Replication of eukaryotic DNA, DNA replication apparatus and its regulation

Replication of Eukaryotic DNA, Replication Apparatus and Its RegulationWhy DNA Needs to Be Copied

Before a cell divides, it has to make a copy of all its DNA so that each new cell gets the same blueprint. In humans, that's 6 billion genetic instructions copied. The cell does this incredibly fast and accurately during an S phase of the cell cycle.

Where It Starts

Eukaryotic chromosomes are quite big, and they do not begin replication from a single point. They use many origins of replication that are scattered all over the DNA. This helps the cell to get the job done earlier by working simultaneously in many places.

Each origin is built ahead of time, and not all of them are used simultaneously. Some fire early, some late, according to the needs of the cell.

The Copying Machinery

At each origin, a group of proteins come together to form a machine called the replisome. It is the cell's machine for copying DNA. Each part has a function:

Helicase unwinds the DNA double helix, like a zipper.

Single-stranded binding proteins move the strands apart.

DNA polymerase alpha starts by adding a short primer (a starting point).

DNA polymerase epsilon continues adding one strand. This is referred to as the leading strand.

DNA polymerase delta adds the other strand in tiny fragments. This is the lagging strand, made up of Okazaki fragments.

PCNA is a doughnut-shaped clamp that places the polymerase on the DNA.

Topoisomerase removes tension ahead of the fork by opening and closing the DNA.

Ligase seals the small pieces of DNA together into one strand.

Each replication fork has this entire team working to get the job done right.

How It's Controlled

To avoid duplicating the same DNA twice, the cell uses a careful system of control.

In the initial cell cycle (G1 phase), the origins are "licensed", they receive special proteins like ORC, Cdc6, and MCM, marking them as available.

Upon the transition of the cell to S phase, other proteins initiate the licensed origins and replication begins. After that, the licensing mechanism is severed until the next cycle, meaning no origin is reused.

This specific timing ensures DNA is duplicated once, and only once, before cell division.

References

1. Strachan T, Read AP. Human Molecular Genetics, 4th ed. Garland Science, 2010.
2. Watson JD et al. Molecular Biology of the Gene, 7th ed. Pearson, 2013.