DNA replication or DNA synthesis is the synthesis of new DNA. This is a complicated process, which involves multiple enzymes.
The overall process is illustrated in the picture to the right. The double helix DNA unzips, and a copy is made of each strand, using the base pairing rule. You’ll notice in the picture to the right that the newly formed DNA strands are identical to each other.
This process of copying each strand is referred to as semi-conservative.
There were multiple theories about DNA replication. The one of the initial thoughts was the entire double stranded DNA makes a copy of its self. This was the conservative model. Another hypothesis was the dispersal model. This is where DNA makes copies of its self in sections.
The original strand is referred to as the Parental or template strand. The newly synthesised DNA is known as the daughter strand.
Before reading about DNA synthesis, make sure you have read the basic s of DNA (click here).
When DNA undergoes replication the double stranded DNA must open. The point where it starts too opens is know as the origin of replication. As the DNA splits, it creates the replication fork.
This is shown in the animation to the right.
Scroll over each word bellow to understand its position is during DNA replication.
Taking a closer look at the DNA synthesis process we see the enzyme ‘Helicase’ unzipping the strands, at the replication fork.
Helicase breaks the hydrogen bonds between the bases, then specialised proteins will bind to the single DNA strand preventing it from Re-coiling too soon.
As you can see the DNA is split into 2 strands. A ‘Leading’ strand an ‘Lagging’ strand.
Lets first look at the Leading strand.
DNA polymerase adds new nucleotides to the growing strand. DNA polymerase can only add nucleotides to the 3’ end.
The leading strand synthesises the daughter strand continuously.
Now lets look at the Lagging strand.
This is more complicated than the leading strand because it is discontinuous. Meaning it synthesises a new piece of DNA in sections.
The first step is for RNA Primase to lay down a Primer. This is a small section of RNA which tells DNA polymerase where to start.
The animation to the left is how DNA replication would look at a distance.
The red strands are the newly formed DNA, while the black are the parent strands.
Notice how division starts at specific point in the DNA and expands outwards from both ends.
The final step is to get rid of the RNA. DNA Polymerase 1 cuts out the RNA section and replaces it with the matching DNA segment. Creating on continues DNA piece
DNA polymerase then starts to synthesise a new DNA strand backwards. It does this in segments called Okazaki fragments.
As you can see in the animation to the left, the small green DNA segments synthesised are Okazaki fragments.
