1. Chapter 12: DNA
The Blueprint of Life

2. Importance of DNA video clip
The Importance of DNA video clip – 1:30 minutes (good introduction)
Importance of DNA video clip

3. DNA = Deoxyribonucleic Acid
Role of DNA
Molecule that stores genetic information in living cells
Genetic information can be copied for new cells
Information is transmitted to new offspring
DNA – molecule that stores genetic information in living cells
Proteins – building blocks; enzymes that control chemical rxns; etc.

4. 12.2 Structure of DNA Objectives
Identify the chemical components of DNA.
Discuss the experiments leading to the identification of DNA as the molecule that carries the genetic code.
Describe the steps leading to the development of the double-helix model of DNA.

5. Components of DNA DNA is a polymer of nucleotides
These nucleotides have three parts:
Phosphate group
5 carbon sugar: deoxyribose
Nitrogen base: A, G, C, T
A = adenine G = guanine
C = cytosine T = thymine
Covalent bonds join nucleotides

6. Check-in What are the three roles of DNA?
DNA is a polymer of _______________.
A nucleotide is made of what three things?
What type of bond joins nucleotides together?
Storing, copying and transmitting genetic information
nucleotides
Phosphate group, deoxyribose sugar, nitrogenous base
Covalent bond

7. Solving the Structure of DNA
Chargaff’s Rule – true for all organisms
Amount of A = Amount of T
Amount of C = Amount of G
Rosalind Franklin
Used X-ray diffraction
Took picture of DNA in 1952
Picture gave clues about DNA structure:
Strands twisted into helix (like coiled spring)
Two strands in DNA
Nitrogenous bases in center
Franklin used x-ray diffraction (beam of X rays is directed at an object; X rays bounce off object and are scattered in pattern onto film) to take a picture of DNA
In May 1952 – got this picture of DNA called Photo 51

8. Solving the Structure of DNA
James Watson and Francis Crick
Built 3-D models of DNA
Shown Franklin’s Photo 51
Led to discovery of double helix shape in 1953
Watson (American biologist) and Crick (British physicist) were trying to understand structure of DNA
built 3-D models made of cardboard and wire
Wasn’t explaining properties of DNA
Picture (known a Photo 51) was shown to Watson and Crick (unknown to Franklin) – this was the crucial piece of information that they needed to determine that DNA was shaped like a double helix
Published their findings in one-page paper in April 1953 when Franklin’s paper describing her X-ray work was also published
Drawing is Crick’s original sketch of DNA

9. Check-in Describe Chargaff’s rule?
What clues did Rosalind Franklin’s picture give about the structure of DNA?
What method did she use to take the picture?
Looking at DNA in all organisms, amount of adenine equals amount of thymine and amount of guanine equals amount of cytosine.
DNA was helix shaped and contained two strands; bases are in the center of DNA molecule.
X-ray diffraction

10. Check-in Who is credited with the discovery of the structure of DNA?
What method did they use to discover its structure?
James Watson and Francis Crick
Built three dimensional models.

11. Double-Helix Model DNA is shaped like a double helix
Looks like: spiral staircase or twisted ladder
Sides = Phosphate groups and sugars
Steps or rungs = Nitrogen bases
Bases are held together by weak hydrogen bonds

12. Double-Helix Model
Stands are antiparallel – run in opposite directions
Which bases are complementary to each other?
A with T
G with C
Called base pairing
Explains Chargaff’s rule
Strands are antiparallel – run in opposite directions (like two way street)
Have students practice pairing with some examples:
TGC: ACG
GGTA: CCAT
TCGAT: AGCTA

13. Chromatin, Chromosomes and DNA subunits video clip
Structure of DNA
Chromatin Chromosomes and DNA subunits video (3:45)
Chromatin, Chromosomes and DNA subunits video clip

14. Check-in What does a double helix look like?
What forms the backbone of DNA or sides of the ladder?
What forms the rungs of the ladder?
What type of bond holds the bases together?
Twisted ladder or spiral staircase
Sugar (deoxyribose) and phosphate groups.
Nitrogenous bases
Hydrogen bonds

15. Check-in What base bonds with A (adenine)?
What base bonds with C (cytosine)?
What is the complementary DNA strand for the following: AGGTCT
T (thymine)
G (guanine)
TCCAGA

16. 12.3 DNA Replication Objectives
Summarize the events of DNA replication.
Compare DNA replication in prokaryotes with that of eukaryotes.

17. DNA Replication What is it? When does it happen? Why does it happen?
Process by which chromosomal DNA is copied before mitosis and meiosis
When does it happen?
Interphase – S phase
Why does it happen?
So each new cell gets a
complete copy of DNA

18. Steps of DNA Replication
Strands of DNA Separate:
Hydrogen bonds between bases are broken by an enzyme and the strands separate
Base Pairing:
Free-floating nucleotides pair up with bases on the exposed strand (matches with the complementary base – AT & CG)

19. Steps of DNA Replication
Bonding Nucleotides
Backbone (sugar/phosphates) of the once-free nucleotides are bonded together by an enzyme (DNA polymerase) to form a new complementary strand
Result of Replication
Two DNA molecules, each with a new strand and an old strand
Animation is short YouTube clip (very basic)
Animation of DNA Replication

20. DNA Replication Role of enzymes
One enzyme unzips the strands (breaks hydrogen bonds)
DNA polymerase creates sugar-phosphate bonds and “proofreads” new DNA strand
Telomerase adds short DNA sequences to telomeres
Enzymes involved in the process of DNA replication
- one enzyme (helicase) unzips the strands by breaking the hydrogen bonds
- another enzyme (DNA polymerase) is involved in creating the sugar-phosphate bonds as well as “proofreading” the new DNA strand to ensure that it is near-perfect
- telomerase adds short repeating DNA sequences to telomeres (DNA at tips of chromosomes)
DNA in telomeres is difficult to replicate
addition of DNA sequences helps prevent genes from being lost/damaged during replication
telomerase often switched off in adult cells, but turned on in stem cells and embryonic cells (that divide rapidly)
Telomere

21. Prokaryotes vs. Eukaryotes
Single circular DNA molecule in cytoplasm
Replication starts at one point and continues in both directions
Two chromosomes often attached at different points in cell and are separated when cell splits to form 2 new cells

22. Prokaryotes vs. Eukaryotes
Chromosomes much bigger than prokaryotes
Replication starts at many places and continues in both directions
Replicated copies of DNA remain close to each other (sister chromatids) – separate during anaphase of mitosis so that each cell has a complete set of genes

23. Check-in Describe the three steps of DNA replication?
What is the result of DNA replication?
What are the two functions of DNA polymerase?
DNA strands separate
Free-floating nucleotides pair with exposed bases
Sugar-phosphate backbone bonds together
Two DNA molecules, each with one new and one old strand
Bond sugar and phosphate together
Proofread new DNA strand

24. Check-in What is the function of telomerase?
What is the major difference in DNA replication between prokaryotes and eukaryotes?
Add DNA sequences to tips of chromosomes to prevent genes from being lost or damaged during replication
In prokaryotic cells, replication starts in one spot on the chromosome, while in eukaryotic cells it starts in multiple spots.

25. References
The Importance of DNA. United Learning Discovery Education. 10 December 2009 <