Download Chapter 13 DNA_Honors Biology

DNA
Chapter 13
The History of DNA
Evidence That DNA Can Transform
Bacteria
 The discovery of the genetic role of DNA began
with research by Frederick Griffith in 1928
 Griffith worked with two strains of a bacterium,
one pathogenic and one harmless
1. Fredrick Griffith
 A British microbiologist- He was studying a
bacterium called Streptococcus pneumoniae
(pneumonia) and trying to develop a vaccine

Some strains of this bacterium can cause lung disease pneumonia
in mammals
 He used virulent (S) and nonvirulent (R) bacterial
cells to show that the heredity material can pass
from cell to cell

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Experiment 1 & 2: he injected either the live R or live
S cells into mice. He found that only the live S cells
killed the mice
Experiment 3:injected heat killed S cells and the mice
lived
Experiment 4: injected mice with both heat killed S
cells and R cells and found the mouse died
Griffith concluded

From his 4 experiments that heat-killed virulent
bacterial cells release a heredity factor that
transfers the disease-causing ability to the live
harmless cells
 This type of transfer of genetic material from
one cell to another cells is called
transformation, now defined as a change in
genotype and phenotype due to assimilation
of foreign DNA
2. Avery’s Experiments
 Early 1940’s, American researcher Oswald Avery
set out to test whether the transforming agent in
Griffith’s experiment was protein (DNA or RNA)
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
He used enzymes to separately destroy each of the 3
molecules in the heat-killed S cells
Then they separately mixed the 3 experimental bathes of
heat killed S with live R cells & injected them in mice
The Experiment
 Experiment 1: used protease enzyme to
destroy protein in the heat-killed cells
 Experiment 2: used RNase enzyme to destroy
RNA
 Experiment 3: used DNase enzyme to
destroy DNA
 Experiment 4: separately mixed the 3
experimental batches of heat-killed S cells
with live R cells & injected mice with the
mixtures
Avery’s
Experiment
His findings
 Cells missing protein & RNA were able to
transform R cells into S cells and kill the
mice
 But, cells missing DNA did not transform R
cells into S cells so the mice lived
 They concluded that DNA is responsible for
transformation in bacteria
3. Hershey-Chase Experiment
 1952, Two American researchers, Martha
Chase & Alfred Hershey set out to test
whether DNA or protein was the hereditary
material viruses transferred when viruses
enter a bacterium
 Viruses that affect bacteria is called a
bacteriophage
Bacteriophage
 Bacteriophage is a virus
that infects bacteria T-4
Step 1 of their experiment
 In Step 1: they used radioactive isotopes to
label the protein and DNA in the phage


Radioactive Sulfur to label the protein
Radioactive Phosphorus to label DNA
 Then the Protein-labeled & DNA-labeled
phage to separately infect the E.coli bacteria
Step 2 & 3 of their experiment
 In Step 2: they removed the phages coats
from the cells in a blender
 In Step 3: they used a centrifuge to separate
the phage from the E.coli

They found that all of the viral DNA & little of
the protein had entered the E.coli cells
 They concluded that DNA is the hereditary
molecule in viruses
Hershey-Chase Experiment
4. Watson & Crick
 In the 1950’s, James
Watson (American
biologists) and Francis
Crick (British graduate
student) teamed up to
determine the structure
of DNA
 In 1953, James Watson and Francis Crick
introduced an elegant double-helix model for the
structure of deoxyribonucleic acid, or DNA
 DNA, the substance of inheritance, is the most
celebrated molecule of our time
 Hereditary information is encoded in DNA and
reproduced in all cells of the body (DNA
replication)
 They proposed that DNA is made of 2 chains
that wrap around each other in the shape of a
double helix
 They relied on other scientists’ work to
develop their DNA model
5. The work of Rosalind
Franklin & Maurice Wilkins
 Part of that work was X-ray diffraction photographs
of DNA crystals
 Photographs were produced by Rosalind Franklin
and Maurice Wilkins
 In 1962, Watson, Crick, & Wilkins received the
Nobel Prize in medicine for their work on DNA

