1. Chromosomes & DNA Replication
TSW explain how the genetic code is contained within DNA

2. Discovering DNA: A Team Effort
Frederick Griffith (1928)
Question: How do bacteria make people sick?
Answer: Transformation

3. Transformation
Process in which one strain of bacteria is changed by a gene(s) from another strain of bacteria

4. Oswald Avery (1944)
Question: Which molecule is important for transformation?
Answer: DNA stores & transmits genetic info from 1 generation to the next

5. Hershey & Chase (1952) Question: Are genes made of DNA or protein?
Experiment: Which part of a bacteriophage enters the bacterium?
Answer: DNA

6. Hershey/Chase Experiment

7. Erwin Chargaff
DNA base-pairing rules
A-T
G-C

8. “Race for the Double-Helix”
Rosalind Franklin (1952): Used X-ray to determine that DNA is spiral-shaped
Watson & Crick (1953): Created 1st accurate model of DNA (double-helix), won the Nobel Prize

9. DNA’s Structure Adenine Thymine Guanine Cytosine DNA is a double helix
4 Nucleotide bases
Adenine
Thymine
Guanine
Cytosine

10. Chargaff’s Rule
Bases form the rungs on the “DNA ladder” by complementary pairing

11. More DNA Structure Sugar-Phosphate backbone
Nucleotides held together by H bonds

12. DNA & Chromosomes Prokaryotes: single, circular chromosome
Eukaryotes: DNA in nucleus of the cell in the form of multiple chromosomes

13. Chromosome # Varies widely between species Example: Humans: 46
Giant Sequoia Tree: 22
Drosophila:8

14. DNA Length
Extremely long (more than 1m in the nucleus of a human cell!)
Must be tightly folded to fit inside of the nucleus
Q: How does this work?
A: Chromosomes

15. Chromosome Structure
CHROMATIN: DNA that is tightly coiled around proteins called HISTONES
NUCLEOSOME: DNA+Histone
Pack together to form a thick fiber with loops and coils
Allows a lot of DNA to fit in such a small space

16. Chromatin
Usually dispersed until MITOSIS where the chromatin condenses into individual chromosomes
Do you remember which phase?

17. DNA Replication
Each strand of the DNA double-helix has all the info needed to construct the other half via “base-pairing”
2 strands are complementary

18. Duplicating DNA Copying DNA called REPLICATION
Results in 2 DNA molecules that are identical to each other & the original strand

19. DNA Replication Replication = Duplication
Happens in the S phase of Interphase
Must replicate before mitosis or meiosis I
Part of the double helix is unwound
Replication in small pieces (Okazaki fragments)
Enzyme stitches pieces together later

20. How does replication occur?
Enzymes “unzip” the DNA molecule
“unzipping”: H bonds break between nucleotides & the 2 strands of DNA unwind

21. DNA Polymerase Enzyme (-ase) Joins individual nucleotides
“proofreads” new strand to maximize odds that each molecule is a perfect copy

22. What does replication look like?

23. What is a Gene? A length of DNA on a chromosome
May be in several parts
Exon – the expressed parts of the DNA sequence
Intron – the intervening, “junk DNA”, not expressed
Parts are cut, put together and then used to make RNA and proteins

24. Telomere
Poly A tail
Telomerase
Cancer
Fountain of Youth