DNA
Structure and replication of DNA - syllabus content
Structure of DNA — nucleotides contain deoxyribose sugar, phosphate and base.
DNA has a sugar–phosphate backbone, complementary base pairing — adenine with thymine and
guanine with cytosine.
The two DNA strands are held together by hydrogen bonds and have an antiparallel structure, with
deoxyribose and phosphate at 3' and 5' ends of each strand.
Chromosomes consist of tightly coiled DNA and are packaged with associated proteins.
Structure of DNA – what you should know
Structure of DNA
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Composed of nucleotides
Nucleotides contain deoxyribose sugar, phosphate and base.
DNA has a sugar–phosphate backbone,
Complementary base pairing — adenine with thymine and guanine with
cytosine.
• The two DNA strands are held together by hydrogen bonds and have an
antiparallel structure, with deoxyribose and phosphate at 3' and 5' ends of
each strand.
Chromosomes consist of tightly coiled DNA and are packaged with
associated proteins.
DNA
DNA is a nucleic acid
Like other nucleic acids it is composed of smaller molecules called nucleotides
A DNA nucleotide molecule has 3 parts
• deoxyribose sugar
• phosphate
phosphate
• a base
Diagram representing a DNA
nucleotide
Deoxyribose sugar
base
DNA bases
A DNA nucleotide can have one of 4 possible bases:
• Adenine (A)
• Thymine (T)
• Guanine (G)
• Cytosine (C)
Therefore there are 4 possible DNA nucleotides each with a different
base
G
A
T
C
DNA nucleotides are linked by covalent bonds between sugar and phosphate to form DNA strands
Position of sugar to
phosphate bonds linking
nucleotides to form a DNA
strand
A DNA molecule has two strands of nucleotides
The two strands are antiparallel (run in opposite directions)
Phosphate is found at the 5’ end of a strand and deoxyribose at the 3’ end
The two strands held together by hydrogen bonds between complementary
bases
Adenine bonds to thymine
Guanine bonds to cytosine
The two strands are coiled into a double helix
Arrangement of DNA in Chromosomes
Chromosomes are made from tightly coiled DNA.
The DNA is wrapped around molecules of protein.
DNA
Protein
DNA replication – syllabus content
Replication of DNA by DNA polymerase and primer.
DNA is unwound and unzipped to form two template strands.
DNA polymerase needs a primer to start replication and can only add complementary DNA nucleotides
to the deoxyribose (3') end of a DNA strand.
This results in one strand being replicated continuously and the other strand replicated in fragments
which are joined together by ligase.
DNA replication – what you should know
DNA molecule is unwound and hydrogen bonds break to allow the strands to
separate
DNA polymerase is the enzyme that joins nucleotides to form a DNA strand
Primer is a short section of joined nucleotides
Primers bind to the separated DNA template strands
DNA polymerase can only begin replication by adding nucleotides to these
short nucleotide strands
DNA polymerase can only add nucleotides to the 3’ end of a strand
This results in one new DNA strand, the leading strand, being formed
continuously
The other strand, the lagging strand, is formed in short fragments which are
then joined together by the enzyme DNA ligase.
DNA replication
Unwinding and unzipping the parent DNA molecule
The DNA molecule is unwound by an enzyme.
Another enzyme breaks the hydrogen bonds between base pairs and the two
strands separate.
The bases of each strand are now exposed at a Y shaped replication fork - the
exposed bases act as templates for making new complementary DNA strands
Need for primers
The enzyme DNA polymerase joins nucleotides to make a new DNA strand.
This enzyme can only add nucleotides to a pre-existing nucleotide chain – so for DNA polymerase to join nucleotides a
primer must be present.
A primer consists of a short sequence of nucleotides.
5’ 3’
Leading and lagging strands
Since the two strands of the DNA molecule are antiparallel,
one of the separated strands ends with a deoxyribose 3’ end
(the complementary strand made from this is called the
leading strand),
the other strand ends with a 5’ end (the complementary
strand made from this is the lagging strand)
3’
5’
DNA copied here is DNA copied here is
the leading strand the lagging strand
DNA polymerase
Nucleotides are joined together to make a DNA strand by the enzyme DNA polymerase.
DNA polymerase can only add nucleotides to the 3’ end of a DNA strand.
Formation of the leading DNA strand
5’
A primer forms at the 3’ end of the parental DNA strand.
3’
Individual nucleotides for the new DNA strand form
hydrogen bonds with their complementary bases on the
DNA template strand.
DNA polymerase joins a nucleotide to the primer and adds
the other nucleotides to the growing DNA strand
This leading strand is formed continuously.
Individual DNA nucleotides bind by
hydrogen bonds between bases to
DNA template strand
Hydrogen bonds
Primer binds to 3’ end
of parent DNA strand
3’ end of primer
3’
5’
5’ end of primer
Formation of the lagging DNA strand
Since DNA polymerase can only add nucleotides to the 3’ end
of a strand, primers bind at various points to the DNA template
The DNA template strand that has the 5’ end is replicated in
fragments, each starting at the 3’ end of a primer.
When completed these fragments are joined together by an
enzyme called ligase.
‘the strand formed in this way is called the lagging strand.
Formation of the lagging strand
Since DNA polymerase can only add nucleotides to the 3’ end of a
strand, primers bind at various points to the DNA template
The DNA template strand that has the 5’ end is replicated in
fragments, each starting at the 3’ end of each primer.
When completed these fragments are joined together by
an enzyme called ligase.
The strand formed in this way is called the lagging strand.
When DNA replication occurs
DNA replication occurs before a cell divides.
It results in a copy being made of the genes so that each daughter cell receives a complete
copy of the genetic information.
DNA replication results in the chromatids being copied so that each chromosome has two
chromatids when the cell is about to divide
result
1 chromatid
2 identical
chromatids