1. DNA Structure and Replication
Chapter 16

2. You must know The structure of DNA
The knowledge about DNA gained from the work of Griffith; Avery, MacLeod, and McCarty; Hershey and Chase; Wilkins and Franklin; and Watson and Crick
Replication is semi-conservative and occurs 5’ to 3’
The roles of DNA polymerase III, DNA polymerase I, ligase, helicase, primase, and topoisomerase in replication
The general differences between bacterial chromosomes and eukaryotic chromosomes
How DNA packaging can affect gene expression

3. Frederick Griffith
Next question: Was that substance protein or DNA?

4. Avery, mccarty, & mcleod - 1944

5. Hershey & chase
DNA is the
genetic
material!

6. Franklin & Wilkins – early 1950s
Franklin deduced that DNA was helical and consisted of 2 or 3 chains

7. Watson & crick
Used models to determine the structure of DNA

8. Dna structure Sugar-phosphate backbone with rungs of nitrogenous bases
Strands are antiparallel
1 nucleotide

9. Dna structure Sugar-phosphate backbone with rungs of nitrogenous bases
Strands are antiparallel
1 nucleotide

10. Nitrogenous base pairing
Nitrogenous Bases:
-Purines – Adenine (A) and Guanine (G)
-Pyrimidines – Cytosine (C), Thymine (T), Uracil (U)

11. Semi-Conservative Replication
A. New strands are composed of 1 strand of parental DNA and 1 strand of newly formed DNA
B. Free-floating nucleotides
Can be DNA or RNA
Triphosphates – reactions to remove extra two phosphates are exergonic – provide the energy to build the new strand

12. Replication Enzymes Enzyme Function Helicase Unzips & unwinds DNA
Topoisomerase
Relieves strain of unwound DNA
SSBs
Help hold DNA open and stabilize it
Primase
Builds RNA Primer
DNA Polymerase III
Builds new DNA strand
DNA Polymerase I
Replaces RNA primer with DNA
DNA Ligase
Joins Okazaki fragments together

13. 1. Helicase unzips DNA (breaks hydrogen bonds) creating a replication bubble at the origin of replication
Multiple origins per chromosome in eukaryotes
Each side of bubble has replication fork
Bubble enlarges as replication proceeds until bubbles meet

14. 2. Primase builds a short primer of RNA nucleotides (5 – 10 bases)

15. 3. DNA Polymerase III builds the complimentary strand of DNA in the 5’  3’ direction
Free-floating DNA nucleotides move in to match up with parent strand, DNA polymerase III moves along and binds them together (forms covalent bonds between phosphate of one nucleotide and 3’carbon of next)

16. 4. DNA Polymerase I replaces RNA primer with DNA nucleotides

17. Leading and Lagging Strands
New nucleotides must be added on to the 3’ end
Leading strand – Bases easily added as DNA is unzipped

18. Lagging Strand – has a delay
Section unzips, then strand is built back towards origin
Results in chunks called Okazaki fragments
DNA ligase bonds Okazaki fragments together after primer is replaced

19. Speed and Accuracy ~4000 nucleotides per second
Nucleotide excision repair (mismatch repair)– enzymes called nucleases cut out incorrect nucleotides and then gap is filled in with correct nucleotide

20. telomeres
Lose a small portion of the chromosome every time it is replicated
Telomeres consist of highly repetitive sequences in order to protect coding genes
Cancer cells (ex. HeLa) – telomerase is activated – prevents degradation of telomeres and renders cells “immortal”
Interesting article on HeLa cells:

21. DNA packaging
Prokaryotes – one circular chromosome associated with very few proteins
Eukaryotes – linear chromosomes associated with many proteins
Histones – proteins that associate with DNA to help it coil
DNA is negatively charged, histones are positively charged

22. Chromatin – the packaged DNA and proteins
The more tightly coiled the DNA is, the less accessible it is to transcription enzymes = coils control gene expression
Euchromatin – very extended and accessible to transcription – form of most DNA during interphase
Heterochromatin – more condensed (like during mitosis) and generally not transcribed (Barr bodies are also an example)