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CHAPTER 9
DNA, RNA, Translation and
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The Discovery Of DNA
 Mendel studied the pea plants in the late
1800’s
 Scientists wanted to know what it was
that contained hereditary factors
 An epidemic of Pneumonia in London in
the 1920’s sparked research which
eventually leads to the discovery of
DNA.
GRIFFITH’s EXPERIMENTS
 1928,
London, England: Fredrick
Griffith was attempting to discover a
vaccine for the virulent strain, or
disease causing strain of the
Streptococcus pneumoniae
bacterium.
 Streptocuccus pneumoniae was
causing pneumonia
GRIFFITH’s EXPERIMENT
By studying the bacterium he discovered
that:
 The virulent form was surrounded by a
capsule made of polysaccarides that
protects it from the body’s defenses. So,
when grown in a petri dish, they grow as
smooth colonies. (S strain)
 There was an another strain that did not
cause pnuemonia. This harmless strain
lacked a capsule and as a result grew into
rough colonies in the petri dish. (R strain)

GRIFFITH’s EXPERIMENTS
Griffith experimented with mice.
 He injected a mouse with the S strain and
it died
 He injected another mouse with the R
strain, it lived
 He heated-killed the S strain and injected
it into the mouse, it lived
 He then combined heat-killed S strain and
live R strain and injected it into the
mouse.

GRIFFITH’s RESULTS
 When
injected with the heat killed S
strain combined with the R strain, the
mouse died.
 Griffith concluded that transformation
occurred.
 Transformation is a type of transfer
of genetic material from one cell to
another OR from one organism to
another organism.
Avery’s Experiments
1940’s, United States: Oswald Avery
wanted to test whether the transforming
agent in Griffith’s experiments was
protien, RNA or DNA.
 The scientists used enzymes to destroy
separately each of the 3 molecules in the
Heat-Killed S strain.

Avery’s Experiments

Experiment #1: Protease was used to destroy
the protien in one batch of the heat-killed S
strain.
 Experiment #2: RNase was used to destroy
the RNA in another batch of heat-killed S
Strain
 Experiment #3: DNase was used to destroy
the DNA in another batch of heat-killed S
strain.
 Each batch was combined with a batch of live
harmless R strain and injected into different
mice
Avery’s Results
The strain missing the protein and the
RNA were still able to transform the R
strain into the S strain and kill the mice.
 The strain missing DNA did NOT
transform the R strain and the mouse
lived.
 Result: DNA was responsible for
transformation in the bacteria

HERSHEY-CHASE EXPERIMENT
1952, United States: Martha Chase &
Alfred Hershey tested whether DNA or
protein was the hereditary material
viruses transfer when viruses enter a
bacterium.
 Bacteriophages – viruses that infect
bacteria

HERSHEY-CHASE EXPERIMENT




Radioactive isotopes was used to label the
protein and DNA in the bacteriophage.
Radioactive Sulfur (35S) was used to label
protein
Radioactive phosphate (32P) was used to
label DNA
Then, they allowed the protein-labeled
and the DNA-labeled bacteriophages to
independently infect E-coli bacteria.
HERSHEY-CHASE RESULTS




The scientists removed the coats of the
viruses by putting the solution in a
blender
The scientists then separated the
bacteriophages from the E coli by
centrifuge
They discovered that all of the viral DNA
and only a little bit of the protein had
entered the E coli.
Conclusion: DNA is the hereditary
molecule in Viruses
SECTION 2: DNA STRUCTURE
1950’s: By now most scientists
understood and accepted that DNA was
molecule that contained the hereditary
information.
What they wanted to discover was how
this molecule could replicate, store &
transmit hereditary information
THE DOUBLE HELIX
1950’s, James Watson, Maurice Wilkens
and Francis Crick
Won the Nobel Prize in Medicine for their
theory about the structure of DNA.
Structure was developed with the help of
Rosalind Franklin and her x-ray
photographs of DNA crystals
DNA NUCLEOTIDES
DNA is made up of TWO long chains (or
strands) of repeating subunits called
nucleotides
Nucleotides contain THREE parts:
– Deoxyribose: A 5-carbon sugar
– A phosphate group
– A nitrogen base – contains a Nitrogen,
Carbon and accepts hydrogen: Adenine,
Guanine, Cytosine, and Thymine
Bonds Hold DNA together
The DNA double helix is similar to a
spiral ladder.
 The alternating sugar and phosphate
molecules form the sides of the ladder
 Nucleotides are held together by
covalent bonds between the sugar of
one nucleotide and the phosphate of
the next nucleotide
 There are 10 nucleotide pairs for each
spiral turn of the DNA helix

