DNA Structure and
Replication
Structure of DNA
DNA is composed of a two strands of nucleotides each
containing a deoxyribose sugar, a phosphate, and a
nitrogenous base
These structures then twist together to form a double
helix shape like a twisted ladder
Structure of DNA
Structure of DNA
The bases of one DNA strand are paired to the bases of
the other
A purine (2 rings) is ALWAYS paired with a pyrimidine
(1 ring)
Adenine pairs with thymine and guanine pairs with
cytosine
Structure of DNA
Why can’t A bond with C or T with G?
Structure of DNA
Hydrogen bonds hold the strands together
Single hydrogen bonds are weak, but the collective
bonding strength of DNA makes it a very stable
molecule
Structure of DNA
The sequence of bases on any one strand of DNA
varies greatly between species but the opposite strand
will always complement the first strand
5’-ATGCCGTTA-3’
3’-TACGGCAAT-5’
Structure of DNA
The two strands of DNA are said to be antiparallel parallel in opposite directions
The direction of the strands are very important when
enzymes interact with the DNA as they tend to read or
copy DNA in only one direction
DNA Replication
DNA replication is the process by which a cell makes an
exact copy of its DNA
The main stages of replication are the same in both
prokaryotic (bacteria) and eukaryotic (plant and
animal) cells
DNA Replication
Replication is semiconservative which means that two
parent strands are separated and two daughter strands
are created using them
Hydrogen bonds between the bases break completely
allowing the DNA helix to unzip
DNA Replication
Each of the parent strands acts as a template to create
a complementary strand
Separating the Strands
DNA is separated using the enzyme DNA helicase
DNA helicase unwinds the helix by breaking hydrogen
bonds between the bases
To prevent the bonds from re-forming, proteins binds
to the DNA molecule keeping the strands apart
Separating the Strands
The point where the strands have separated is known
as the replication fork
The leading strand moves toward the replication fork
and the lagging strand moves away from the
replication fork
Building the Complementary
Strands
New strands of DNA are synthesized using an enzyme
called DNA polymerase III (DNA pol III)
DNA pol III links together free nucleotides that are
complementary to the template strand
Building the Complementary
Strands
A short piece of single stranded RNA called a primer is
attached to the template strand
This gives DNA pol III somewhere to start making the
new DNA strand
DNA pol III only works in ONE direction (5’ → 3’)
Building the Complementary
Strands
The leading strand (towards replication fork) is
synthesized continuously during DNA replication
The lagging strand (away from replication fork) is
synthesized in short fragments that are later joined
together - these short fragments are known as Okazaki
fragments
Building the Complementary
Strands
The lagging strand requires multiple RNA primers to
complete replication
These primers are then cut from the lagging strand and
replaced by DNA nucleotides using DNA polymerase I
(DNA pol I)
DNA ligase also is used to link the backbones of the
fragments together
DNA Repair
As new DNA strands are synthesized and new nucleotides are
added on, errors can occur
DNA pol I and DNA pol III act as quality control checkers by
proofreading the newly synthesized strands
If there is an error, they will cut it out and replace it with the correct
base
Repair must occur immediately to ensure it is not passed on
Questions
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