DNA contains the genetic instructions for life. It exists in the cell nucleus, associated with proteins, and a small amount is found in mitochondria and chloroplasts. DNA has three main components - deoxyribose sugar, phosphate groups, and four nitrogenous bases (adenine, guanine, cytosine, thymine). The bases bond with each other through hydrogen bonding - adenine pairs with thymine, and cytosine pairs with guanine. DNA exists as a double helix structure, with the two antiparallel strands coiling around each other and connected by these hydrogen bonds between bases.

1. DNA Structure

2. Occurrence
• In eukaryotes: in nucleus
• DNA in nucleus is associated with simple
proteins (deoxyribonucleo proteins)
• Small quantity occurs in mitochondria and
chloroplast. (extranuclear DNA has no
proteins associated).
C-value is the amount, in picograms, of DNA
contained within a haploid nucleus (e.g. a gamete)
or one half the amount in a diploid somatic cell of a
eukaryotic organism.
C-value (quantity of DNA)

3. DNA Structure (an overview)
• DNA has three main components
– 1. deoxyribose (a pentose sugar)
– 2. base (there are four different ones)
– 3. phosphate

4. #2. The Bases
They are divided into two groups
Pyrimidines and purines
Pyrimidines (made of one 6 member ring)
Thymine
Cytosine
Purines (made of a 6 member ring, fused to a 5
member ring)
Adenine
Guanine
The rings are not only made of carbon (specific
formulas and structures are not required for IB)

5. Purines
• 1. Adenine 2. Guanine

6. Pyrimidines
• Thymine Cytosine

7. DNA Double Helix and Hydrogen Bonding
1- 3
6 - 4

8. 6 - 4
1 - 3
2 - 2

9. Phospho-diester bond

10. Glycosidic bond
Bond between nitrogenous base and deoxyribose sugar
P-5’- sugar (C-1)- N-1 (Py)=(Pu)- N-9- C- 1 (sugar-C-5’- P

12. •Adenine always pairs
with thymine because
they form two H bonds
with each other
•Cytosine always pairs
with guanine because
they form three
hydrogen bonds with
each other
DNA Double Helix and Hydrogen Bonding

13. #6. Antiparallel strands
The strands
run opposite
of each
other.
The 5’ end
always has
the
phosphate
attached.
5’ 3’
3’ 5’

14. • The DNA has a negatively charged backbone
(because of the phosphate groups).
• Duameter= 20 A (2nm).
• 1 turn of spiral = 34 A contains 10 bases.
• The proteins (the histones) are positively
charged.
• DNA is dexrorotatory.
• The DNA and proteins are electromagnetically
attracted to each other to form chromatin

15. Charguff’s rule
• no. of A and T bp = G and C
• Purines and pyrimidines are equal in amount.
• Amount of A=T and G=C.
• Base ratio A+T/G+C is constant for a species.
• Deoxyribose sugar and phosphate occur in
equal proportions

16. The Structure of DNA.mp4