1. Chapter 12
DNA

2. The Genetic Code
Genetic Code – the way in which cells store the program that they seem to pass from one generation of an organism to the next generation
Evidence that DNA is the Genetic Material

3. 1928 – Fred Griffith studied pneumonia caused by bacteria
1928 – Fred Griffith studied pneumonia caused by bacteria. He worked with 2 strains of bacteria, each containing different genetic information.
S - strain – virulent produced capsule
R- strain – nonvirulant – did not have capsule

4. 12.1 Identifying the Substance of Genes “The Role of DNA”
Frederick Griffith Conclusion: Some of the genetic material from the dead, virulent bacteria (S, had entered the living, nonvirulent bacteria (R) changing them to the virulent form. This is called BACTERIAL TRANSFORMATION (one strain of bacteria had been transformed into another)

5. Discovery of the Role of DNA (cont’d)
Oswald Avery, Maclyn McCarty, and Colin MacLeod wanted to know – What factor had transformed the bacteria?
Made “juice” from heat killed bacteria and treated “juice” with enzymes to destroy lipids, proteins, carbs, and RNA  transformation still occurred BUT when the treated the “juice” with enzymes to destroy DNA  transformation did not occur therefore, DNA was the TRANSFORMING FACTOR
Scientists were still skeptical about the genetic material of higher organisms.

6. Discovery of the Role of DNA (cont’d)
Hershey-Chase 1952 – hypothesized that bacteriophages (made of DNA and a protein coat) don’t enter bacteria intact, but the phages protein coat attaches to the bacterial cell wall and the phage then injects its DNA into the bacterial cell.

8. Hershey-Chase Experiment: Infected cells make more virus by injecting their DNA
animation 1

9. Discovery of the Role of DNA (cont’d)
Conclusion: DNA and not protein entered the bacteria – strong evidence that the genetic material of bacteriophages is DNA. DNA was the molecule that carried the genetic code

10. Discovery of the Role of DNA (cont’d)
1. Storing Information
2. Copying Information
3. Transmitting Information 10

11. 12.2 The Structure of DNA
1. Rosiland Frankiln (& Maurice Wilkins) (early 1950’s)– produced photographs (using X-ray diffraction) showing DNA is twisted into a spiral or HELIX with the bases perpendicular to the length of the molecule. Picture also showed that DNA must be composed of more than one strand and that sugar-phosphate backbone is on the outside of the helix

12. Erwin Chargaff –Chargaff’s Rule – number of nucleotides containing A (adenine) equals the number of nucleotides containing T (thymine) and that the number of G (guanine) equals the number of C (cytosine)

13. James Watson and Francis Crick 1953 – used Franklin and Wilkins x-ray crystallography picture of DNA and information from Chargaff to make a model of DNA (still used today)

14. The Double Helix DNA is made of nucleotide subunits
5 C-sugar – DEOXYRIBOSE Sugar
One or more phosphate groups Phosphate
One of 4 possible nitrogen-containing bases Base

15. Model suggests that there are 2 strands of DNA and that the 2 strands are arranged like a ladder.
Sides of the ladder = sugar and phosphate backbone (phosphodiester bonds)
Rungs of the ladder = nitrogen bases – each rungs consists of 2 nitrogen bases COMPLEMENTARY Base Paired. (A=T) (C=G) (held together by H bonds)
Purine – Adenine & Guanine- Double Ring (It’s 2x’s as good to be pure)
Pyrmidine – Thymine & Cytosine- single ring

16. DNA (deoxyribonucleic acid) bases:
Adenine, Thymine, Cytosine, and Guanine
pyrimidines
purines
Pyrimidines: single ring bases
Purines: double ring bases
(2xs as good to be pure)
Complimentary binding pattern:
Adenine + Thymine (share 2 Hydrogen bonds)
Cytosine + Guanine (share 3 hydrogen bonds)

17. The Structure of DNA
Two polynucleotide strands wrapped around each other in a double helix
A sugar-phosphate backbone
Steps/”rungs” made of hydrogen-bound bases (A=T, C=G)
Twist

18. 12.3: DNA Replication
Semi-conservative model was suggested by Watson and Crick but proven by Matthew Meselson and Franklin Stahl (1958) – each parent strand is a template for a new complimentary strand – end result is two identical DNA molecules each consisting of one old side “conserved” from parent and one new side

19. Takes place in the nucleus in eukaryotes
Four Easy steps to remember:
1.Unwind
2. Unzip
3. Add new parts
4. 2 new molecules of DNA rewind

20. 12.3 DNA REPLICATION: During “S phase” of interphase DNA Replication
UNWIND/UNZIP
1.DNA helicase separates parental DNA and exposes bases (unwinds/unzips)
2.Single Stranded Binding Proteins (SSBP) hold strands apart, preventing them from recoiling
Adding New Parts/Elongation
3. RNA primase lays down short segments of RNA (RNA primer) to which new strands of DNA can be made

21. 4. DNA polymerase- attaches to the RNA primer and begins to elongate (attach free nucleotides to exposed bases) the strands. Done continuously on the leading strand, in short pieces (Okazaki fragments) on the lagging strand.

22. Why is there a Leading and a Lagging Strand?????
The 2 strands of DNA are antiparallel
DNA polymerase can only add nucleotides to the 3’ end (5’  3’) therefore both strands can not be made continuously

23. DNA replication begins at specific sites on double helix
5. DNA Polymerase replaces RNA primer with DNA nucleotides, it also proofreads new strand for base-pair errors (lagging strand requires many RNA primers)
6. DNA ligase joins sugar-phosphate backbone (“glues”) of Okazaki fragments (phosphodiester bonds link fragments)
2 New IDENTICAL Molecules of DNA REWIND
replication forks
Animation/tutorial
Secret of Life Video

24. *Telomeres – tips of chromosomes are difficult to copy, the enzyme telomerase adds short repeated DNA sequences to telomeres, lengthening the chromosomes – this makes it less likely that important gene sequences will be lost during replication
animation

26. The Big Picture
Two strands unwind and unzip (HELICASE) splits H bonds between bases
Add new parts – new nucleotides are added to the exposed strands by DNA POLYMERASE (RNA PRIMASE – first adds RNA nucleotides)
DNA LIGASE “glues”
2 new identical molecules of DNA rewind

27. When DNA can repair mistakes and when it can’t
DNA Repair enzymes work like a spell checker
Cut out wrong sequences
Undamaged strand is template
Only 2 or 3 stable changes per year
Mutations: some severe, others are not
Inheritable changes occur in gametogenesis
Now the “wrong” sequences are copied
Ex: cystic fibrosis (CF): a deletion of 3 nucleotides in a certain gene
Ex: sickle cell anemia: one nucleotide substitution in the hemoglobin gene
Mutagen: a mutation causing substance (can break DNA)
Ex: X-Rays, radioactivity, nicotine

28. Prokaryotic Cells vs. Eukaryotic Cells: DNA Replication