Bonds Hold DNA together: the rungs
The bases (nitrogen bases) face toward
the center of the DNA molecule.
 The bases form hydrogen bonds with
bases on the other side and make up
“the rungs of the ladder”
 All pairs are of uniform width: in each
pair, one base has a two ring structure
and other base has a one ring structure

Nitrogen Bases
While the sugar and phosphate groups
are identical, the nitrogen bases could
be one of four kind (broken down to
two groups by structure):
 Purines: contain a double ring of
carbon & nitrogen: Adenine & Guanine
 Pyrimidines: contain a single ring of
carbon & nitrogen: Thymine & Cytosine

COMPLIMENTARY BASES
 Just before DNA structure was confirmed: it was
found that the percentage of adenine is equal to the
percentage of thymine
 It was also found that the percentage of cytosine
was equal to the percentage of guanine
 Upon Observation, Base-Pairing Rules was
uncovered – cytosine pairs only with guanine, and
adenine pairs only with thymine
COMPLIMENTARY BASES
 These pairs are known as Complementary
base pairs – one double-ringed purine and
one single-ringed pyrimidine
 Because of these base pairing rules, the
order if bases on one side of a chain of
DNA is complementary to the order of
bases on the other side.
 Base Sequence: the order of nitrogen
bases on a chain of DNA
Complementary Bases
 WHY IS THE BASE PAIRING IMPORTANT?
 Hydrogen bonds b/t the base pairs help hold the
two strands of a DNA molecule together
 The complementary nature of DNA helps explain
how it is that DNA replicates before a cell divides
– One strand of a DNA molecule can serve as a
template for making a new complimentary
strand.
DNA MODELS
 You will see the structure of DNA simplified
from the actual double helix model to a
straight ladder model or just the base
pairings
 This is because the only variable is the base
pairs, the sugar and phosphate groups are
identical in all DNA
Section 3: DNA REPLICATION
 The discovery of the double helix structure
of DNA explains how it can replicate exactly
each time a cell divides, the key feature of
hereditary material.
How DNA replicates
 DNA Replication is the process by which
DNA is copied in a cell before a cell divides
by mitosis, meiosis or binary fission.
 Because the two strands of DNA are
complimentary, each serve as a template to
make a NEW COMPLIMENTARY STRAND
 After replication, the 2 identical doublestranded DNA molecules separate & move
to new cells formed during cell division.
STEPS OF DNA REPLICATION
 Step 1
 Helicases: Enzymes that separate the DNA
strands
 Helicase move along the strands and breaks
the hydrogen bonds between the
complimentary nitrogen bases
 Replication Fork: the Y shaped region that
results from the separation of the strands
STEPS..
 Step 2
 DNA Polymerase: enzymes
that ADD complimentary
nucleotides.
 Nucleotides are found floating
freely inside the nucleus
 Covalent bonds form between
the phosphate group of one
nucleotide and the deoxyribose
of another
 Hydrogen bonds form between the complimentary
nitrogen bases
STEPS..
 Step 3
 DNA polymerases finish replicating the DNA
and fall off.
 The result is two identical DNA molecules
that are ready to move to new cells in cell
division.
 Semi-Conservative Replication: this type of
replication where one strand is from the
original molecule and the other strand is new
http://www.johnkyrk.com/DNAreplication.html
Something about Step 2
 In step 2, remember that each strand is
making its own new strand.
 DNA synthesis is occurring in two different
directions
 One strand is being made towards the
replication fork and the other is being made
away from the fork. The strand being made
away from the fork has gaps.
 Gaps are later joined by another enzyme,
DNA ligase
www.lewport.wnyric.org/JWANAMAKER/animations/DNA%20Replication%20-%20long%20
Prokaryotic Replication
 Prokaryotic Cells have one circular
chromosome.
 Two replication forks are formed in the same
area of the chromosome.
 They proceed in opposite directions – like
two zippers
 Replication continues along each fork until
they meet and the entire the entire molecule
is copied
DNA ERRORS
 Usually DNA replication occurs without any errors.
Only about one error in about every billion
replications occurs.
 DNA polymerases have a repair function that
“proofreads” the DNA. It will replace a wrongly
placed nitrogen base
 Mutation: a change in the nucleotide sequence.
 A mutation can have serious effects on the
function of an important gene and disrupt an
important cell function
DNA Errors
 Chemicals and Ultraviolet radiation from the sun
and tanning booths can change DNA
 Some mutations can lead to cancer. Mutations in
the way the cell divides can lead to tumors.
 An effective mechanism for the repair of damaged
DNA is very important to the survival of an
organism.
 Studying DNA replication is important to
understanding and treating various types of
cancer.
Section 4: Protein Synthesis
DNA contains genes that code for a
hereditary characteristic, example: hair
color.
 The gene that codes for hair color directs
the making of a protein called melanin in
the hair follicle.
 The protein is made through an
intermediate (middle man) – a nucleic acid
called RNA, Ribonucleic Acid

