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Chapter 12
DNA and Genes
DNA: The Molecule of
Heredity
• Vocabulary:
•
•
•
•
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Transformation
Bacteriophage
Nucleotide
Base pairing
Double helix
Key Concepts:
•What did scientists discover about the
relationship between genes and DNA?
•What is the overall structure of the
DNA molecule?
What we’ve learned so far…
•Cells make proteins
•Genetic information is passed on through chromosomes
•Compacted DNA and proteins= chromosomes
•Genetic information is stored in the nucleus
•Genetic information is essential so that each cell is
identical (mitosis)
To truly understand genetics,
biologists first had to discover the
chemical nature of the gene.
 How do genes control what you
look like?
Animal Husbandry
Goat herders 5000
years ago knew …
Offspring tend to look
like their parents.
DNA’s “Experiment” History
Fredrich Miescher (1869)

Studied pus cells he collected
from bandages in hospitals

White blood cells are huge.

Used an enzyme to digest
away the proteins in nucleus

A strange phosphorous
molecule remained

Molecule of heredity?
Fredrick Griffith (1928)
 Griffith:
How do certain types of
bacteria cause pneumonia?


The experiment that tested this
question led to new knowledge.
Genetic information could be
transformed from one bacterium
to another.
Fredrick Griffith (1928)

He injected mice with the one strain of bacteria
and they died.
 When he did the same with another strain of
bacteria they didn’t die.
 He could use heat to kill the deadly bacteria so it
wouldn’t kill mice.
 Injected mice with a mixture of harmless bacteria
and the heat killed bacteria…
Genetic Info.
Transferred
 The
mice died!!
 Something had passed from the heat killed
deadly bacteria to the living harmless
bacteria.
 He called this process
TRANSFORMATION.
Avery & DNA

Avery and a group of scientists decided to repeat
Griffith’s experiment
determine which molecule in the heat killed bacteria was
needed for transformation
Made an extract (juice) from heat killed bacteria and added
enzymes that destroy proteins, carbohydrates, & lipids




Transformation still occurred!
Repeated the experiment but added enzymes that destroy
DNA

Transformation DID NOT occur!!
Avery’s Conclusion
 What
was the “transforming factor”?
 DNA
DNA is the nucleic acid that
stores and transmits genetic info from
generation to generation
 Conclusion:
Hershey-Chase Experiment
(1952)

Studied viruses; specifically bacteriophages




Viruses—made up of:


Bacterio = bacteria
Phage = eater
Bacteria eater
DNA + protein coat
Research Question: Which part of the virus
enters the infected bacteria cell?
Hershey & Chase
Bacteriophage




Wanted to REALLY make
sure it was DNA
Viruses are made of only
protein and DNA
Proteins have sulfur but
no phosphorous.
DNA has phosphorous
but no sulfur
 Grew
viruses in the presence
of radioactive markers P32 and
S35.


Sulfur is found in protein coat
Phosphorus is found in DNA
Hershey-Chase Conclusion
 Genetic
information of the bacteriophage
that they infected with bacteria was DNA

NOT protein
What DNA does…
 Deoxyribonucleic
Acid = DNA
 Carries information from one generation to
the next
 Put that information to work by determining
heritable characteristics
 Must be easily copied since almost every
new cell gets a copy
DNA Structure

Made up of nucleotides (monomer)
 Each composed of 3 parts:



Deoxyribose: sugar
Phosphate group
Nitrogenous (nitrogen containing)
base
• Adenine (A)
• Thymine (T)
• Guanine (G)
• Cytosine (C)
DNA Structure =
Double Helix

Backbone


Deoxyribose & Phosphate
Group of each nucleotide
“Steps of Ladder”

bases
 Any
base sequence
is possible
Chargaff’s Rules
Percentage of Bases in Four
Organisms
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
What does this data suggest to you?
Oooooohhhh!!
Aaaaaahhhhh!!!!
A
=T
C = G
(A pairs with T)
(C pairs with G)
What DNA Looks Like
 Rosalind
Franklin: used X-Ray
diffraction patterns to discover that DNA
twisted around each other like a helix
 Watson
& Crick: while
trying to build a 3-D
model of DNA, Watson
was shown Franklin’s
photograph and within
weeks, figured out the
structure of DNA
What’s holding the strands
together?
 HYDROGEN BONDS!!!
 Hydrogen
Bonds form
between A and T base
pairs as well as
between C and G base
pairs