1. The Molecular Basis of Inheritance
Chapter 16
Biology – Campbell • Reece

2. History of DNA Scientists knew…
Genes are located on chromosomes
Chromosomes are made of DNA and Protein
Proteins are more complex than DNA
So the question is: Is it protein or DNA that is the genetic material?

3. History of DNA
Frederick Griffith (1928) – studied the bacteria, Streptococcus pneumoniae, to show transformation
Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944) – isolated various chemicals to try to identify which one caused transformation
Concluded that it was DNA

4. Transformation

5. History of DNA
Alfred Hershey and Martha Chase (1952) – used bacteriophages to prove that DNA is the transforming factor
Erwin Chargaff (1947) – found that the number of adenines = thymines and cytosines = guanines
Rosalind Franklin – used X-ray crystallography to show that DNA is a helix

6. Protein or DNA?

7. Chargaff’s Data
Source of DNA A T G C
Streptococcus
29.8
31.6
20.5
18.0
Yeast
31.3
32.9
18.7
17.1
Herring
27.8
27.5
22.2
22.6
Human
30.9
29.4
19.9
19.8

8. X-ray photo of DNA

9. History of DNA James Watson and Francis Crick (1953)
Proposed the double-helix model of DNA
The phosphate-sugar chains were the backbone
The base pairing followed Chargaff’s Rules (A-T and C-G)
Always paired a purine with a pyrimidine

10. Double-Helix DNA Model

11. Purines Paired With Pyrimidines

12. 5’ to 3’

13. DNA Replication
Watson and Crick then proposed that each strand of DNA would act as a template to make new copies of DNA

14. The Process of Replication…
Origins of Replication – special sites where DNA replication begins
Proteins recognize this sequence and attach to the DNA, separating the two strands, forming a replication “bubble”
Replication then proceeds in both directions, until the entire molecule is copied

15. Origins of Replication

16. Primers
Primer - A short stretch of RNA that initiates synthesis (about 10 bases long)
Primase – the enzyme that makes the primer
The RNA is then later replaced with DNA by DNA polymerase

17. The Process cont.
DNA polymerases – enzymes that add the nucleotides to the new DNA strand
The two DNA strands are antiparallel meaning their sugar-phosphate backbones run in opposite directions
Nucleotides are only added to the 3’ end
Leading strands and lagging strands
The segments of the lagging strand are called Okazaki fragments

18. Leading and Lagging Strands

19. Other Enzymes…
DNA ligase – joins segments of DNA together to make one strand of DNA
Helicase – untwists the double-helix at the replication fork, separating the strands
Single-strand binding protein – holds the DNA strands apart while they are replicated

20. Proofreading
DNA polymerase proofreads each nucleotide as soon as it is added
If the base is incorrect, the DNA polymerase removes it and continues
If DNA polymerase doesn’t catch it, nuclease, will cut out the incorrect base and it will be replaced by DNA polymerase and ligase

21. Nucleotide Excision Repair

22. Telomeres
Since DNA Polymerase can only add nucleotides to the 3’ end, there is no way to complete the 5’ ends of the daughter strands
Each replication results in a shortening of the DNA strand
Regions of repetitive sequences, called telomeres, prevent the loss of genes

23. Shortening of DNA

24. DNA Packing