1. The structure of Nucleic Acids
DNA
&
RNA
The structure of Nucleic Acids

2. D N A
A polymer composed of repeating units (monomers) called NUCLEOTIDES
DNA: Molecule found in all of your cells*, carries instructions for life.
RNA: molecule used to build proteins and carry instructions
*except mature red blood cells

3. Nucleotide Phosphate Nitrogen Base Deoxyribose Sugar*
Watson and Crick discovered the double helix n They relied on info from Rosalind Franklin, x-ray photographs.
Deoxyribose Sugar*
A nucleotide has three components.
* RNA nucleotides contains ribose sugar

4. Structure
The “backbone” of DNA and RNA are repeating molecules of sugar and phosphate.
Sugar and Phosphate are connected by covalent bonds.
* Phosphate: *Sugar (notice ring shape)

5. Sugar Phosphate Sugar Phosphate Sugar Phosphate Sugar
Note: Sugar and Phosphate repeating along the “backbone” of the molecule.
Notice the sugar is always attached to the middle molecule, a nitrogen base. PHOSPHATE DOES NOT ATTACH HERE!
Sugar

6. Note: four types of bases in the middle.
Pyrimidine: smaller (one ring)
Purine: larger (two rings)

7. Notice the sugar-phosphate backbone.
Note the size of nitrogen bases that pair up.

8. Nitrogen Bases These form the “rungs” of the DNA
They are connected by weak hydrogen bonds.
There are FOUR different bases in DNA.
Adenine (A) * Cytosine (C )
Guanine (G) * Thymine (T)
There are FOUR different bases in RNA.
Guanine (G) * Uracil (U)
G and A: purine
T and C: pyrimidine
Chargraff showed that in organisms the amount of Adenine and Thymine was always equal, and the amount of Guanine and Cytosine was always equal.

9. Complimentary Base Pair Rule
Edwin Chargaff
A always with T
G always with C

10. Using Chargaff’s Ratios
Organism
% Adenine
% Guanine
% Cytosine
% Thymine
Grasshopper
29.3
20.5
Yeast
32.1
17.9
E. Coli
24.1
25.9
Octopus
32.4
17.6
Human 20
Can you fill in the missing amounts?
*Numbers for organisms are simplified

11. Note: Anti-parallel
One side of the DNA is arranged from 5’ (five prime) to 3’ (three prime) The sugar is oriented so the Oxygen is pointed upward. On the other side, the DNA is arranged from 3’ to 5’, and the oxygen is pointed downward.

12. Which color is the sugar?
Which color is phosphate?
If lime green is cytosine, what nitrogen base is the purple??
If green is adenine, what is the red nitrogen base?

13. What bond connects the phosphate and the sugar?
What bond connects the nitrogen bases?

14. The mystery of DNA
You need to know the following scientists and their contribution to our understanding of DNA

15. Frederick Griffith (1928)
Provided evidence that some factor in cells passes information to other cells

16. Avery, McLeod, McCarty 1944
Supported griffith’s work by showing that something inside cells can transform other cells

17. Alfred Hershey & Martha Chase 1952
Provided evidence that DNA (not proteins) is the source of genetic information

18. Rosalind Franklin Maurice Wilkins
X-ray crystallography
Her photo 51 gave Watson the information needed to decipher the double helix shape of DNA

19. James Watson and Francis Crick
Won Nobel Prize for discovering the structure of DNA
James Watson and Francis Crick

20. DNA Replication replication
“ It has not escaped our notice that the pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
Watson & Crick
Nature 1953

21. replication Replication
First, ladder separated as Helicase Enzyme breaks hydrogen bonds
Replication Fork

22. replication Replication DNA Polymerase Adds complementary nucleotides
Lots of DNA polymerase

23. replication Replication 1 error in every 10,000 base pairs
DNA polymerase proofreads the DNA
Ensures all bases are correctly matched

24. replication Mutations
DNA proofreading drops the actual number of errors to 1 in a billion nucleotide pairs.
A human cell takes a few hours to replicate the 6 billion base pairs.
DNA polymerase proofreads

25. replication Telomeres The tip of eukaryotic chromosomes Hard to copy
Telomerase is a special enzyme that adds repeating DNA sequences to the ends of chromosomes
DNA polymerase proofreads

26. replication Eukaryotic Starts in multiple locations
DNA is in long strands
DNA polymerase proofreads

27. replication Prokaryotic Replication Start at a single spot
DNA is in a single circle
DNA polymerase proofreads