RNA STRUCTURE & FUNCTION
DNA and RNA are similar in that they are
both made up of nucleotides.
DNA and RNA differ in Four Ways:
1. RNA has ribose, DNA has deoxyribose
2. RNA contains a nitrogen base uracil
instead of thymine
3. RNA is single stranded*
4. RNA is much shorter than DNA. It
contains the information for one gene.
TYPES OF RNA

1.
Three Major Types of RNA
Messenger RNA (mRNA) ~ single
stranded RNA molecule that carries the
instructions from a gene to make a
protein. Carries the genetic “message”
from the DNA in the nucleus to the
ribosomes in the cytosol
TYPES OF RNA
Three Major Types of RNA
2. Ribosomal RNA (rRNA) ~
part of the structure of
ribosomes, where protein
synthesis occurs
3. Transfer RNA (tRNA) ~
transfers amino acids to the
ribosome to make a protein.

FLOW OF GENETIC INFORMATION
 How the information goes from DNA to
the Ribosomes and into protein form.
1. Transcription – DNA acts as a template
for the synthesis of RNA
2. Translation – RNA directs the assembly
of the proteins.
3. Protein Synthesis – proteins are formed
based on information in DNA and
carried out by RNA in the ribosomes
TRANSCRIPTION
 Transcription means that the information within
DNA is transcribed/“rewritten” as an RNA
molecule
 Occurs in three steps:
1. RNA polymerase, an enzyme that catalyzes
(starts) the formation of RNA on a DNA
template.
a. A promoter is a specific nucleotide sequence
of DNA where RNA polymerase binds and
initiates transcription.
b. After RNA polymerase binds to the promoter,
DNA strands unwind and separate
TRANSCRIPTION
 THREE STEPS OF TRANSCRIPTION
2. RNA polymerase adds free RNA nucleotides that
are complementary to the nucleotides on one of the
DNA strands. The resulting chain is an RNA
molecule.
a. Complementary base-pairing determines the
nucleotide sequence in the newly made RNA.
b. Transcription only occurs in a specific area (one
gene) of the DNA. RNA polymerase moves past
the area and DNA rewinds
TRANSCRIPTION
 THREE STEPS OF TRANSCRIPTION
3. RNA polymerase reaches the terminal
signal, a specific sequence of nucleotides
that marks the end of the gene.
a. Upon reaching this mark, RNA polymerase
releases both the DNA & the newly formed
RNA.
b. The newly formed RNA can be any type of
RNA, free to perform it’s job within the cell.
The Genetic Code
• What makes up a protein?
– Amino Acids
• Instructions on which amino acids to
assemble are coded within the sequence of
nucleotides.
• Genetic Code – the term for the rules that
relate how a sequence of nitrogen bases
corresponds to a particular amino acid
• There are 20 different amino acids found
in living things
The Genetic Code
• Codon – each 3-nucleotide sequence in
mRNA that encodes an amino acid or
signifies a start or stop sequence
• RNA Polymerase unwinds and unzips DNA
(but does not proof-read… why not?)
• Complementary NTP’s add to template
DNA strand from 5’ to 3’
• RNA Polymerase begins transcribing
the DNA at a specific point
• RNA strand is identical to the noncoded
DNA (and complementary to
