DNA replication is the process by which genetic information is faithfully copied from one DNA molecule to another before cell division. It ensures that daughter cells receive an identical copy of the genetic material. Replication occurs in a 5′ → 3′ direction, following a semiconservative mechanism where each new DNA molecule contains one parental and one newly synthesized strand.
Phases and Components of DNA Replication
| Component | Function / Key Point | Clinical / Pharmacologic Note |
|---|---|---|
| Origin of Replication | Specific AT-rich sequence (TATA box) where DNA replication begins. | Single in prokaryotes, multiple in eukaryotes. |
| Replication Fork | Y-shaped area where new strands are synthesized. | Leading (continuous) and lagging (discontinuous / Okazaki fragments) strands. |
| Helicase | Unwinds DNA at the replication fork. | Deficiency → Bloom syndrome (BLM gene mutation). |
| Single-Stranded Binding Proteins (SSBP) | Stabilize unwound DNA; prevent reannealing. | Protect from nuclease degradation. |
| DNA Topoisomerases (I & II) | Relieve supercoils ahead of the replication fork. | Eukaryotes: Irinotecan/topotecan inhibit Topo I; Etoposide/teniposide inhibit Topo II. Prokaryotes: Fluoroquinolones inhibit DNA gyrase (Topo II) and Topo IV. |
| Primase | Synthesizes short RNA primers for DNA polymerase III. | Provides a 3′-OH group to initiate synthesis. |
| DNA Polymerase III | (Prokaryotes only) Adds deoxynucleotides to the 3′ end; the main elongating enzyme. | Proofreads with 3′ → 5′ exonuclease activity. Blocked by drugs with modified 3′-OH (chain termination). |
| DNA Polymerase I | (Prokaryotes only) Removes RNA primer, replaces it with DNA. | Has 5′ → 3′ exonuclease activity. |
| DNA Ligase | Seals nicks between Okazaki fragments by forming phosphodiester bonds. | “Ligase links DNA.” |
| Telomerase | (Eukaryotes only) Adds TTAGGG repeats to 3′ ends of chromosomes. | Upregulated in cancer and stem cells; downregulated in aging and progeria. |
Key Points to Remember
- DNA replication is bidirectional and semiconservative.
- Helicase defects → chromosomal instability (Bloom syndrome).
- Topoisomerase inhibitors are major chemotherapy drugs.
- Telomerase activity correlates with cellular immortality and tumorigenesis.
- Okazaki fragments form on the lagging strand and require ligase to join them.
Learning Objective
By the end of this session, the learner should be able to describe the enzymes and steps involved in DNA replication, explain how each contributes to fidelity and genome stability, and identify pharmacologic or pathologic relevance (e.g., topoisomerase inhibitors, Bloom syndrome, telomerase in cancer).
