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Chapter 13
DNA, RNA and Proteins
Pre-class question 4/27
If you were to take a chromosome, decompress
it into chromatin, separate the nucleosomes and
remove the histones, what would you have?
DNA
Homework – Due Tuesday (4/28)
Active Reading: The Structure of DNA
DNA is Genetic Material Found in Cells
3 Major Experiments
1. Griffith’s Discovery of Transformation
2 Bacteria Types
Rough (R), no capsule, broken down by the body’s
immune system, does not cause pneumonia
Smooth (S), capsule, protected from the body’s
immune system, causes pneumonia
Griffith’s Experiment Conclusions
Heat destroyed the capsule,
but not the S bacteria.
S bacteria has the genetic
code for making the capsule.
The genes of the S bacteria
were taken up by the R
bacteria, which then began
to make the protective
capsule.
The genetic code of the R
bacteria had been
transformed.
DNA, RNA or Protein?
2. Avery’s Experiment
Avery wanted to know what substance, DNA,
RNA or protein, was causing the transformation
in Griffith’s experiment.
When Avery treated the S heat killed bacteria
with enzymes to destroy both RNA and protein,
transformation still occurred.
When Avery treated the S heat killed bacteria
with enzymes to destroy the DNA,
transformation did not occur.
The Genetic Material in Viruses
2. Hershey Chase Experiment
Hershey and Chase worked with
bacteriophages (viruses that infect bacteria,
which in turn produce more viruses).
Bacteriophages are made of DNA and protein.
The scientists wanted to know which substance
the genetic material in the bacteriophage was
made of.
3 Phases
Step 1
Proteins Contain Sulfur and
DNA contain phosphorus. Two
virus samples were incubated
with either radioactive sulfur or
radioactive phosphorus.
Step 2
The virus samples were then
allowed to infect bacteria.
Each mixture was then
separated into two portions,
one containing bacteria and the
other containing only viruses.
Conclusion
Step 3
The infected bacteria
contained radioactive
phosphorus, but did not
contain radioactive sulfur.
Because the Bacteria contained radioactive
phosphorus, and phosphorus is a component of
DNA, Hershey and Chase conclude that DNA was
the genetic material in viruses.
Pre-Class Question 4/28
In the Hershey-Chase experiment, where
DNA was found to be the component of
bacteriophages responsible for “infecting”
bacteria, what radioactive substance was
found in the bacteria?
The Double Helix of DNA
Each strand of DNA is
made of subunits called
nucleotides
Each nucleotide is made
of 3 parts:
A phosphate group
A 5 carbon sugar molecule
(Deoxyribose)
A nitrogen containing base
The Information in DNA
Nitrogen Bases
Purines
Adenine (A) and Guanine (G)
Pyrimidines
Cytosine (C) and Thymine (T)
On opposite strands, A always
pairs up with T and C always
pairs up with G. Base pairs
are held together with hydrogen
bonds. The strands are said to be
complimentary.
Discovering DNA’s Structure
Chargaff’s Observation
In every organism, the amount of thymine always
equals the amount of adenine. Likewise the
amount of cytosine always equals the amount of
guanine.
Photographs of DNA
Rosalind Franklin and Maurice Wilkins
developed x-ray diffraction images of
strands of DNA.
Discovering DNA’s Structure, Cont.
Watson and Crick’s Model of DNA
By combining Chargaff’s observation and the
photographs taken by Franklin and Wilkins,
Watson and Crick created a 3-dimensional double
helix model of DNA which won them the Nobel
Peace Prize in 1962.
Replication of DNA
In DNA replication, the DNA molecule
unwinds, and the two sides split. Then
new nucleotide sequences are added to
each side until two identical sequences
result.
DNA replication occurs before cell division
so that each new daughter cell has a
complete copy of the parent DNA.
3 Steps of DNA Replication
Step 1 – Unwinding and Strand Separation
DNA helicases break the hydrogen bonds
between base pairs and unwind the helix. As
the strands separate, replication forks form
Step 2 – Adding Complementary Bases
DNA polymerase forms the new DNA molecule
by traveling along each single strand and adding
complementary nucleotides.
