1. NOTES: 12.2 – 12.3 DNA Structure & Replication

2. 12.2: DNA Structure
● it was known that DNA was made up of nucleotides joined into long strands by covalent bonds…but HOW were they connected…what was the structure?

3. DNA Structure ● Made up of nucleotides (monomer)
● Each nucleotide composed of 3 parts:
5-carbon sugar (DEOXYRIBOSE)
Phosphate group
Nitrogen-containing base
● Adenine (A)
● Thymine (T)
● Guanine (G)
● Cytosine (C)

4. Chargaff’s Rules Percentage of Bases in Four Organisms Source of DNA A
Streptococcus
29.8
31.6
20.5
18.0
Yeast
31.3
32.9
18.7
17.1
Herring
27.8
27.5
22.2
22.6
Human
30.9
29.4
19.9
19.8
What do these data suggest to you?

5. ● A = T (A pairs with T)
● C = G (C pairs with G)

6. What DNA Looks Like:
● Rosalind Franklin: used X-ray diffraction patterns to discover that DNA strands twisted around each other like a HELIX

7. ● Watson & Crick: while trying to build a 3-D model of DNA, Watson saw Franklin’s photograph and measurements and within weeks, he and Crick figured out the structure of DNA

8. DNA Structure = DOUBLE HELIX (“twisted ladder”)
● Backbone
deoxyribose & phosphate group of each nucleotide
●“Steps of Ladder”
bases (A, G, C, T)
● Any base sequence is possible!

11. What’s holding the strands together?
● HYDROGEN BONDS!!!
● Hydrogen Bonds form between A and T base pairs as well as between C and G base pairs

12. Vocabulary: Chromatin Replication DNA polymerase
DNA Replication
Vocabulary:
Chromatin
Replication
DNA polymerase
Key Concept:
What happens during DNA Replication?

13. The Review ● Prokaryotic cells do not have a nucleus
DNA molecules are located in the cytoplasm
Usually a circular DNA molecule and it is referred to as the cell’s chromosome
● Eukaryotic cells have a nucleus
Can have 1000x more DNA than prokaryotic cells
DNA is located in the form of a number of chromosomes
# of chromosomes varies widely from species to species

14. DNA molecules are long…how does DNA fit in the nucleus?
● It forms chromosomes!
● DNA coils around proteins called histones and then this chromatin supercoils around itself until chromosomes form

15. DNA Replication
● before a cell divides, it must copy its DNA so that all cells have a copy of the genetic instructions
● this process involves A LOT of ENZYMES!
● each half of the DNA double helix can serve as a "template" for the replication of another DNA double helix molecule
The strands are said to be complementary

16. DNA REPLICATION: THE PROCESS
1) An enzyme unwinds & "unzips" DNA (separates the 2 DNA strands) by breaking the hydrogen bonds between base pairs
Hydrogen Bonds Breaking!

17. 2) a complementary base is inserted to each side of the DNA strand with the help of DNA POLYMERASE
Why does the “blue” always pair with the “green?”

21. 3) The sugar-phosphate groups are covalently bonded to the growing DNA chain (new "backbone")
1 nucleotide
COVALENT BOND

23. 4) an enzyme (DNA polymerase) will "proof-read" the order of bases & make corrections

24. A SIMPLIFIED VERSION…
                                 

25. A Little More Complicated…