DNA Replication Steps to Know for AP Biology

DNA replication is a vital process that ensures genetic information is accurately copied for cell division. Understanding the steps involved, from unwinding the DNA to sealing the final strands, is essential in fields like genetics and molecular biology.

1. DNA unwinding by helicase

   • Helicase enzymes bind to the DNA at the replication fork and separate the two strands.
   • This unwinding creates two single-stranded DNA templates for replication.
   • Energy from ATP hydrolysis is used to break the hydrogen bonds between base pairs.

2. Primer synthesis by primase

   • Primase synthesizes short RNA primers complementary to the DNA template.
   • These primers provide a starting point for DNA polymerases to begin synthesis.
   • Primers are essential for both leading and lagging strand synthesis.

3. Leading strand synthesis by DNA polymerase III

   • DNA polymerase III adds nucleotides continuously in the 5' to 3' direction on the leading strand.
   • It requires only one RNA primer to initiate synthesis.
   • This process is fast and efficient, allowing for rapid DNA replication.

4. Lagging strand synthesis and Okazaki fragments

   • The lagging strand is synthesized discontinuously in short segments called Okazaki fragments.
   • Each fragment requires a new RNA primer for initiation.
   • DNA polymerase III synthesizes these fragments in the 5' to 3' direction, moving away from the replication fork.

5. Removal of RNA primers

   • RNA primers are removed by the enzyme RNase H and other exonucleases.
   • This step is crucial to ensure that only DNA remains in the newly synthesized strand.
   • The removal of primers creates gaps that need to be filled in.

6. Gap filling by DNA polymerase I

   • DNA polymerase I fills in the gaps left by the removal of RNA primers with DNA nucleotides.
   • It also has exonuclease activity to proofread and correct any errors during this process.
   • This ensures the integrity of the newly synthesized DNA strand.

7. Nick sealing by DNA ligase

   • DNA ligase seals the nicks between Okazaki fragments, creating a continuous DNA strand.
   • It catalyzes the formation of phosphodiester bonds between adjacent nucleotides.
   • This step is essential for the structural integrity of the DNA molecule.

8. Proofreading and error correction

   • DNA polymerases have proofreading capabilities to correct mismatched nucleotides.
   • They can remove incorrectly paired bases and replace them with the correct ones.
   • This process significantly reduces the mutation rate during DNA replication.

9. Telomere replication

   • Telomeres are repetitive DNA sequences at the ends of linear chromosomes.
   • They protect chromosomes from degradation and prevent loss of genetic information during replication.
   • Telomerase extends telomeres in certain cells, allowing for continued cell division.

10. Topoisomerase action to relieve supercoiling

    • Topoisomerases alleviate the torsional strain generated ahead of the replication fork.
    • They create temporary breaks in the DNA strands to unwind supercoiled regions.
    • This action is crucial for maintaining the stability and integrity of the DNA during replication.