1. More on DNA Chromosomes and Replication

2. Chromosomes A single molecule of DNA
Eukaryotes usually have many linear chromosomes
Prokaryotes usually have one circular chromosome
Prokaryotes also have plasmids

3. Eukaryotic Chromosomes
Are highly condensed and coiled many times to fit a lot of DNA into a tiny nucleus
The length of all the DNA in your body would stretch to the sun and back
70 times

4. Histone Proteins
Helix only the first level
DNA is wrapped tightly around histone proteins
Control access to DNA

5. Fibers
When genes are less active they get coiled more tightly into fibers
Changes in gene expression are caused by coiling or uncoiling of chromosome

6. Fibers Loop and Coil Scaffold proteins coil and condense DNA further
During replication extra scaffold proteins condense the DNA into chromosomes

7. Chromatin v. Chromosome

9. DNA Replication
Watson and Crick noticed the huge benefit of double strands
Each strand can serve as a template for making the other

10. Semiconservative Model
Each strand serves as a template for the creation of a new strand of DNA
2 DNA molecules are created, each containing 1 strand of the original DNA

11. Semiconservative Replication

12. How Do We Know?
Meselson-Stahl experiment

13. DNA Replication is Remarkably Fast and Accurate!
Humans have 46 chromosomes, and thus 46 DNA molecules
About 6 billion base pairs
DNA replication takes just a few hours, even in humans
Only 1 error per 1 billion nucleotides

15. The Beginning Topoisomerase unwinds the helix
Helicase separates the strands
Single stranded binding proteins (SSBs) keep the strands separated
This forms a replication fork

16. Topoisomerase, Helicase and Single-Strand Binding Protein

17. DNA Polymerases Each nucleotide is added one by one by DNA polymerases
Nucleoside Triphosphates are added
Nucleoside loses 2 phosphate groups, providing energy to synthesize new strand

18. The Leading Strand
Synthesis always occurs in the 5' to 3' direction (the 5' end of the new strand is synthesized first)
(attaches to the 3’ end of the template)
One strand can grow continuously as the fork opens in front of it

19. The Lagging Strand
Built discontinuously in the opposite direction of replication
Built in fragments, called Okazaki fragments
DNA ligase connects fragments

21. Priming DNA
DNA polymerases can only add nucleotides to an existing strand
A RNA primer, first binds to the template strand with the help of primase (aka RNA Polymerase)
Eventually replaced by DNA molecules

22. Primer continued... The leading strand requires only one primer
For the lagging strand, each fragment requires a new primer

24. DNA Polymerase – An Amazing Enzyme
DNA pol proofreads each nucleotide that it adds against the template
If an error is made, the enzyme deletes the nucleotide and continues synthesizing DNA
Other proteins do the same thing
DNA is also repaired after damage, such as exposure to X-rays
Over 100 DNA repair enzymes!
Extremely, extremely important

25. DNA Repair/ Excision Repair
Nucleases cut out (incise) the incorrect nucleotide
DNA polymerase adds the correct nucleotide
Ligase connects the new nucleotide to the strand

26. Topoisomerase
Helicase
Single Stranded Binding Proteins

27. 5’ 3’ DNA Ligase Lagging Strand Okazaki Fragments Leading Strand
DNA Polymerase
Primase