DNA is a molecule composed of two strands twisted into a double helix shape.
Each strand consists of a sequence of four chemical bases:
A (Adenine)
C (Cytosine)
G (Guanine)
T (Thymine)
The two strands are complementary:
A pairs with T
C pairs with G
Each strand has a 5' end and a 3' end, and they run in opposite directions.
DNA Replication Process
Unzipping the DNA
The first step in DNA replication is the separation of the two strands, known as unzipping.
This is facilitated by the enzyme helicase, resulting in a formation called the replication fork.
Template Creation
The separated strands serve as templates for creating new strands of DNA.
An enzyme called primase synthesizes a small piece of RNA called a primer, marking the starting point for new DNA synthesis.
DNA Strand Synthesis
DNA polymerase binds to the primer to begin synthesizing the new DNA strand.
DNA polymerase can only add bases in the 5' to 3' direction.
Leading and Lagging Strands
Leading Strand
Synthesized continuously as DNA polymerase adds bases one by one in the 5' to 3' direction.
Lagging Strand
Cannot be synthesized continuously due to its opposite direction.
Synthesized in small segments called Okazaki fragments.
Each fragment starts with an RNA primer, and DNA polymerase adds DNA bases in the 5' to 3' direction until the next primer is added.
Finishing the New DNA
Once new DNA strands are synthesized, the enzyme exonuclease removes all RNA primers from both strands.
Another DNA polymerase fills in the gaps left by the removed primers with DNA.
Finally, the enzyme DNA ligase seals the fragments to form a continuous double strand.
Semiconservative Nature of DNA Replication
DNA replication is described as semiconservative because each new DNA molecule consists of one original (conserved) strand and one newly synthesized strand.