Essential for cell replication and the cell cycle.
Cell cycle phases: G1, S phase (where DNA replication occurs), G2, Mitosis.
Key Concepts of DNA Replication
1. Semi-Conservative Model
Each new DNA molecule consists of one old (parental) strand and one new (daughter) strand.
Complementarity Rule: A pairs with T, G pairs with C.
2. Direction of Replication
DNA replication occurs from the 5' end to the 3' end.
Important to know the structure of nucleotides:
5' end = phosphate group
3' end = hydroxyl (OH) group.
3. Bi-Directional Replication
DNA unwinding creates replication forks at both ends.
Enzymes: Helicases unwind DNA, and DNA polymerases synthesize new DNA.
Steps of DNA Replication
1. Initiation
Origin of replication: Region rich in adenine and thymine, easier to separate.
Pre-replication protein complex binds to the origin and separates DNA strands.
Single-stranded binding proteins stabilize the separated strands and prevent re-annealing.
2. Elongation
Primase and RNA Primers
Primase lays down RNA primers for DNA polymerase to build off of.
Synthesizes RNA in a 5' to 3' direction using the template strand (reads from 3' to 5').
DNA Polymerase III uses the RNA primers to synthesize DNA continuously on the leading strand and in fragments (Okazaki fragments) on the lagging strand.
3. Removing RNA Primers
DNA Polymerase I removes RNA primers and replaces them with DNA nucleotides; includes proof-reading ability.
DNA Ligase connects Okazaki fragments on the lagging strand to form a continuous DNA strand.
4. Termination
Occurs when DNA polymerases reach the end of the DNA or meet at replication forks.
Telomeres
Telomeres shorten with each replication cycle; protect genes from being lost.
Hayflick limit: Maximum number of times a cell can replicate before it impacts genes.
Telomerase: Enzyme that extends telomeres, especially in stem cells and cancer cells.
Performs reverse transcription to add DNA to the ends of chromosomes.
Clinical Significance
Drugs targeting topoisomerases are used in cancer treatment (e.g., Irenotecan, Etoposide).
Nucleoside reverse transcriptase inhibitors (NRTIs) used in HIV treatment inhibit DNA replication in infected cells.
Cancer cells may upregulate telomerase, allowing them to continue replicating without losing important genes.
Conclusion
Understanding DNA replication is crucial in cell biology and medicine, particularly in cancer and genetic research.