3.3 and 3.4 DNA
Structure and Replication
3.3 Assessment Statements
3.3.1 Outline DNA nucleotide structure
3.3.2 State the names of the 4 bases in DNA
3.3.3 Outline how DNA nucleotides are linked
together
3.3.4 Explain how a DNA double helix is
formed using complementary base pairing
3.3.5 Draw and label a simple diagram of the
molecular structure of DNA
A little bit of DNA History
Francis Crick and James Watson
The Discovery of DNA (& other stuff)
DNA and RNA Structure
In 1868, Meischer
first isolated
deoxyribonucleic acid
from cells in pus and
from fish sperm.
No one knew its
function.
In 1928, Griffith was working with smooth
(S,virulent) and rough (R, non-virulent) strains
of a pneumonia-causing bacterium.
In 1944, Avery
showed that the
substance was DNA.
Hershey and Chase
confirmed that DNA is
the hereditary material
(not protein).
3.3.1 Outline DNA nucleotide structure
3.3.1 DNA Nucleotide Structure
DNA is composed of four kinds of
nucleotides.
Each consists of
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a five carbon sugar (deoxyribose)
a phosphate group (PO4-)
one of four bases
adenine (A)
thymine (T)
guanine (G)
cytosine(C)
3.4.2 Name the 4 bases in DNA
The nucleotides
are similar, but
T and C are
single ringed
pyrimidines
A and G are
double ringed
purines.
Edwin Chargaff, in 1949,
noted two critical bits of
data.
1) The four kinds of
nucleotide bases making
up a DNA molecule differ
in relative amounts
from species to
species
2) The amount of A = T, and the amount of C = G.
3.3.4 Explain how a DNA double helix is
formed using complementary base
pairing
Complementary base pairing
Rosalind Franklin used x-ray diffraction
techniques to produce images of DNA
molecules.
She concluded:
DNA exists as a long, thin
molecule of uniform diameter
The structure is highly repetitive
DNA is helical
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Franklin’s Story
Watson and Crick used numerous sources of
data to build models of DNA.
The following features were incorporated into
their model:
The bases were hydrogen bonded (a weak
bond) together in the center of the helix
T (a pyrimidine) had two hydrogen bonds with
A (a purine)
C (a pyrimidine) had three hydrogen bonds
with G (a purine)
The nucleotides are joined together using
phosphodiester bonds
Each sugar of the backbone is covalently
bonded to the nitrogenous base off of carbon 1
of the ribose sugar.
covalent bond
carbon 1
There are 10 base pairs per turn of the helix.
The two sides are anti
parallel, meaning that the
sugar and phosphates are
running in opposite directions.
Each side ends in a phosphate
(5’ end) and a sugar (3’ end)
Since the sides are anti
parallel, one side goes in the
3' to 5' direction, and the other
goes in the 5' to 3' direction.
3.3.5 Draw and label a simple diagram of the
molecular structure of DNA
3.3.5 Draw and label a simple diagram of the
molecular structure of DNA
3.4 Assessment Statements
3.4.1 Explain DNA replication in terms of
unwinding of the double helix and separation of
strands by helicase followed by formation of
new complementary strands by DNA
polymerase
3.4.2 Explain the significance of complementary
base pairing in conservation of the base
sequence of DNA
3.4.3 State that DNA replication is
semiconservative.
3.4.1 Explain DNA replication
Helicase
Breaks hydrogen
bonds between bases,
unzips and unwinds the
double helix.
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Is an enzyme (a protein
that speeds up chemical
reactions)
Is made during G1
Overview of
DNA
replication
DNA
replication in
real time!
DNA polymerase adds DNA nucleotides in a 5’
to 3’ direction. The enzyme is very specific and
can only add nucleotides to the 3’ end.
5’ 3’
DNA polymerase adds
nucleotides using the
original strand as a
template
replication
fork
3’ 5’
Helicase enzyme
unzips the double
helix
Free nucleotides from
cytoplasm are added
using complementary
base pairing
3’ 5’
3.4.3 State that DNA replication is
semiconservative.
The mechanism of semi-conservative replication
produces two descendent double helices that
each contain one of the original polynucleotide
chains
Meselson and Stahl carried
out an experiment which showed
the semi conservative nature of
DNA replication.
3.4.2 Explain the significance of
complementary base pairing
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The DNA molecule is copied precisely from one
cell generation to the next.
In a unicellular organism this means that the
total genome is successfully copied into each
new generation.
In a multicellular organism all cells contain an
exact copy of the total genome (even though not
all genes are expressed in every cell).
Genes are faithfully passed from one generation
to the next.
1.Which molecules form the nucleotide marked in the
diagram?
A.
B.
C.
D.
phosphate, deoxyribose and nitrogenous base
phosphorus, ribose and nitrogenous base
phosphorus, deoxyribose and guanosine
phosphate, ribose and guanine
2. Which of the following are connected by hydrogen bonds?
A.
B.
C.
D.
Hydrogen to oxygen within a molecule of water
Phosphate to sugar in a DNA molecule
Base to sugar in a DNA molecule
Hydrogen to oxygen between two different molecules
water
3. (a) Draw a labelled diagram to show how two nucleotides
are joined together in a single strand of DNA (3)
4. What principle is necessary to preserve the sequence
of DNA during replication?
A.
B.
C.
D.
Base pairing is complementary.
One gene codes for one polypeptide.
Substrates are specific to enzymes.
The genetic code is universal.
5.(a) Explain why DNA must be replicated before mitosis
and the role of helicase in DNA replication (4)
(b) Explain how the base sequence of DNA is conserved
during replication (5)