Microcheck

DNA polymerases synthesize DNA in the 5' to 3' direction, using one strand as a template to generate the complementary strand. Replication of DNA begins at a specific sequence, called the origin of replication, and then proceeds bidirectionally, creating two replication forks.

■ Why is a primer required for DNA synthesis?

■ How does synthesis of the lagging strand differ from that of the leading strand?

■ If DNA replication were shown to be "conservative,'' what would this mean?

Synthesis of the leading strand proceeds continuously.

RNA primer

1. Primase synthesizes an RNA primer

Synthesis of the lagging strand is discontinuous; synthesis is reinitiated periodically, generating a series of fragments that are later joined.

Synthesis of the leading strand proceeds continuously.

RNA primer

1. Primase synthesizes an RNA primer

Synthesis of the lagging strand is discontinuous; synthesis is reinitiated periodically, generating a series of fragments that are later joined.

2. DNA polymerase adds nucleotides onto the 3' end of the fragment

2. DNA polymerase adds nucleotides onto the 3' end of the fragment

3. DNA polymerase replaces the RNA primer with deoxynucleotides

4. DNA ligase ' seals the gaps between adjacent fragments

Figure 7.6 The Replication Fork This simplified diagram of the replication fork highlights the key steps in the synthesis of the lagging strand.

3. DNA polymerase replaces the RNA primer with deoxynucleotides

4. DNA ligase ' seals the gaps between adjacent fragments

Figure 7.6 The Replication Fork This simplified diagram of the replication fork highlights the key steps in the synthesis of the lagging strand.

Replication

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