\textbf{DNA Replication in Living Cells}
In living cells, such as $E. coli$, the process of replication requires a set of enzymes. The main enzyme is DNA-dependent DNA polymerase. It catalyzes the polymerization of DNA using a DNA template.
$E. coli$ has $4.6 \times 10^6$ base pairs $(bp)$ and completes replication in $18$ minutes, meaning the rate of polymerization is approximately $2000$ $bp$ per second.
Replication is an energy-expensive process. Deoxyribonucleoside triphosphates serve dual purposes: they act as substrates and provide energy for polymerization (the two terminal phosphates are high-energy bonds).
Besides DNA polymerase, other enzymes are required to complete the process:
$1.$ Helicase: Unwinds the DNA helix at the replication fork.
$2.$ Primase: Synthesizes a short RNA primer to initiate replication.
$3.$ DNA ligase: Joins the discontinuous Okazaki fragments on the lagging strand.
DNA polymerase can only catalyze polymerization in the $5' \rightarrow 3'$ direction. Consequently, on the template strand with $3' \rightarrow 5'$ polarity, replication is continuous, while on the template strand with $5' \rightarrow 3'$ polarity, it is discontinuous.
DNA replication does not initiate randomly; it starts at specific regions called the origin of replication $(ori)$.
In eukaryotes, replication occurs during the $S$-phase of the cell cycle. Failure to undergo cell division after DNA replication results in polyploidy (chromosomal abnormalities).