1. DNA and The Genome Structure and Organisation of DNA
What are we learning?
To describe in detail the structure of DNA.
To describe how DNA is organised in eukaryotes and prokaryotes.
Textbook Chapter 1: Pages 2-14

2. What you should already know

3. Structure of DNA

4. Structure of DNA
DNA is a complex molecule present in all living cells. DNA – deoxyribonucleic acid. DNA stores information in its sequence of bases which determines an organism’s genotype and the structure of its proteins.

5. Nucleotides
A molecule of DNA is made up of two strands, each made of repeating units called nucleotides.
Phosphate
Deoxyribose sugar
Organic base

6. Organic bases
DNA nucleotides contain one of four organic bases: Adenine Thymine Cytosine Guanine

7. Base Pairing Rule
In order for 2 strands of DNA to become joined to one another, the bases must pair together following the base pair rule. Adenine can only pair with Thymine. Guanine can only pair with Cytosine. The bases are linked by weak hydrogen bonds.

8. Base Pairing Rule

9. Deoxyribose sugar
If we take a closer look at a molecule of deoxyribose sugar, we can see its structure. The sugar contains 5 carbons as shown in the diagrams above. 1 2 3 4’ 5’

10. Deoxyribose sugar
Organic bases are attached to carbon number 1 on a deoxyribose sugar molecule. Phosphate molecules are attached to carbon 5 on a deoxyribose sugar molecule.

11. Deoxyribose sugar

12. Sugar phosphate backbone
Strong chemical bonds form between the phosphate group of one nucleotide and carbon 3 of the deoxyribose sugar of the next nucleotide. By this means, neighbouring nucleotides become joined together permanently forming a DNA molecule with a sugar-phosphate backbone.

14. Antiparallel DNA Strands
A molecule of DNA is only able to grow by adding DNA nucleotides to the end of DNA that begins with carbon 3 of its sugar. This is called the 3’ (3 prime) end. A complimentary strand runs in the opposite direction, and has carbon 5 at the end. This is called the 5’ ( 5 prime end).

15. We say that DNA strands are antiparallel: the two strands of DNA run in opposite directions.

16. Double helix
In order for base pairs to align with each other, the two strands of a DNA molecule take the shape of a twisted double helix. The sugar phosphate backbone is on the outside and the base pairs on the inside.

18. Organisation of DNA