1. DNA
Chapter 4 – Biology 12 textbook
Molecule of Life

2. Deoxyribonucleic Acid
DNA...
Deoxyribonucleic Acid

3. DNA contains all the information for the formation of an organism
Chromosomes – are made of condensed DNA
DNA controls the production of proteins – which form the structure of the cell & control chemical reactions

4. Gregor Mendel – 1865 Father of Genetics
Discovered that organisms pass their characteristics to their offspring
Also: genes are found in pairs
Using experiments with pea plants, he determined the laws of inheritance – without knowing anything about DNA

5. Friedrich Miescher – 1871 Isolation of DNA
Isolated DNA for the 1st time – found in the nucleus
Called it “nuclein”
Did not realize that it was genetic information
DNA = carbon, hydrogen, oxygen, nitrogen & phosphorous

6. Other scientists improved DNA extraction & found that DNA is an acidic long chain molecule (polymer) made of nucleotides
1 nucleotide:
Pentose sugar
Phosphate group
Nitrogenous base
Nucleotide

7. Sugar-phosphate Backbone
2 components of the backbone:
Pentose sugar: deoxyribose
Phosphate group: phosphoric acid

8. Deoxyribose sugar 5 carbons
Carbons are numbered clockwise 1’ – 5’ (1 prime to 5 prime)
3’ & 5’ carbons links to
phosphate groups with
ester bonds
1’ carbon links to the
nitrogenous base with
a glycosyl bond

9. Phosphate group
Negatively charged
Free Oxygen links to the sugar

10. 4 Nitrogenous Bases
Adenine
Guanine
Thymine
Cytosine

11. Purines  double ringed molecules:
Adenine
Guanine
Pyrimidines  single ringed molecules
Thymine
Cytosine

12. Phoebus Levine discovered that each of the nitrogenous bases is attached to the sugar & the sugar is attached to the phosphate group
These 3 components make up nucleotides

13. Nucleotide structure

14. Watson & Crick
In 1953, thanks to an X-ray image by Rosalind Franklin, Watson & Crick discovered that DNA is in the form of a spiral
Called a double helix

15. An unrolled helix looks like a ladder
The sugar phosphate backbone forms the 2 sides
The nitrogenous bases form the rungs
The bases are linked to each other with hydrogen bonds

16. In a molecule of DNA, the quantities of adenine & thymine are equal & guanine = cytosine
Watson & Crick found that the structure of the bases determines which other base they bond with
Complimentary bases
are:
A & T
G & C

17. Hydrogen bonds link complimentary bases
The 2 strands are complimentary & antiparallel
One strand runs 3’- 5’, the other strand runs 5’ – 3’

18. If you know the sequence of one strand, you also know the sequence of the other strand
5’ – TCGAATTGCGCGATA – 3’
3’ - _________________ - 5’
Purines bond with pyrimidines so the diameter of DNA is a constant 2nm
DNA is twisted clockwise – “right-handed helix”
One complete turn (3.4 nm) every 10 nucleotides

19. Hydrogen bonds: individually weak, collectively strong
3 H bonds between G & C
2 H bonds between A & T
G cannot bond with T & A cannot bond with C because the molecule would be unstable – too few H bonds

20. Homework questions:
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