Basic Molecular Biology: – DNA Replication
How DNA Replicates
Have you ever wondered how DNA gets passed on from cell to cell? Before a cell divides,
the DNA strands unwind and separate. Each strand then makes a new complementary
strand by adding the appropriate nucleotides. This process is known as replication.
In molecular diagnostics, replication plays a crucial role in a technique called Polymerase
Chain Reaction (PCR).
The Process of Replication
The replication process begins when helicase enzymes unwind the double helix structure of
DNA, exposing two single DNA strands and creating two replication forks. For teaching
purposes, we will focus on one replication fork.
To prevent the single DNA strands from reannealing or snapping back together, single-
strand binding proteins (SSBs) coat them. The DNA strand that runs in the 3' to 5' direction
toward the fork is called the leading strand, while the opposite strand is called the lagging
strand.
Initiating Replication
To give DNA Polymerase 3 a starting point, an enzyme called DNA primase copies a short
stretch of the DNA strand, creating a complementary RNA segment known as a primer.
Using this primer, DNA Polymerase 3 can now copy the DNA strand. It starts at the 3' end
of the RNA primer and, guided by the original DNA strand, synthesizes a new
complementary DNA strand.
Please note that DNA Polymerase 3 can only synthesize DNA in the 5' to 3' direction. The
sliding clamp helps hold DNA Polymerase 3 in place as it moves down the strand.
The Leading Strand
Due to the orientation of the leading strand, it can be replicated continuously by DNA
Polymerase 3.
The Lagging Strand
On the other hand, due to the orientation of the lagging strand, DNA Polymerase 3 is
forced to release and slide further upstream to begin extension from another RNA primer.
This results in the lagging strand being made discontinuously in short pieces called
Okazaki fragments, which are later joined together.
As replication proceeds, RNase H recognizes and removes RNA primers bound to the DNA
template. DNA Polymerase 3 then fills in the gap left by RNase H. The process of DNA
replication is completed when the enzyme DNA ligase joins the short DNA pieces together
into one continuous strand.
By- Topper_Notes
How DNA Replicates
Have you ever wondered how DNA gets passed on from cell to cell? Before a cell divides,
the DNA strands unwind and separate. Each strand then makes a new complementary
strand by adding the appropriate nucleotides. This process is known as replication.
In molecular diagnostics, replication plays a crucial role in a technique called Polymerase
Chain Reaction (PCR).
The Process of Replication
The replication process begins when helicase enzymes unwind the double helix structure of
DNA, exposing two single DNA strands and creating two replication forks. For teaching
purposes, we will focus on one replication fork.
To prevent the single DNA strands from reannealing or snapping back together, single-
strand binding proteins (SSBs) coat them. The DNA strand that runs in the 3' to 5' direction
toward the fork is called the leading strand, while the opposite strand is called the lagging
strand.
Initiating Replication
To give DNA Polymerase 3 a starting point, an enzyme called DNA primase copies a short
stretch of the DNA strand, creating a complementary RNA segment known as a primer.
Using this primer, DNA Polymerase 3 can now copy the DNA strand. It starts at the 3' end
of the RNA primer and, guided by the original DNA strand, synthesizes a new
complementary DNA strand.
Please note that DNA Polymerase 3 can only synthesize DNA in the 5' to 3' direction. The
sliding clamp helps hold DNA Polymerase 3 in place as it moves down the strand.
The Leading Strand
Due to the orientation of the leading strand, it can be replicated continuously by DNA
Polymerase 3.
The Lagging Strand
On the other hand, due to the orientation of the lagging strand, DNA Polymerase 3 is
forced to release and slide further upstream to begin extension from another RNA primer.
This results in the lagging strand being made discontinuously in short pieces called
Okazaki fragments, which are later joined together.
As replication proceeds, RNase H recognizes and removes RNA primers bound to the DNA
template. DNA Polymerase 3 then fills in the gap left by RNase H. The process of DNA
replication is completed when the enzyme DNA ligase joins the short DNA pieces together
into one continuous strand.
By- Topper_Notes
