DNA Replication Process Overview

DNA Replication: Steps of Formation

• DNA replication is the process where DNA creates a copy of itself when a cell divides, providing the daughter cells with a copy of the parent cell's DNA.
• In eukaryotes, DNA replication occurs in the nucleus during the S-phase of interphase.

Rules of DNA Replication in Eukaryotes

• DNA replication is semiconservative: Each DNA strand serves as a template to synthesize a new strand. Each new DNA molecule consists of one original and one new strand.
• Replication begins at multiple origins and usually proceeds bidirectionally. Multiple origins allow for rapid replication of eukaryotic DNA.
• Replication exhibits polarity: DNA synthesis proceeds in a 5' to 3' direction, and is semi-discontinuous.
• Replication is highly accurate: It proceeds with a high degree of fidelity.

Identification of Origins of Replication

• A human chromosome contains millions of base pairs. Replication would be too slow with only a single origin.
• Replication starts at numerous sites, called origins of replication, spaced 30,000 to 300,000 base pairs apart on the chromosome.
• A replication fork (where DNA strands unwind) forms at each origin, and proceeds in both directions until it reaches the end of the chromosome.

Steps of DNA Replication

• Step 1: DNA Unwinding: DNA helicases break hydrogen bonds between nitrogenous bases to separate DNA strands. Single-stranded DNA binding proteins prevent the separated strands from rejoining. DNA topoisomerases relieve supercoiling ahead of the replication fork.
• Step 2: RNA Primer Synthesis: RNA primase synthesizes short RNA primers (8-12 base pairs long) providing a 3' hydroxyl group for DNA polymerase to start adding nucleotides.
• Step 3: DNA Polymerization: DNA polymerase adds nucleotides to the 3' end of the RNA primer, extending the new DNA strand in a 5' to 3' direction. The leading strand is synthesized continuously, and the lagging strand is synthesized discontinuously in short fragments (Okazaki fragments).
• Step 4: Degrading RNA Primers: RNA primers are removed by a 5' to 3' exonuclease.
• Step 5: Ligation: DNA ligase joins Okazaki fragments, completing the lagging strand.

DNA Replication: Protective Measures Against Abnormalities

Proofreading

• DNA polymerases (ε, δ, and γ) proofread base-pair mismatches.
• Mismatches result in irregularities, activating a 3' to 5' exonuclease activity in the DNA polymerase.
• The incorrect nucleotide is removed and replaced with the correct one, minimizing errors.

Telomeres and Telomerase

• Telomeres are repetitive DNA sequences at the ends of linear chromosomes protecting them from damage.
• Telomerase is a reverse transcriptase enzyme that synthesizes telomere sequences at chromosome ends. This prevents shortening during replication.
• Absence of telomerase activity during cell division leads to the loss of telomere fragments.
• Telomeres are like a mitotic clock, their length relates to a cell's reproductive potential.

Clinical Implications

• Different steps of DNA replication are targets for anticancer and antimicrobial drugs affecting replication, inducing errors or preventing fragments of new DNA from joining.
• Drugs impact DNA replication by interfering with chain elongation or ligation.
  • Some examples: acyclovir against Herpes simplex virus, quinolone antibiotics.
• Reverse transcriptases are RNA-directed DNA polymerases, seen in retroviruses like SARS-CoV-2 (COVID-19), HIV, or hepatitis C, where RNA is transcribed into DNA. The reverse transcription step can be targeted by drugs to inhibit viral replication.

This quiz covers the essential steps of DNA replication, particularly in eukaryotic cells. Learn about the rules governing replication, the importance of origins of replication, and the semi-conservative nature of DNA synthesis. Test your understanding of how DNA accurately copies itself during cell division.