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6.2 DNA Replication

Syllabus
2025
Topic
6.2
Level
—

Learning objectives

6.2A—Describe the mechanisms by which genetic information is copied for transmission between generationsDescribe the mechanisms by which genetic information is copied for transmission between generations.• DNA replication ensures continuity of hereditary information.- i. DNA is synthesized in the 5’ to 3’ direction.- ii. R eplication is a semiconservative proc ess, meaning one strand of DNA serves as the template for a new strand of complementary DNA.- iii. Helicase unwinds the DNA strands.- iv. T opoisomerase relaxes supercoiling in front of the r eplication fork.- v. DNA polymerase requires RNA primers to initiat e DNA synthesis.- vi. DNA polymerase synthesizes new strands of DNA c ontinuously on the leading strand and discontinuously on the lagging strand.- vii. Ligase joins the fragments on the lagging strand.- Exclusion: The names of the steps and particular enzymes involved, excluding DNA polymerase, ligase, RNA polymerase, helicase, and topoisomerase, are beyond the scope of the AP Exam.

How a Replication Fork Copies DNA

DNA replication is semiconservative: each completed DNA molecule contains one parental strand and one newly synthesized complementary strand. This preserves hereditary information by using each original strand as a template.

Component Role at the replication fork
Helicase Unwinds and separates the parental DNA strands
Topoisomerase Relaxes supercoiling ahead of the replication fork
RNA primer Provides the starting point required by DNA polymerase
DNA polymerase Adds complementary DNA nucleotides to synthesize new DNA in the 5′→3′ direction
Ligase Joins DNA fragments made on the lagging strand

Because the parental strands are antiparallel but DNA polymerase synthesizes only 5′→3′, the two new strands cannot be made in the same physical pattern. The leading strand is synthesized continuously toward the advancing fork. The lagging strand is synthesized discontinuously as short fragments, which ligase joins into one continuous strand.

Follow one fork: helicase opens the strands; topoisomerase reduces twisting ahead of the fork; RNA primers provide starting points; DNA polymerase extends complementary DNA 5′→3′; ligase seals the lagging-strand fragments. The result is two DNA molecules, each with one old strand and one new strand.

Both new strands are synthesized 5′→3′. ‘Lagging’ means discontinuous synthesis, not synthesis in the 3′→5′ direction. For the AP Exam, additional step names and enzymes beyond DNA polymerase, ligase, RNA polymerase, helicase, and topoisomerase are outside the required scope.