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DNA and RNA
Chapter 12
Griffith
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Studied disease in
humans in 1928
Wanted to know
how certain types
of bacteria cause
pneumonia
Found two strains
of pneumonia
bacteria
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Smooth coated
bacteria caused
the disease to
appear in humans
Rough coated
bacteria of the
same type did not
Rough
Strain
Smooth
Strain
Heat-killed
smooth
Live rough
and heat
killed smooth
Transformation
Somehow the heat-killed bacteria
had passed their disease-causing
ability to the harmless strain.
 One strain of bacteria was
“transformed” into a disease-causing
strain.
 Griffith called the process
transformation
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Bacteriophage
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Phage-eat
BacterioComposed of a
DNA or RNA core
and protein coat
Injects viral genes
into bacteria and
“reprogram” the
bacteria to make
more viruses
Hershey-Chase
If they could determine which part
of the virus, the protein coat or the
DNA core, entered the infected cell
they would know whether genes
were made of protein or DNA
 They grew viruses in cultures of
radioactive isotopes found in protein
coats and in DNA

Sulfur 35
DNA
Long molecule made up of units
called nucleotides
 Each nucleotide is made up of three
basic components

Sugar (deoxyribose)
 Phosphate group
 Nitrogenous (nitrogen-containing) base

DNA, It’s not
just for cells
anymore
Nucleotides
Deoxyribose
Nucleic Acid
H - Bonds
Return!
Chargraff’s Rules
 C=G in almost any
sample of DNA
 A=T in almost any
sample of DNA
 From bacteria to
humans, it’s
always true
Hydrogen Bonds Return!
Hydrogen bonds can form between
certain nitrogenous bases and provide
just enough force to hold the two
strands together
 H-bonds form only between given
pairs A-T and C-G
 This is known as base pairing
 Adenine-Thiamine, Cytosine-Guanine

Chromosomes
Eukaryote
Prokaryote
Prokaryotic DNA
The prokaryotic DNA molecule is
located in the cytoplasm.
 It is usually a single circular
molecule
 It contains nearly all of the cell’s
genetic information
 Usually referred to as the cell’s
chromosome.

Miles and Miles of Info!
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E. coli contains
4,639,221 base pairs
and is around 1.6mm
In larger terms, that’s
like stuffing 300m of
rope into your book
bag
Bacterial DNA is
usually about 1000
times as long as the
bacteria itself
Chromosome Structure
Many eukaryotes have as much as
1000 times the amount of DNA as
prokaryotes.
 The nucleus of a human cell
contains more than 1 meter of
DNA. (3.28 feet or 39.37 inches in
each nucleus!)
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Eukaryotic chromosomes contain both DNA
and protein, packed together to form
chromatin.

Chromatin is DNA
that is tightly
coiled around
proteins called
histones.
Histone
To make a chromosome
The DNA and histone molecules
form a bead-like structure called a
nucleosome.
 During most of the cell cycle the
fibers are dispersed and not visible.
 During mitosis, the fibers are drawn
together to form a tightly wound
chromosome.

Supercoils
Coils
Chromosome
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Nucleosomes seem to be
able to forld enormous
lengths of DNA into the
tiny space available in
Supercoils
the cell.
The histone proteins
Coils
have hardly changed
during evolution,
probably because
mistakes in DNA folding Nucleosome
could be devastating to a
DNA
cell
Histone
DNA Replication:
Why Multiply?
Mitosis and Meiosis both require
replication of DNA molecules
 Growth
 Healing from injury
 Replacing worn molecules

Creating Sex Cells for sexual
reproduction
Replication
1.
2.
Before a cell divides, it duplicates its DNA
in a copying process called replication.
The two DNA strands are “uncoiled” and
“unzipped” from each other by enzymes
regulating replication
1.
3.
“Unzipped” means the hydrogen bonds holding
the opposing bases together are broken
DNA polymerase builds a new
complementary strand onto each of the
old unzipped strands
DNA Replication
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In prokaryotes,
DNA replication
begins at a single
point in the
chromosome
Replication
proceeds in two
directions around
the circle
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In eukaryotes, DNA
replication occurs at
hundreds of places
Replication proceeds
in both directions
Sites where
separation and
replication occur are
called replication
forks
Original DNA strand
New DNA strands
“Unzipping” breaking
Hydrogen
bonds
Replication Fork
DNA Polymerase
Polymerase
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Joins individual
nucleotides to
produce a DNA
molecule (which is
a polymer)
“Proofreads” each
new DNA strand to
maximize odds of
correct pairing
DNA