Rosalind Franklin died in 1958 and could not be named
to the award, but Cambridge Univ. recognizes her work
 Rosalind Franklin's
original X-ray
diffraction photo
revealed the physical
structure of DNA.
OREGON STATE
UNIVERSITY
LIBRARIES SPECIAL
COLLECTIONS
The DNA Structure, & Replication,
Welcome to Jurassic Park Mr. DNA
DNA Nucleotides
 DNA is a nucleic acid made of two long chains of
repeating subunits called nucleotides
 Each nucleotide consist of 3 parts:
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1. Deoxyribose: a 5 carbon sugar
2. Phosphate Group: consist of phosphorus atom bonded
to 4 oxygen atoms
3. Nitrogen Base: contains nitrogen & carbon atoms & is
a base (accepts hydrogen ions)
Bonds hold DNA together
 The DNA double helix is like a spiral staircase
alternating sugar and phosphate molecules
 Nucleotides are connected by covalent bonds
 Each full turn of the DNA helix has 10 nucleotide
pairs
 The nitrogen bases face toward the center and form
hydrogen bonds between the bases on the other
strand
Parts of the Nucleotide
 A nitrogenous base is a carbon ring structure that
contains one or more atoms of nitrogen.
 In DNA, there are four possible nitrogenous bases:
adenine (A), guanine (G), cytosine (C), and
thymine (T).
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
The Nitrogen Bases
 The nitrogen bases and their chemical
structures called rings .
 Nitrogen bases have a double ring of carbon
and nitrogen atoms <ie: Adenine and guanine
are called purines
 Nitrogen bases that have a single ring of
carbon and nitrogen atoms <ie: Cytosine and
thymine are called pyrimidines
Erwin Chargaff
 In 1949, Erwin Chargaff observed that the
percentage of adenine equals the percentage of
thymine, and the percentage of cytosine equals the
percentage of guanine in DNA
 This observation lead to the base-pairing rules: In
DNA, cytosine on one strand pairs with guanine on
the opposite strand; and adenine pairs with thymine
Complementary Base Pairs
 These pairs of bases are called
complementary base pairs
 The order of the base (base sequence) of a
chain of nucleotides of DNA is
complementary to the order of bases on the
opposite chain
2 Reasons
 Complementary base pairing is important in
DNA structure & function
1. Hydrogen bonds between the base pairs
help hold the two strands together
2. The complementary nature of DNA helps
explain how DNA replicates before a cell
divides (one stand can serve as a template for
making a new complementary strand)
 The structure of DNA
DNA Replication
Is the process by which DNA is copied
in a cell before the cell divides by
mitosis, meiosis or binary fission
Steps of Replication #1
 Enzymes called helicases separate the DNA
strands
 Helicases move along the DNA molecule
breaking the H-bonds between the
complementary bases
 This allows 2 DNA strands of the double
helix to separate from each other
 The Y-shaped region that results when the
two strands separate is called a replication
fork
Steps of Replication #2
 DNA polymerases (enzyme) add complementary
nucleotides (found floating freely inside the
nucleus) of each original strand
 Covalent bonds form between the deoxyribose
sugar of one nucleotide and phosphate group to the
next nucleotide on the growing strand
 H-bonds form between the complementary nitrogen
bases on the original & new strand
Step of Replication #3
 DNA polymerases finishes replicating the
DNA and fall off
 The result is 2 separate and identical DNA
molecules that are ready to move to new cells
in cell division
 Each new DNA double helix (one original,
one new strand) is called semi-conservative
replication because each of the new DNA
molecules has kept one of the two original
DNA strands
Action at the replicating fork
 DNA synthesis occurs in different directions on
each strand
 As the replicating fork moves along the original
DNA, synthesis follows the movement
 Synthesis on the other strand moves in the opposite
direction away from the replication fork which
leaves a gap
 The gaps are later joined together by an enzyme
called DNA ligase
Action at the replicating fork
DNA Replication
http://www.ncc.gmu.edu/dna/repanim.
htm
DNA Errors in Replication
 Errors: only about 1 in every billion paired
nucleotides
 Accuracy: DNA polymerase have repair
functions that proofread DNA