Step 3 – Formation of Two Identical DNA Molecules
Two identical DNA molecules are produced, each consisting
of one new strand
Prokaryotic and Eukaryotic Replication
Prokaryotic Replication
Eukaryotic Replication
 Prokaryotes have circular DNA  Eukaryotes have linear DNA
 Replication occurs at one site,  Replication occurs at many
producing 2 replication forks
sites, producing 2 replication
and proceeds in opposite
forks and proceeds in
directions
opposite directions.
Eukaryotic Replication
Because replication occurs at more than one
site on the DNA, eukaryotic DNA can be
replicated at a faster rate than prokaryotic
DNA. An entire human chromosome can be
replicated in about 8 hours.
Gene Expression and RNA
Stage I – Transcription
• Takes place in the nucleus
• RNA is made from DNA
Stage II – Translation
• Takes place in the cytoplasm on
ribosomes
• Uses RNA to make proteins
How is RNA different from DNA?
1. RNA is single stranded
2. An RNA nucleotide contains
ribose sugar, not deoxyribose.
3. RNA contains the nitrogenous
base, Uracil, but does not
contain Thymine.
3 Types of RNA
Messenger RNA (mRNA)
DNA is transcribed into messenger RNA. Messenger RNA
transports the genetic code to ribosomes for translation.
Transfer RNA (tRNA)
Transfer RNA reads the mRNA sequence and translates it
into amino acids which are used to make proteins.
Ribosomal RNA (rRNA)
Ribosomes are comprised of ribosomal RNA and proteins/
Transcription (3 Steps)
Step 1 – RNA polymerase binds to a “promoter” gene or
“start” location on the DNA.
Step 2 – RNA polymerase unwinds the DNA
Step 3 – RNA polymerase moves along the DNA adding
complementary RNA bases (A-U, T-A, C-G, G-C)
As RNA base pairs are added, the DNA strands
close up and reform the double helix. Eventually
the polymerase reaches a stop signal in the DNA.
Transcription vs. Replication
In both processes, DNA is used as a
template. However, transcription results in
the formation of mRNA whereas replication
results in the formation of DNA. During
replication, both DNA strands are involved
while only one DNA strand is involved in
transcription.
Pre-class Question 5/11
Which part of a nucleotide contains genetic
information?
Nitrogenous base
(A,T,C,G,U)
Homework
Chapter 13 Vocab Quiz Wednesday
Chapter 13 Test Friday
The Genetic Code: Three-Letter Words
• Each 3-nucleotide
sequence is called
a codon
• Each codon
contains the code to
make 1 of 20 amino
acids (building
blocks of proteins).
Translation: RNA to Proteins
Step 1-Binding of tRNA
to mRNA
The anti-codon of tRNA
binds to the mRNA start
codon at the ribosome.
Each tRNA carries one
amino acid. The amino
acid carried by this tRNA is
methionine.
Translation continued
Step 2 – Formation of peptide bonds
A second tRNA anti-codon binds to the next
sequence of mRNA. As a peptide bond is formed
between the two adjacent amino acids, the first
tRNA is released from the ribosome.
Translation Continued
Step 3 –
The ribosome moves one codon down the
mRNA. The mRNA and the tRNA move as
a unit.
Step –
The process of translating mRNA into an
amino acid chain continues until stop
codon is reaced
The Final Step of Translation
Step 5 –
The newly made polypeptide falls off the
ribosome, the ribosome complex falls apart
and the ribosome is then free to begin
translation again with the same mRNA
molecule or with a different transcript.
Pre-class Question 5/14
What is the anti-codon
of the mRNA
sequence AUG?
UAC
What amino acid
corresponds to this
anti-codon?
Tyr = Tyrosine
Homework (Due 5/15/09)
Read Article “When Darwin Got Sick of Feathers” and answer
questions
Translation Activity Instructions
1. Every pair of students should have …



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Ribosome Sheet
6 tRNA molecules
6 Amino acids
mRNA strip from transcription activity
2.Assigned Roles
 Cytoplasm (Provides tRNA for translation)
 Ribosome (Assembles Protein)