DNA Structure
DNA is a polymer made up of
individual units called nucleotides
Nucleic Acids
• Polymer
• Individual unit called a (mono)nucleotide
• DNA is a type of nucleic acid
– deoxyribonucleic acid
Nucleotide Structure
3 components:
– Pentose sugar
(ribose in DNA)
– Phosphoric acid
– Organic base
Phosphate
(always contains nitrogen)
Stay the same
base
sugar
Pentose sugar
(5 Carbon atoms)
Changes
Contains
nitrogen &
carbon
4 Bases
• Purines –
– Double ringed structure
• Pyrimidines –
– Single ringed structure
Bondings
• The base and sugar join with a:
glycosidic bond
• The phosphate and sugar join with an:
ester bond
Both require a condensation reaction to
occur

Phosphate
2 condensation reactions
occur in the formation of
a nucleotide:
H
Ester
bond
OH
Sugar
Glycosidic
OH
H
bond
Base
Compl mentary Base Pairing
*Purines always pair with pyrimidines*
A joins to T (2 hydrogen bonds)
C joins to G (3 hydrogen bonds)
Therefore, when discussing the proportions of a particular
base, you will always find A=T and C=G
or!
Charagaff’s Rule!
A+C = G+T
Type of Base
Purine
Pyrimidine
Adenine
Thymine
Guanine
Cytosine
Structure
Bonding
relationships
= hydrogen bond
Phosphodiester Bonds:
the sugar-phosphate-backbone
5’
3C
5C
G
C
5C
3’
3C
5C
C
G
0.34nm
5C
3C
5C
T
A
5C
3’
3C
3C
5C
A
5’
T
5C
3C
2nm
Strands run anti parallel
Points to remember:
• DNA stands for deoxyribonucleic acid
• It is a polymer of mono
• Each nucleotide has three components:
• pentose sugar (deoxyribose)
• phosphate group
• organic base (4 types - purines/pyrimidines)
• 2 condensation reactions occur when the
• base and sugar form a glycosidic bond
• phosphate and sugar form an ester bond
• Phosphodiester bonds build a sugar phosphate backbone on
each strand of DNA, with the bases all facing inwards
• The 2 strands of the DNA molecule form anti-parallel to one
another; one strand runs 5’ to 3’ and the other strand runs 3’ to 5’
• The strands wind around one another into a double helix (like a
twisted ladder)
Tell me…
4 Names of bases in DNA
3 Components of a nucleotide
2 Place where a condensation reaction
occurs
1
Difference between purine and
pyrimidine bases
4 Bases
• Purines –
– Double ringed structure
• Pyrimidines –
– Single ringed structure
Chromosomes
• DNA is located in the cell nucleus is the
form of chromosomes
– The number of chromosomes varies
Chromosome Structure
Chromosome Structure
• Chromatin: Tightly
packed together DNA
and proteins
• Histones: Proteins
• Chromatin consists of
DNA that is tightly coiled
around Histones.
Nucleosomes
• Together, the DNA and histone
molecules form a beadlike
structure called a nucleosome
• Function of nucleosomes is to
fold enormous lengths of DNA
into the tiny space available in
the cell nucleus
DNA Replication – why?
• Cells divide for an organism to grow or
reproduce,
– Every new cell needs a copy of the DNA or
instructions to know how to be a cell!
• DNA replicates right before a cell divides.
Movie time!
https://www.khanacademy.org/science/biology/crash-course-biology-science/v/crash-course-biology110
Key words to look out for:
– Replication fork
– Template
– Complementary base pairing
– Leading strand
– Lagging strand
– Okazaki fragments
– Enzymes:
– helicase
– dna polymerase
– ligase
Replication
• New strand synthesized in
5’ -> 3’ direction b/c DNA
Polymerase can only work
in 3’ -> 5’ direction
• DNA Template: previously
existing DNA on which the
new DNA is to be synthesized
• Initiation: DNA is unwound by
DNA helicase
– Results in the formation of
a Y shaped structure
known as the replication
fork
Replication Cont.
• DNA helicase
completes the splitting
of the strand
• Meanwhile, free
nucleotides that have
been activated are
attracted to their
complementary bases
– Each chain acts as a
template
DNA Polymerase adds
nucleotides (and proofreads, and replaces RNA
primers with DNA
nucleotides)
Leading and Lagging Strands
• DNA Polymerase only reads in
the 3’ -> 5’ direction
• Therefore, during replication
the leading strand is replicated
continuously (3’ -> 5’ on PS)
• Lagging-strand replication is
discontinuous, with short
Okazaki fragments being
formed and later linked
together (5’ -> 3’ on PS).
Primase catalyzes the synthesis of
a short RNA segment called a
primer complementary to the DNA
template. Primase is of key
importance in DNA replication
because no known DNA
polymerases can initiate the
synthesis of a DNA strand without
an initial RNA or DNA primer).
Replication Cont.
• The new strand must be sealed up
– DNA Ligase: the enzyme that seals the
phosphate-sugar backbone of DNA together
– DNA Replication
Enzymes of Replication
Rep. Enzymes
DNA Helicase
DNA Polymerase
Primase
DNA Ligase
Function
Enzymes of Replication
Rep. Enzymes
Function
DNA Helicase
• Unzips the DNA base pairs at initiation
DNA Polymerase
• Adds nucleotides (and proof-reads, and
replaces RNA primers with DNA
nucleotides)
• Can only read in the 3’ -> 5’ dir.
Primase
• RNA primase sticks an RNA primer onto the singlestranded DNA, and the DNA polymerase can then
elongate the new strand by sticking a DNA
nucleotide onto the end of the RNA primer, and then
continuing to stick DNA nucleotides onto the end of
the newly forming DNA strand.
DNA Ligase
• Enzyme that seals the phosphate-sugar
backbone of DNA together
Proposed Methods of Replication
Conservative Model: DNA didn’t
split open at all, keeps the parent
strands intact while creating an
entirely new and separate copy
Semi-Conservative Model: Half
of the parent DNA is conserved
in each new DNA molecule
Dispersive Model: DNA only
copied itself for short chunks at a
time, producing new strands that
alternated parent and daughter
DNA
Meselson-Stahl Experiment
• Discovered that DNA was replicated in a
semi-conservative method
• Meselson-Stahl Experiment YouTube
Big Ideas
• DNA is used to store information because it is:
– Durable
– Easy to use
• When DNA replicates, the double helix strand is
split in half be DNA Helicase
• Free nucleotides rush in and bond with their
opposite bases, creating a new complementary
DNA strand
• When DNA is replicated, half of the original DNA
is in one new strand and half is in the other
Exit Ticket
• This process is technically known as
‘semiconservative replication’,
EXPLAIN WHY