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Understanding DNA Structure and Replication
Oct 30, 2024
DNA Structure and Replication
DNA Structure
DNA is a molecule composed of two strands.
Strands are twisted in a double helix shape.
Comprised of four chemical bases: A, C, G, T.
Strands are complementary:
T pairs with A
C pairs with G
Each strand has a 5' end and a 3' end.
Strands run in opposite directions.
DNA Replication Process
Separation of Strands
Done by the enzyme
helicase
.
Results in a
replication fork
.
Template Creation
Separated strands serve as templates for new DNA.
Initiation by Primase
Enzyme
primase
synthesizes a small piece of RNA called a
primer
.
Primer marks the starting point for new strand construction.
DNA Synthesis by Polymerase
DNA polymerase
binds to the primer to create the new DNA strand.
Can only add bases in the
5' to 3' direction
.
Leading and Lagging Strands
Leading Strand
Synthesized continuously.
DNA polymerase adds bases one by one in the 5' to 3' direction.
Lagging Strand
Synthesized in small chunks called
Okazaki fragments
.
RNA primers are added to start each fragment.
DNA polymerase adds bases in the 5' to 3' direction for each fragment.
Finalization of DNA Replication
Removal of RNA Primers
Exonuclease
removes all RNA primers from both DNA strands.
Filling in Gaps
Another DNA polymerase fills in gaps left by removed primers.
Sealing Fragments
The enzyme
DNA ligase
seals all fragments to form a continuous double strand.
Semi-Conservative Nature
DNA replication is described as
semi-conservative
.
Each new DNA molecule consists of one old (conserved) strand and one new strand.
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