
Homework:
 Finish Molecular Genetics Review Packet Part A
 Problem Set 12 #1

Do Now:
1. Look over your midterm exam.
2. Read the intro of the Molecular Genetics review packet and
complete as much of Part A as you can

Homework:
 Finish Molecular Genetics Review Packet Part A
 Problem Set 12 #1

Goals for Today:
 Interpret the results of historical experiments in molecular
genetics
 Describe the structure of DNA

Agenda:
 Review Midterm Exams
 Lecture: Historical Experiments with DNA
Exam Scores:
100-90
2
89-80
6
79-70
6
69-60
2
59-0
0
A History of Experiments in
Molecular Genetics
Important questions of the 20th century:
• What is the genetic material?
• What is its structure?
• How does it replicate?
Griffith’s classic experiment
Britain, 1928
Griffith’s conclusions
• Live, nonpathogenic bacteria have taken
up some chemical from the dead,
pathogenic bacteria (transformation)
• The offspring of the nonpathogenic
bacteria inherited this material, becoming
pathogenic
Next question: What is the transforming
material?
What is the genetic material?
Previous studies have revealed:
• Genes are on chromosomes
• Chromosomes are passed from one generation
of cells to the next during cell division
• Chromosomes are made of proteins and
nucleic acids
Therefore, the genetic material could be
– Protein
– DNA
– RNA
Whatever this material is, it must be able to
– Store genetic info
– Replicate

Homework:
 Problem Set 12 #1, 2, 3

Do Now: (to think about, not necessarily write down)
1. What is the difference between replication and

transcription?
2. Take out your notes… What was the major conclusion from
Avery, McCarty, and McLeod’s experiment?
Goals for Today:
 Interpret the results of historical experiments in molecular
genetics
 Describe the structure of DNA
 Explain the major steps and enzymes involved in DNA
replication
Avery, McCarty, McLeod
United States, 1944
What is the genetic material of bacteria?
Conclusion ?
Hershey & Chase
1952
What is the genetic material of viruses?
Background:
• Bacteriophage (“phage”) are viruses that infect bacteria
& re-program the bacteria cell to make more viruses
• Phage are made of
– Protein shell
-- DNA (or RNA) inside
Hypothesis:
• If protein is the genetic material, then bacteria infected
with phage containing radioactive protein will become
radioactive
• If DNA is the genetic material, then bacteria infected
with phage containing radioactive DNA will become
radioactive
Hershey & Chase
1952
Conclusion?
Early 1950s: DNA is genetic
material
Evidence:
• Bacteria: Avery, McCarty, MacLeod
• Viruses: Hershey & Chase
• Eukaryotes:
– DNA (but not protein) content of cells doubles exactly
during mitosis
– DNA (but not protein) content of diploid cells is
exactly double that of haploid cells
Next question:
What is the structure of DNA?
We already
know (1950)
• DNA is a polymer of
nucleotides
• Each nucleotide has
three parts:
– Pentose sugar
(deoxyribose)
– Phosphate group
– Nitrogenous base (4
types)
Adenine
Guanine
Cytosine
Thymine
We don’t know:
• What is the polymer
structure?
• How does DNA encode
complex genetic info?
• How does its structure
enable replication?
4 Nitrogenous Bases
PYRIMIDINES
Sugar
PURINES
Sugar
Sugar
Sugar
What was Watson & Crick’s
evidence??
• Erwin Chargaff (1947)
– Percentage of DNA bases (A, G, T, C) varies
between species
– But always %A = %T and %G = %C
• Rosalind Franklin (1952)
– Made an X-ray diffraction
image of DNA that
suggested a double helix
The
Breakthrough:
Watson & Crick
1953
• DNA is a doublestranded polymer
• A H-bonds w/ T
• C H-bonds w/ G
Biochemical reasons
(width, H-bonding)
• DNA molecules
twist to form a
DOUBLE HELIX
Next question:
How does DNA replicate?
A quote from Watson & Crick’s Nobel Prizewinning paper:
“It has not escaped our notice that the
specific pairing we have postulated
immediately suggests a possible copying
mechanism for the genetic material.”
(1953)
3 Hypotheses for DNA Replication
Meselson-Stahl
experiment
• Parent DNA
molecules
are “heavy”
• Newlysynthesized
DNA
molecules
are “light”
1st round of
replication
2nd round of
replication
Meselson-Stahl Conclusion:
DNA replication is semi-conservative.
DNA Structure

Homework:
 Problem Set deadline extended until next Tuesday
 Tomorrow’s classwork (pre-lab and worksheet) also due Tuesday
if you don’t finish in class

Do Now:
1. What is the difference between DNA replication and making
RNA (transcription)?
2. From your review packet, what are 3 major steps of DNA
replication?
3. From yesterday’s notes, what are 3 enzymes important in
starting the process of DNA replication?

Goals for Today:
 Explain the major steps and enzymes involved in DNA
replication
 Explain how scientists use knowledge of DNA structure and
replication to study and manipulate DNA:
▪ How can we…
▪ Separate pieces of DNA based on their size
▪ Cut DNA at specific places
▪ Make many copies of a specific gene
▪ Find out the sequence of a DNA strand
▪ Attach different pieces of DNA (possibly from different
species) together in order to genetically engineer an
organism
DNA Replication:
Key enzymes in starting the process
DNA Replication:
Elongation
DNA Replication:
Leading vs. Lagging Strands
Lagging Strand
Elongation
DNA Replication:
Summary of Key Molecules
• Helicase – Splits apart the template DNA strands (unwinds and
unzips)
• RNA Primer – short strand of RNA on daughter strand that DNA Pol
uses to get started
• DNA Polymerase – assembles nucleotides on daughter strand
(complementary to template strand)
• Leading strand – daughter strand that is synthesized continuously
• Lagging strand – daughter strand that is synthesized discontinuously
• Okazaki fragment – pieces of lagging strand that need to
be connected
• DNA Ligase – enzyme that connects fragments of DNA
together
DNA Sequencing
Adel
Fatima
Kimberly
Aaron
Gel Electrophoresis
Rishab
Nehemie
Jasmine
Talia
PCR
Imad
Klara
Drishti
Nicole
Restriction Enzymes
Christina
Poppy
Kira
Djinnie
1. Research your technology in the textbook
2. Summarize on the handout. Use bullet
points no longer than 2 lines each! The
goal is to SUMMARIZE with the
ESSENTIAL info!
3. Create a powerpoint or a Word doc
(template on AP Bio blog)
4. Also be ready to DRAW a simple picture
on the board