the template
strand)
EXCEPT FOR...
• Same process as Prokaryotes!
• After mRNA is transcribed from DNA
then the mRNA has a different
fate
in prokaryotes and
eukaryotes
• Prokaryotes immediately begin
translating the mRNA.
Eukaryotes must process it first.
No mRNA Processing
mRNA Processing:
• intron/exon
• methyl cap
• poly-A tail
• Viral DNA injected into cells
• Cells evolve nucleases in cytoplasm that chomp
up any RNA or DNA out there
• Nucleases can’t get through the nuclear
envelope so DNA is safe
• mRNA sent out into the cytoplasm must be
protected
– Methyl cap is a block
– Poly A tail is a fuse
• mRNA is still chomped up into NTP’s and
recycled, but the Poly A tail gives it some time
• Eukaryotic DNA is composed mostly of “noncoding DNA” (or “junk DNA”)
– We’re still not entirely sure what it does
– Was probably inserted by different viruses over time
– The ultimate selfish gene just hitching a ride on a
successful group of genes…
• The introns are the sections of DNA not
expressed, the exons are the sections that are
expressed (ex-ons are ex-pressed, get it?)
• Spliceosome loops out the introns and snips them
out
TRANSLATION
the making of a protein
 The start codon: AUG – a specific
sequence (Adenine, Uracil, Guanine) of
nucleotides in mRNA that indicates where
translation should begin – Methionine
 UGA – stop codon - doesn’t code for an
amino acid but signals for translation to
stop
TRANSLATION
 Every protein is made up of one or
more polypeptides.
 Polypeptides are amino acid chains
which are linked by peptide bonds
 Each polypeptide chain may consist of
hundreds of thousands of the 20
different amino acids – arranged in a
sequence that is specific to THAT
PARTICULAR PROTEIN
TRANSLATION
STEP ONE
 Initiation: tRNA and mRNA join
together. The tRNA carries a
anticodon – three nucleotides that
are complimentary to the sequence of
codon on the RNA (the start codon)
 The start amino acid is methinonine
but this would be removed later
TRANSLATION
STEP TWO
 Elongation – the chain is put
together
 Another tRNA carrying the
second appropriate amino acid
pairs with the mRNA
 A peptide bond forms between
the first amino acid and this one
(and so on). The first tRNA
detaches
TRANSLATION
Step Three
 The polypeptide chain continues to
grow
 Another tRNA moves in, carrying the
amino acid for the next mRNA codon
 The growing chain moves from one tRNA
to the amino acid attached to the next
tRNA
TRANSLATION
Step Four
 The ribosome reaches the stop codon
 The newly made polypeptide falls off.
Step Five
The ribosome complex falls apart. The newly
made polypeptide chain is released
 The last tRNA leaves the ribosome
 The ribosome is free to translate the same
or another mRNA
THE HUMAN GENOME
The Human Genome
 Genome – the complete genetic content
 In 1999, scientists have mapped the entire
gene sequence of the human genome
 They now know the order of the 3.2 billion
base pairs in the 23 human chromosomes
 The next challenge is to learn what
information the DNA sequences actually
code for.
