1. REVISION: DNA STRUCTURE & REPLICATION
Topic 3
IB Biology
Miss Werba

2. DNA STRUCTURE

3. 3.3.1
NUCLEOTIDE STRUCTURE
Nucleotides are the building blocks of RNA and DNA
Each nucleotide consists of :
a nitrogenous base (either Adenine, Thymine, Cytosine, Guanine or Uracil)
a pentose sugar (either deoxyribose or ribose sugar)
a phosphate group
J WERBA – IB BIOLOGY 3

4. NUCLEOTIDE STRUCTURE Purines = 2 ring bases = A and G
3.3.1
NUCLEOTIDE STRUCTURE
Covalent bonds link each part of the nucleotide to the other
Covalent bonds also link each nucleotide to one another
Purines = 2 ring bases = A and G
Pyrimidines = 1 ring bases = C, T and U
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5. COMPLEMENTARY BASE PAIRING
3.3.2
3.3.4
COMPLEMENTARY BASE PAIRING
Base pairing rule across the strands is:
Adenine always joins with Thymine (A=T)
Guanine always joins with Cytosine (C≡G)
This is due to the length of the bases and the number of hydrogen bonding sites
Uracil replaces thymine in RNA only!!!!!!!!!!!!!!!
DNA contains A, T, G & C RNA contains A, U, G & C
J WERBA – IB BIOLOGY 5

6. DOUBLE HELIX STRUCTURE
3.3.3
3.3.4
3.3.5
DOUBLE HELIX STRUCTURE
Nucleotides form long chains or strands
2 strands form a twisted ladder  double helix
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7. DOUBLE HELIX STRUCTURE
3.3.3
3.3.4
3.3.5
DOUBLE HELIX STRUCTURE
Sugar-phosphate backbone runs along the sides of the ladder:
Made of alternating sugar and phosphate molecules
Held together by strong covalent bonds
Nitrogen base pairs runs along the centre:
Follow base pairing rule
Held together by weak hydrogen bonds
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8. DNA REPLICATION

9. PROCESS DNA double helix “unwinds” Two strands of DNA “unzip”
3.4.1
PROCESS
DNA double helix “unwinds”
Two strands of DNA “unzip”
helicase enzyme catalyses this step
helicase breaks the hydrogen bonds between the bases
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10. PROCESS Free DNA nucleotides pair up with exposed bases
3.4.1
PROCESS
Free DNA nucleotides pair up with exposed bases
nucleotides float freely within the nucleus
form complementary pairs with the nucleotides of both strands of DNA
A new complementary strand forms
nucleotides react with each other & form a strand
DNA polymerase enzyme catalyses this step
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11. 3.4.1
3.4.3
RESULT
The result of DNA replication are two identical DNA double helices, each consisting of:
one “old” strand
and one “new” strand
DNA replication is therefore referred to as being semi- conservative (b/c part is conserved, part is new)
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12. CONSERVATION OF THE CODE
3.4.2
CONSERVATION OF THE CODE
Complementary base pairing means that the new strand of DNA will match one strand of the parent DNA molecule.
Means that exact copies of the DNA can be made in preparation for cell division.
Mistakes resulting from errors in replication are called mutations.
J WERBA – IB BIOLOGY 12

13. Ref: Biology for the IB Diploma, Allott

14. Ref: IB Biology, OSC