Example: if A bonds with G instead of T, DNA
polymerase can repair the error by removing the
mispaired G and replacing it with T.
Mutation
 A change in the nucleotide sequence of a
DNA molecule is called a mutation
 Mutations have serious effect on the
functions of important genes and disrupt cell
function
Point Mutation
 A point mutation is a change in a single base pair in
DNA
 A change in a single nitrogenous base can change
the entire structure of a protein because a change in
a single amino acid can affect the shape of the
protein.
example
 Normal:
THE DOG BIT THE CAT
Mutation:
THE DOG BIT THE CAR
The effects of point
mutations
mRNA
Normal
Protein
Stop
Replace G with A
mRNA
Point
mutation Protein
Stop
Frameshift Mutation
 What would happen if a single base were lost from
a DNA strand?
 Frameshift mutation is an addition or
deletion of a base in a DNA strand
Frameshift Mutation
Deletion of U
mRNA
Protein
Causes of Mutations
 Could be spontaneous, or caused from
environmental factors
 Any agent that can cause a change in DNA is called
a mutagen
* Radiations
*Chemicals
* High Temperatures
RNA & Protein Synthesis
Ribonucleic Acid
RNA vs. DNA
1. Ribose
2. Single Helix
3. A = U
Adenine = Uracil
1. Deoxyribose
2. Double Helix
3. A = T
Adenine = Thymine
Protein Synthesis
 Transcription: DNA acts as a template for the
synthesis of RNA
 Translation: RNA directs the assembly of
proteins (translates RNA to codons)
 Protein Synthesis: forming proteins based on
information in DNA and carried out by RNA.
AKA: gene expression
3 Types of RNA: each plays a
different role in protein synthesis
1. Messenger RNA (mRNA): a singlestranded RNA molecules that carries the
instruction from a gene to make a protein
 In Eukaryotic cells, mRNA carries the
genetic “message” from DNA in the
nucleus to the ribosome in the cytosol
3 Types of RNA: each plays a
different role in protein synthesis
2. Ribosomal RNA (rRNA): is part of the
structure of ribosomes

Remember:
Ribosomes-organelles in the cells that make
protein
Ribosomes-are made of rRNA’s and many
proteins
3 Types of RNA: each plays a
different role in protein synthesis
3. Transfer RNA (tRNA): transfers amino acids
to the ribosome to make protein
From RNA to protein Synthesis
http://www.youtube.com/watch?v=NJx
obgkPEAo
Transcription
 During transcription, the enzyme of RNA
polymerase “reads” one of the chains, the template
strand.
 RNA polymerase adds and joins complementary
RNA nucleotides resulting in an RNA strand
 AGCTACC
 UCGAUGG
(Transcription)
The Genetic Code
 Amino acids are assembled based on
instruction encoded in the sequence of
mRNA.
 3 adjacent nucleotides “letters” in mRNA
specify an amino acid “word” in a
polypeptide
 Each three-nucleotide sequence that encodes
an amino acid, start, or stop is a codon
The Genetic Code
 All organisms use the same genetic code
 This provides evidence that all life on Earth
evolved from a common origin
Codon’s
 One special codon: AUG acts as a start
codon
 UAA, UAG, or UGA codes for stop
 There are 64 codon combinations
Facts about Codons
 Some codons do not code for amino acids; they
provide instructions for making the protein
 More than one codon can code for the same amino
acid.
 For any one codon, there can be only one amino
acid
 There are 20 different amino acids found in the
proteins of all living things
Translation
 The making of protein
Human Genome
 In the years since Watson & Crick, biologists have
achieved a milestone in applying this knowledge to
human biology
 The entire gene sequence of the human genome is
now known
 The human genome is so large it would take a
person almost 10 years to read the total sequence
 The challenge now is to learn what information
these sequences actually encode