1. The Molecular Basis of Heredity
Chapter 16

2. 5.5
Nucleic acids store, transmit, and help express hereditary information

3. Nucleotides

4. DNA Structure

5. Anti-parallel Structure
Sugar carbons are numbered 1’-5’
One side of DNA runs in the 3’ direction
The other side runs in the 5’ direction
This is important to replication

6. RNA Structure
Ribose sugar
Uracil replaces thymine
Single stranded

7. 16.1
DNA is the genetic material

8. Protein as the genetic material?
T.H. Morgan – fruit flies
Discovered genes as part of chromosomes
Chromosomes made of protein and DNA
Protein?
More known
Diverse structures
Specificity of function
DNA?
Little known
Seemed too uniform to be the genetic code of all life

10. 16.2
Many proteins work together in DNA replication and repair

11. Making DNA from existing DNA Semi-conservative
DNA Replication
Making DNA from existing DNA
Semi-conservative
At the end of DNA replication, each daughter molecule has one old strand (from the parent DNA) and one new strand (synthesized during replication)
Model proposed by Meselson and Stahl

12. Meselson & Stahl

13. DNA Replication
Step 1:
Helicases unwind DNA at origin of replication by breaking hydrogen bonds between nitrogen bases
Replication bubble forms as two parental strands separate
Replication fork forms at end of each replication bubble

14. DNA Replication
Step 2:
Single-strand binding proteins hold the unpaired DNA strands apart while new DNA strands are being synthesized
Topoisomerase protein binds to parental DNA to relieve strain untwisting puts on replication fork

15. DNA Replication
Step 3:
Primase creates a short RNA primer that binds to the parent DNA to signal DNA polymerase III where to begin adding nucleotides
RNA primer will later be replaced with DNA nucleotides

16. DNA Replication
Step 4:
DNA Polymerase III adds nucleotides to exposed bases in 5’-3’ direction at the RNA primer
Leading strand
Produced continuously in 5’-3’ direction
Elongation moves towards replication fork
Lagging strand
Produced in pieces
Okazaki fragments
Elongation moves in opposite direction of replication fork (5’-3’)

17. DNA Replication
Step 5:
Lagging strand is completed as DNA ligase seals Okazaki fragments

19. Proofreading & Repair
Initial error rate in replication is 1 in 100,000 nucleotides
DNA polymerases proofread and correct errors
Error rate in completed replication is 1 in 10 billion bases

20. Mismatch Repair
For that 1 in 10 billion errors that escapes DNA polymerase or are due to environmental mutations
Many enzymes involved
Cut out damaged section (nuclease)
Replace with new nucleotides (DNA polymerase I)
Seal in place (DNA ligase)

21. 16.3
A chromosome consists of a DNA molecule packed together with proteins