Replication in Prokaryotes

Prokaryotic DNA replication ensures that genetic material is accurately passed to daughter cells during cell division.

It begins when initiator proteins bind to the origin of replication, a specific DNA sequence, and form a complex.

This binding locally unwinds the DNA at the origin, creating a small opening in the double helix.

Then, the enzyme DNA helicase binds to the complex and unwinds the DNA further, forming a replication bubble.

The newly exposed strands are stabilized by single-stranded DNA-binding proteins.

The replication bubble now contains two replication forks that move in opposite directions.

Ahead of the forks, topoisomerase enzymes bind to the DNA and reduce torsional strain as the molecule unwinds.

Once the strands are separated, another enzyme, primase, synthesizes an RNA primer, which is a short stretch of RNA that is complementary to the DNA sequence.

The primer provides a starting point where DNA polymerase can add nucleotides that match the template strand and create a new DNA strand during elongation.

DNA polymerase III synthesizes DNA in the five-prime to three-prime direction, so synthesis of this strand, called the leading strand, proceeds continuously.

The other strand, called the lagging strand, has the opposite orientation to the direction of replication fork movement. DNA polymerase can add nucleotides only in the 5’ to 3’ direction. To overcome this constraint, DNA is synthesized in short pieces called Okazaki fragments. Each fragment starts from a new RNA primer and grows in the direction opposite to the movement of the replication fork.

The RNA primers are then removed by DNA polymerase I and replaced with DNA. The DNA fragments are joined together by the enzyme DNA ligase, forming a continuous strand.

Termination occurs when the two replication forks converge at specific Ter sites, where termination proteins bind and halt further replication, ensuring complete duplication of the DNA.

This process produces two identical circular DNA molecules. Each molecule contains one original strand and one newly synthesized strand, and is described as semiconservative.