1. DNA STRUCTURE
Topic 3.3
IB Biology
Miss Werba

2. CARBS, LIPIDS & PROTEINS 3.5 TRANSCRIPTION & TRANSLATION
TOPIC 3 - BIOCHEMISTRY
3.1 ELEMENTS & WATER
3.2
CARBS, LIPIDS & PROTEINS
3.3
DNA STRUCTURE
3.4
DNA REPLICATION
3.5 TRANSCRIPTION & TRANSLATION
3.6 ENZYMES
3.7 CELL RESPIRATION
3.8 PHOTOSYNTHESIS
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3. THINGS TO COVER DNA nucleotide structure Nitrogenous bases
Formation of DNA strands and double helix
Complementary base pairing
Molecular structure of DNA
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4. STRUCTURE OF DNA
DNA is a large molecule made up of smaller sub-units called nucleotides
Each nucleotide consists of:
a nitrogenous base
a pentose sugar
a phosphate group
There are four different DNA nucleotides

5. STRUCTURE OF DNA Pentose sugars
There are two sugars that are used in nucleotides:
deoxyribose sugar  used in DNA
ribose sugar  used in RNA

6. STRUCTURE OF DNA Nitrogen bases Command term = STATE
There are four nitrogen bases in DNA:
A = Adenine
T = Thymine
G = Guanine
C = Cytosine
There is a fifth nitrogen base found only in RNA:
U = Uracil

7. STRUCTURE OF DNA Nitrogen bases
Adenine and guanine are purines (double ring bases)
Thymine and cytosine are pyrimidines (single ring bases)

8. STRUCTURE OF DNA Chemical bonds
The three components are held together by covalent bonds.
These bonds are strong so that the unit holds together within the larger molecule.
COVALENT BOND
COVALENT BOND

9. BASE PAIRING IN DNA C G A T
The nitrogen bases exhibit complementary base pairing.
This means that each base only join with one other base:
Adenine joins with Thymine (A=T)
Guanine joins with Cytosine (C≡G) C G A T

10. BASE PAIRING IN DNA Reasons behind the rule: Size of bases
Number of hydrogen bonding sites

11. SHAPE OF DNA DNA is double stranded.
Each strand is made up of nucleotides, connected through chemical bonds.
These two strands are twisted around each other.
This is called a double helix.

12. SHAPE OF DNA The structure is similar to a twisted ladder.
The “rungs” of the ladder are made from the complementary nitrogen base pairs.
The “sides” of the ladder are made from alternating sugar and phosphate groups.
This is called the sugar-phosphate backbone.

13. C G A T COMPLEMENTARY NITROGEN BASE PAIRS SUGAR-PHOSPHATE BACKBONE

14. SHAPE OF DNA
The nucleotides are joined by covalent bonds  these are very strong
The two strands are joined by hydrogen bonds  these are very weak,
This allows the strands to be separated during DNA replication and protein synthesis without destroying the DNA completely

15. COVALENTBONDS
HYDROGEN BONDS C G A T

16. DNA MOLECULAR STRUCTURE Command term: DRAW and LABEL5’ 3’ C G A T 3’ 5’

18. Sample questions
Q1. Which of the following are connected by hydrogen bonds?
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 of water.
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19. Sample questions
Q2. The percentage of thymine in the DNA of an organism is approximately 30%. What is the percentage of guanine?
70%
30%
40%
20%
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20. Sample questions
A1. D
A2. D
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21. Sample questions
Q3. Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands.
(5 marks)
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22. Sample questions
A3.
four nucleotides shown in diagram with one nucleotide clearly labeled;
base, phosphate and deoxyribose (shown as pentagon) connected between the correct carbons and labeled at least once;
backbone labeled as covalent bond between nucleotides correctly shown as 3′ to 5′ bond;
two base pairs linked by hydrogen bonds drawn as dotted lines and labeled;
two H bonds between A and T and three H bonds between C and G;
adenine to thymine and cytosine to guanine; do not accept initials of bases antiparallel orientation shown;
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