is a pattern Produces a complementary strand SEMI-CONSERVATIVE- 1 new strand and 1 old Student Misconceptions and Concerns 1. The authors note that although the general process of semiconservative DNA replication is relatively simple, it involves complex biochemical gymnastics. The DNA molecule is unwound, each strand is copied simultaneously, the correct bases are inserted, and the product is proofread and corrected. Before discussing these details, be sure that your students understand the overall process, what is accomplished, and why each step is important. Teaching Tips 1. Demonstrate the complementary base pairing within DNA. Present students with the base sequence to one side of a DNA molecule and have them work quickly at their seats to determine the sequence of the complimentary strand. For some students, these sorts of quick practice are necessary to reinforce a concept and break up a lecture. 2. The authors note that the semiconservative model of DNA replication is like making a photo from a negative and then a new negative from the photo. In each new negative and photo pair, the new item was made from an old item. Copyright © 2009 Pearson Education, Inc.

Figure 10.4B Untwisting and replication of DNA. This diagram shows an overview of DNA replication, emphasizing the semiconservative nature of the process.

Parental molecule of DNA Parental molecule of DNA Figure 10.4A A template model for DNA replication. This figure emphasizes the accuracy of DNA replication, due to the specific base-pairing interactions. When the strand on the left is a template, the complementary strand is identical to the one on the right and vice versa.

Nucleotides Parental molecule of DNA Both parental strands serve Nucleotides Parental molecule of DNA Both parental strands serve as templates Figure 10.4A A template model for DNA replication. This figure emphasizes the accuracy of DNA replication, due to the specific base-pairing interactions. When the strand on the left is a template, the complementary strand is identical to the one on the right and vice versa.

Nucleotides Parental molecule of DNA Both parental strands serve Nucleotides Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA Figure 10.4A A template model for DNA replication. This figure emphasizes the accuracy of DNA replication, due to the specific base-pairing interactions. When the strand on the left is a template, the complementary strand is identical to the one on the right and vice versa.

Two daughter DNA molecules Parental strand Origin of replication Daughter strand Bubble Figure 10.5A Multiple “bubbles” in replicating DNA. Eukaryotic chromosomes have multiple replication origins, while prokaryotic chromosomes have a single origin. In both cases, replication proceeds bidirectionally from the origin. Two daughter DNA molecules

DNA replication- in two directions; many sites Origins of replication DNA unwinds; produces a “bubble” Replicates in both directions Replication ends when products from the “bubbles” merge with each other Student Misconceptions and Concerns 1. The authors note that although the general process of semiconservative DNA replication is relatively simple, it involves complex biochemical gymnastics. The DNA molecule is unwound, each strand is copied simultaneously, the correct bases are inserted, and the product is proofread and corrected. Before discussing these details, be sure that your students understand the overall process, what is accomplished, and why each step is important. Teaching Tips 1. Demonstrate the complementary base pairing within DNA. Present students with the base sequence to one side of a DNA molecule and have them work quickly at their seats to determine the sequence of the complimentary strand. For some students, these sorts of quick practice are necessary to reinforce a concept and break up a lecture. 2. The authors note that the semiconservative model of DNA replication is like making a photo from a negative and then a new negative from the photo. In each new negative and photo pair, the new item was made from an old item. Copyright © 2009 Pearson Education, Inc.

DNA MADE IN 5’ TO 3’ direction 5 Prime and 3 Prime DNA MADE IN 5’ TO 3’ direction Replication is continuous on the 3’ - 5’ template Replication is NOT continuous on the 5’ - 3’ template, short segments Called Okazaki Fragments

5 end 3 end P P P P P P P P 3 end 5 end 5 2 4 3 3 5 end 3 end P 5 2 4 3 3 1 1 2 4 P 5 P P P Figure 10.5B The opposite orientations of DNA strands. This figure emphasizes the opposite polarity of the DNA chains. The 3 end has a free hydroxyl group attached to the 3 carbon of the sugar, while the 5 end has a free phosphate attached to the 5 carbon of the sugar. DNA polymerase enzymes elongate the chain by adding to a free hydroxyl group so that synthesis occurs in the 5  3 direction. P P P 3 end 5 end

Overall direction of replication DNA polymerase molecule 3 5 Daughter strand synthesized continuously Parental DNA 5 3 Daughter strand synthesized in pieces 3 5 5 Figure 10.5C How daughter DNA strands are synthesized. Continuous synthesis and discontinuous synthesis are depicted in this figure. DNA synthesis beginning on the template oriented in the 3′  5′ direction can continue as the unwinding of the replication fork provides additional template in the direction of synthesis. Enzymes synthesizing DNA on the template oriented in the 5′  3′ direction are moving away from the replication fork, so short discontinuous fragments are produced. As more of the template strand is unwound, an enzyme can bind and synthesize the complementary strand. These short Okasaki fragments are joined by DNA ligase to form a continuous nucleotide chain. 3 DNA ligase Overall direction of replication

DNA replication- in two directions; many sites Proteins involved in DNA replication Helicase- scissors, opens like zipper DNA polymerase adds nucleotides to chain DNA ligase joins small fragments into a continuous chain Student Misconceptions and Concerns 1. The authors note that although the general process of semiconservative DNA replication is relatively simple, it involves complex biochemical gymnastics. The DNA molecule is unwound, each strand is copied simultaneously, the correct bases are inserted, and the product is proofread and corrected. Before discussing these details, be sure that your students understand the overall process, what is accomplished, and why each step is important. Teaching Tips 1. Demonstrate the complementary base pairing within DNA. Present students with the base sequence to one side of a DNA molecule and have them work quickly at their seats to determine the sequence of the complimentary strand. For some students, these sorts of quick practice are necessary to reinforce a concept and break up a lecture. 2. The authors note that the semiconservative model of DNA replication is like making a photo from a negative and then a new negative from the photo. In each new negative and photo pair, the new item was made from an old item. Copyright © 2009 Pearson Education, Inc.

3’ 5’ 5’ 3’ HELICASE

5’3’ 3’5’ 3’5’ 5’3’ LEADING DNA LIGASE DNA LIGASE LAGGING DNA POLYMERASE DNA POLYMERASE DNA POLYMERASE DNA POLYMERASE LEADING 5’3’ 3’5’ DNA LIGASE DNA LIGASE DNA LIGASE 3’5’ 5’3’ LAGGING

3’5’ 5’3’ 3’5’ 5’3’ 5’3’ 3’5’