DNA Replication (Updated)

DNA is the ultimate director for cells, coding for traits. When cells divide, new daughter cells need a copy of DNA, which introduces the topic of DNA replication – making more DNA. This process occurs in the nucleus of eukaryotic cells and in prokaryotic cells (without a nucleus) before cell division, specifically during interphase in eukaryotes, prior to mitosis or meiosis.

Several key enzymes facilitate DNA replication. Helicase, the 'unzipping enzyme,' unwinds the two strands of DNA by breaking hydrogen bonds. DNA Polymerase, the 'builder,' replicates DNA molecules to create new strands. Primase, the 'initializer,' creates an RNA primer that DNA polymerase needs to start building. Ligase, the 'gluer,' connects DNA fragments together.

DNA replication begins at a specific origin identified by DNA sequences. Helicase unwinds the DNA at this origin. Single-stranded binding proteins (SSB Proteins) keep the DNA strands separated, preventing them from rejoining. Topoisomerase prevents the DNA from supercoiling, ensuring the strands remain accessible for replication.

DNA strands are complementary and anti-parallel, meaning they run in opposite directions. The direction is determined by the numbering of carbons in the deoxyribose sugar. The 5' carbon is outside the ring structure, and the 3' carbon is part of the ring. Therefore, strands are referred to as running 5' to 3' or 3' to 5'.

Primase places RNA primers on both DNA strands. DNA Polymerase can only build new DNA strands in the 5' to 3' direction. The leading strand is synthesized continuously in the 5' to 3' direction as the DNA unwinds. The lagging strand, however, is synthesized discontinuously in fragments (Okazaki fragments) because DNA polymerase must repeatedly re-attach closer to the unwinding fork.

After Okazaki fragments are made on the lagging strand, the RNA primers are replaced with DNA bases. Ligase then 'glues' these DNA fragments together, sealing any gaps. The result is two identical double helix DNA molecules, each containing one old and one new strand (semi-conservative replication). DNA polymerase also has proofreading abilities, minimizing errors during replication. The detailed understanding of DNA replication has enabled life-saving medical treatments for various diseases.