Mechanism of DNA Replication

During DNA replication, both strands of the double helix act as templates for the formation of new DNA molecules. Copying occurs at a localized region called the replication fork, which is a Y-shaped structure where new DNA strands are synthesized by a multi-enzyme complex. Here, the DNA to be copied enters the complex from the left. One new strand is leaving at the top of frame and the other new strand is leaving at bottom.

The first step in DNA replication is the separation of the two strands by an enzyme called helicase. This spins the incoming DNA to unravel it: at 10,000 RPM in the case of bacterial systems. The separated strands are called three prime (3’) and five prime (5’), distinguished by the direction in which their component nucleotides join up. The 3’ DNA strand, also known as the leading strand, is diverted to a DNA polymerase and is used as a continuous template for the synthesis of the first daughter DNA helix. The other half of the DNA double helix, known as the lagging strand, has the opposite 3’-to-5’ orientation and consequently requires a more complicated copying mechanism. As it emerges from the helicase, the lagging strand is organized into sections called Okazaki fragments. These are then presented to a second DNA polymerase enzyme in the preferred 5’-to-3’ orientation. These sections are then effectively synthesized backwards. When the copying is complete, the finished section is released and the next loop is drawn back for replication.

Intricate as this mechanism appears, numerous components have been deliberately left out to avoid complete confusion. The exposed strands of single DNA are covered by protective binding proteins. And in some systems, multiple Okazaki fragments may be present. The molecular reality is very different from the iconic image of the double helix neatly separating into two DNA copies as so often depicted.

Developed by the DNA Learning Center at Cold Spring Harbor Laboratory in Cold Spring Harbor, New York, USA.


TOP:  Anatomy of a Bacteriophage
MIDDLE:  A Bacteriophage Attacking a Bacterium
BOTTOM:  What a Phage Does to its Host

The cycle begins when the virus uses its tail fibers to attach itself to its victim. The details of what happens next vary, but the process is always the same: the phage’s genetic material, which is located in its head, enters the bacterium.

Here, we’ll use T4, a well-studied phage infecting the Escherica coli bacterium, as an example.

(1) T4 contracts its tail sheath which pushes a tube located within the tail through the membrane of the bacterial cell.

(2) The phage’s DNA is passed through the tube into the cell, where it takes control, brutally stops many of its vital functions and forces it to churn out new virus components – heads, tails, tail fibers – in production-line style.

(3). Finally, enzymes dissolve the wall of the bacterium from the inside and the newborn bacteriophages reach the exterior, ready to attack new victims.

(4)  But these viruses proceed very selectively as they do so. Most of them attack only a subgroup of a single bacterial species. Generally, they don’t touch animal or human cells, which is why they are harmless to human beings.

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