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11.1 DNA Is Reproduced by Semiconservative Replication

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Section 11.1

• The complementarity of DNA strands allows each strand to serve as a template for synthesis of the other (Figure 11.1)

© 2012 Pearson Education, Inc. Figure 11.1

© 2012 Pearson Education, Inc.

Section 11.1

• Three modes of DNA replication are possible: – Conservative

• Original helix is conserved and two newly synthesized strands come together

– Semiconservative• Each replicated DNA molecule consists of one

"old" strand and one new strand

– Dispersive • Parental strands are dispersed into two new

double helices

(Figure 11.2)

© 2012 Pearson Education, Inc. Figure 11.2

© 2012 Pearson Education, Inc.

Section 11.1

• Meselson and Stahl (1958), using 15N-labeled E. coli grown in medium containing 14N, demonstrated that– DNA replication is semiconservative in

prokaryotes– each new DNA molecule consists of one old

strand and one newly synthesized strand

(Figure 11.3 and Figure 11.4)

© 2012 Pearson Education, Inc. Figure 11.3

© 2012 Pearson Education, Inc. Figure 11.4

© 2012 Pearson Education, Inc.

Section 11.1

• DNA replication begins at the origin of replication

• Where replication is occurring, the strands of the helix are unwound, creating a replication fork

• Replication is bidirectional; therefore, there are two replication forks(Figure 11.6)

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11.2 DNA Synthesis in Bacteria Involves Five Polymerases, as well as Other

Enzymes

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Section 11.2

• DNA polymerase catalyzes DNA synthesis and requires a DNA template and all four deoxyribonucleoside triphosphates (dNTPs) (Figure 11.7)

© 2012 Pearson Education, Inc. Figure 11.7

© 2012 Pearson Education, Inc.

Section 11.2

• Chain elongation occurs in the 5' to 3' direction by addition of one nucleotide at a time to the 3' end (Figure 11.8)

• As the nucleotide is added, the two terminal phosphates are cleaved off, providing a newly exposed 3'-OH group that can participate in the addition of another nucleotide as DNA synthesis proceeds

© 2012 Pearson Education, Inc. Figure 11.8

© 2012 Pearson Education, Inc.

Section 11.2

• DNA polymerases I, II, and III can elongate an existing DNA strand (called a primer) but cannot initiate DNA synthesis (Table 11.2)

• All three possess 3' to 5' exonuclease activity, allowing them to proofread newly synthesized DNA and remove and replace incorrect nucleotides

• Only DNA polymerase I demonstrates 5' to 3' exonuclease activity, excising primers and filling in the gaps left behind

© 2012 Pearson Education, Inc. Table 11.2

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Section 11.2

• DNA polymerase III is the enzyme responsible for the 5' to 3' polymerization essential in vivo

• Its 3' to 5' exonuclease activity allows proofreading

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Section 11.2

• DNA polymerases I, II, IV, and V are involved in various aspects of repair of DNA damaged by external forces such as UV light

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Section 11.2

• DNA polymerase III is a complex enzyme (holoenzyme) made up of 10 subunits whose functions are shown in Table 11.3

• The holoenzyme and some other proteins at the replication fork form a complex called the replisome

© 2012 Pearson Education, Inc. Table 11.3

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11.3 Many Complex Issues Must Be Resolved during DNA Replication

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Section 11.3

• There are seven key issues that must be resolved during DNA replication:– Unwinding of the helix – Reducing increased coiling generated during

unwinding – Synthesis of a primer for initiation – Discontinuous synthesis of the second strand

continued

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Section 11.3

continued– Removal of the RNA primers– Joining of the gap-filling DNA to the adjacent

strand– Proofreading

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Section 11.3

• To elongate a polynucleotide chain, DNA polymerase III requires a primer with a free 3'-hydroxyl group

• Primase synthesizes an RNA primer that provides the free 3'-hydroxyl required by DNA polymerase III (Figure 11.10)

• DNA polymerase I removes the primer and replaces it with DNA

• Priming is a universal phenomenon during initiation of DNA synthesis

© 2012 Pearson Education, Inc. Figure 11.10

© 2012 Pearson Education, Inc.

Section 11.3

• As the replication fork moves, only one strand can serve as a template for continuous DNA synthesis—the leading strand

• The opposite lagging strand undergoes discontinuous DNA synthesis (Figure 11.11)

© 2012 Pearson Education, Inc. Figure 11.11

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Section 11.3

• The lagging strand is synthesized as Okazaki fragments, each with an RNA primer

• DNA polymerase I removes the primers on the lagging strand, and the fragments are joined by DNA ligase

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Section 11.3

• Both DNA strands are synthesized concurrently by looping the lagging strand to invert the physical but not biological direction of synthesis (Figure 11.12)

© 2012 Pearson Education, Inc. Figure 11.12

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Section 11.3

• Proofreading and error correction are an integral part of DNA replication

• All of the DNA polymerases have 3' to 5' exonuclease activity that allows proofreading

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11.4 A Coherent Model Summarizes DNA Replication

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Section 11.4

• DNA synthesis at a single replication fork involves– DNA polymerase III– single-stranded binding proteins– DNA gyrase– DNA helicase– RNA primers

(Figure 11.13)

© 2012 Pearson Education, Inc. Figure 11.13

© 2012 Pearson Education, Inc.

Section 11.8

• Gene conversion is characterized by nonreciprocal genetic exchange between two closely linked genes (Figure 11.19)