1. DNA Structure
IB Topics 3.3 and 7.1

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

3. 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)

4. Nucleotide Structure
Nucleotides are formed by the condensation of a pentose sugar, phosphate and one of the 4 bases
The following illustration represents one nucleotide

5. How do the nucleotide and the ribose attach?OH
H2O

6. N-Glycosidic Bonds REMEMBER???
In Nucleic Acid…Glycosidic Bonds form between sugar and the nitrogen base!

7. How does the Phosphate group get on the ribose?

8. Nucleotide Structure
Nucleotides are linked together (or POLYMERIZED) by covalent bonds called phosphodiester linkage

9. Phosphodiester Bonds Polymerized Double Stranded DNA
Hydrogen is removed H
Phosphodiester bond
+ H & Pyrophosphate as Products
Pyrophosphate

10. DNA Double Helix and Hydrogen Bonding
Made of two strands of nucleotides that are joined together by hydrogen bonding
Hydrogen bonding occurs as a result of complimentary base pairing
Adenine and thymine pair up
Cytosine and guanine pair up
Each pair is connected through hydrogen bonding
Hydrogen bonding always occurs between one pyrimidine and one purine

11. #4. DNA Double Helix and Hydrogen Bonding
Complimentary base pairing of pyrimidines and purines

12. #4. DNA Double Helix and Hydrogen Bonding

13. DNA Double Helix and Hydrogen Bonding
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

14. DNA Double Helix
The ‘backbones’ of DNA molecules are made of alternating sugar and phosphates
The ‘rungs on the ladder’ are made of bases that are hydrogen bonded to each other

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

16. Homework…3.3 1. Draw the structure of ribose and number the carbons
2. Draw a schematic representation of a nucleotide. Label the sugar, base and phosphate.
3. What are the complimentary base pairs to a DNA strand that has the following order A T A C C T G A A T?
4. Draw a schematic representation of an unwound DNA double helix using the base pairs from your answer in question 3.
Include the number of hydrogen bonds between each base pair. Be sure to label all of the bases and the 5’ and 3’ ends of the structure.

18. When phosphodiester links are formed . . .
A. When the covalent bonds are formed between nucleotides the attach in the direction of 5’→3’
B. The 5’ end of one nucleotide attaches to the 3’ end of the previous nucleotide

19. Nucleosome structure
Nucleosome are the basic unit of chromatin organization
In eukaryotes DNA is associated with proteins
(in prokaryotes the DNA is naked)
Nucleosomes = basic beadlike unit of DNA packing
Made of a segment of DNA wound around a protein core that is composed of 2 copies of each of 4 types of histones

20. Nucleosome structure Nucleosomes have: 8 histones in the core
DNA wrapped twice around the core
One histone holding the nucleosome together
A DNA ‘linker’ continuing towards the next nucleosome

21. Nucleosome structure
The DNA has a negatively charged backbone (because of the phosphate groups)
The proteins (the histones) are positively charged
The DNA and proteins are electromagnetically attracted to each other to form chromatin

22. Genes Genes=units of genetic information (hereditary information)
Order of nucleotides make up the genetic code
Genes can contain the information for one polypeptide
Genes can also regulate how other genes are expressed
All cells of an organism contain the same genetic information but they do not all express the same genes
THIS IS CELL DIFFERENTIATION
Cells differentiate by genes that are activated

23. Genes Repetitive sequences-part of the non-coding section of DNA
Function-unknown
Can be used in DNA profiling (DNA fingerprinting)