DNA & Replication
Bell Ringer
1. On a sheet of paper
2. Write down everything you know about DNA
-- Who discovered DNA?
-- What is the structure of DNA?
-- How does DNA differ from RNA?
The Beginnings
 T. H. Morgan’s group showed that genes are located on
chromosomes
 The two components of chromosomes—DNA and
protein— were candidates for the genetic material
 The role of DNA in heredity was first discovered by
studying bacteria and the viruses that infect them
 The discovery of the genetic role of DNA began with
research by Frederick Griffith in 1928
Griffin & Transformation
 Griffin looked at bacteria & mice
 Major finding = Transformation
 Killed disease-causing bacteria + live, harmless bacteria
 Result: Live, disease-causing bacteria
 Transformation – change in genotype and phenotype
due to uptake of foreign DNA
Living S
cells
(control)
Living R
cells
(control)
Heat-killed
S cells
(control)
Mixture of
heat-killed
S cells and
living R cells
Living S cells
Healthy Mouse Healthy Mouse
Mouse dies
Mouse dies
Hershey & Chase
 What is responsible for transformation?
 Dumb question now, but not so at the time
 Bacteriophages – viruses than infect bacteria
 Consist of protein + DNA
 Which is the transformative agent?
EXPERIMENT
Phage
Radioactive
protein
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Radioactive
DNA
Batch 2:
radioactive
phosphorus (32P)
EXPERIMENT
Phage
Empty
Radioactive protein
shell
protein
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Radioactive
DNA
Batch 2:
radioactive
phosphorus (32P)
EXPERIMENT
Phage
Empty
Radioactive protein
shell
protein
Radioactivity
(phage
protein)
in liquid
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive
DNA
Batch 2:
radioactive
phosphorus (32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
Chargaff’s Rules
In DNA:
[A] = [T]
&
[G] = [C]
 Why would this be true?
Structural Model of DNA?
 M. Wilkins & R. Franklin
use X-ray crystallography
to study molecular
structure
 Watson & Crick
“deduced” that DNA was
2-stranded
 Double Helix
DNA Replication
 DNA  DNA
 2 strands separate, and each one is paired using
complementary bases
Themes (meh.)
 Science as a process
 Regulation
 Interdependence in nature
 Science, Technology, & Society
Semiconservative Replication
 Each daughter molecule = 1 old strand (Conserved from
parent strand) + 1 newly replicated strand
 Called Semiconservative
 Half of the strands are conserved from the parent
 Competing (Incorrect models): Conservative &
Dispersive
DNA Replication
 16_05DNAandRNAStructure
 16_07DNADoubleHelix
 16_09Overview
Telomeres
 Eukaryotic chromosomal DNA molecules have at their
ends nucleotide sequences called telomeres
 Telomeres do not prevent the shortening of DNA
molecules, but they do postpone the erosion of genes
near the ends of DNA molecules
 It has been proposed that the shortening of telomeres
is connected to aging
 Telomerase – enzyme that lengthens the telomere
 Telomerase in cancerous cells?
PROKARYOTE
EUKARYOTE