Higher Biology
DNA
Deoxyribonucleic Acid
Structure and Replication
1
DNA and Genes
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A gene determines an organisms
characteristics.
Genes are found on chromosomes .
Chromosomes are found in the nuclei of
all cells.
Chromosomes are threadlike structures
made from
DNA .
DNA
nuclei
chromosomes
gene
2
DNA Structure
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In 1953 the structure of the DNA
molecule was explained for the first time
by two scientists in Cambridge- James
Watson and Francis Crick.
They showed that DNA:
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Consists of two strands
Each strand is made of repeating units (like
monomers) called nucleotides
The two strands are antiparallel (run in
opposite directions)
The two strands wrap around each other to
form a double helix.
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DNA Structure
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DNA structure 1
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DNA structure 2
4
Nucleotides
Nucleotide
structure
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Insert and label the “nucleotide diagram”,
from your pack, with the following:
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Nitrogenous or organic base
Deoxyribose sugar
Phosphate
These structures are held together by
strong chemical bonds.
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Nucleotides
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In DNA there are 4 different organic
bases.
They are:
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Adenine
Thymine
Cytosine
Guanine
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Nucleotides
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In DNA the bases ALWAYS bond (or pair)
together according to the following rule

Adenine
Thymine

Cytosine
Guanine
The bonds that hold the complementary bases
together are weak hydrogen bonds.
Insert and complete the “Structure of DNA”
diagram from your diagram pack.
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Quick Questions
1.
2.
3.
4.
5.
What shape is the DNA molecule?
What type of bonding holds the two DNA
strands together?
Name the three components of a
nucleotide.
Name the organic base which pairs with
cytosine.
Name the organic base which pairs with
thymine.
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Quick Questions
6.
What do the following symbols represent?
phosphate
P
sugar
S
A
adenine
thymine
T
guanine
G
cytosine
C
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DNA Replication
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During cell division DNA must be
replicated (or copied) so that each
daughter cell carries an identical set of
instructions to those in the parent cell.
DNA is a unique molecule because it can
copy itself exactly.
The replication is described as semiconservative because each of the 2
resulting DNA molecules is composed of
one original (or conserved) and one new
strand.
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DNA Replication
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Replication of DNA is a fast but complex
process that requires:
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A DNA template
Several enzymes including DNA polymerase
(the enzyme which joins new nucleotides to the
growing DNA strand)
A supply of the 4 types of DNA nucleotides
A supply of ATP to provide energy
Watch the DNA replication animation.
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DNA Replication
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1.
2.
3.
Briefly the stages in DNA replication are:
The DNA molecule unwinds.
The DNA molecule unzips (as the weak
hydrogen bonds, between complementary
bases, break).
Free nucleotides align themselves with
their complementary nucleotide on the
open chain.
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DNA Replication
4.
5.
6.
New weak hydrogen bonds form between
complementary bases (remember A-T, GC).
Adjacent new nucleotides are linked
through the sugar and phosphate molecules
by strong chemical bonds to form the
“backbone” of the new strand.
The new molecule winds up into a double
helix.
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DNA Replication
14
DNA Replication
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Insert and complete the “DNA replication”
diagram from your pack.
Use 2 different colours to draw a diagram
to illustrate what is meant by semiconservative replication.
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Practice Questions
From the Torrance textbook:
1. TYK page 47 Q4
2. TYK page 48 Q1-4
3. AYK page 49 Q1-6.
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