The Molecular Basis of Inheritance

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Each strand acts as a template for building a new strand in replication

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In DNA replication

The parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

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DNA replication is semiconservative

Each of the two new daughter molecules will have one old strand, derived from the parent molecule, and one newly made strand

(a)

(b)

(c)

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Experiments performed by Meselson and Stahl

Supported the semiconservative model of DNA replication

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DNA Replication: A Closer Look

The copying of DNA

Is remarkable in its speed and accuracy

More than a dozen enzymes and other proteins

Participate in DNA replication

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Getting Started: Origins of Replication

The replication of a DNA molecule

Begins at special sites called origins of replication, where the two strands are separated

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A eukaryotic chromosome

May have hundreds or even thousands of replication origins

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Elongating a New DNA Strand

Elongation of new DNA at a replication fork

Is catalyzed by enzymes called DNA polymerases, which add nucleotides to the 3 end of a growing strand

Nucleoside

triphosphate

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Antiparallel Elongation

How does the antiparallel structure of the double helix affect replication?

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DNA polymerases add nucleotides

Only to the free 3end of a growing strand

Along one template strand of DNA, the leading strand

DNA polymerase III can synthesize a complementary strand continuously, moving toward the replication fork

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To elongate the other new strand of DNA, the lagging strand

DNA polymerase III must work in the direction away from the replication fork

The lagging strand

Is synthesized as a series of segments called Okazaki fragments, which are then joined together by DNA ligase

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