DNA
&
The Language of Life
(Chapter 11)
Griffith’s Transformation Experiment
1928 – Frederick Griffith is studying how certain
strains of bacteria cause pneumonia and
inadvertently makes a discovery about how genetic
information is passed from organism to organism
His Experiment:
 Grow two slightly different strains (types) of bacteria
 One strain proven harmless and other deadly
 Laboratory mice are injected with these strains
Griffith’s Results
What caused Griffith’s results?
• The heat-killed strain passed on its disease-causing
ability to the live harmless strain.
• If Griffith’s words, one strain of bacteria was
TRANSFORMED into another.
Oswald Avery
•Further demonstrated
that DNA was the
molecule that carried the
genetic code
•Destroyed other
components of virulent
pneumonia (protein,
lipids, carbohydrates and
RNA– infection still
occurred unless the
nucleic acid DNA, was
destroyed.)
The Hershey-Chase Experiment
• Alfred Hershey & Martha Chase studied viruses,
which are non-living particles smaller than a cell that
can infect living organisms.
• Bacteriophages: specific group of viruses that infect
bacteria.
• OBJECTIVE: To determine which part of the virus
(protein or DNA) enters a bacteria it is infecting.
How do bacteriophages infect bacteria?
o When a bacteriophage enters a bacterium, the virus
attaches to the surface of the cell and injects its genetic
information into it.
o The viral genes replicate to produce many new
bacteriophages, which eventually destroy the bacterium.
o When the cell splits open, from viral overload, hundreds
of new viruses burst out and can infect surrounding cells
The Hershey Chase Experiment
• Goal: determine which part of the virus entered an
infected cell to they would learn whether genes were
made of protein or DNA.
• Method: grew viruses in cultures containing
radioactive isotopes of phosphorus-32 (32P) and
sulfur-35 (35S).
– Some viruses had P-32 in their DNA,
– Some had S-35 in their protein coat.
• If S-35 is found in the bacteria: viruses release their
protein as the genetic material
• If P-32 is found in the bacteria: viruses release their
DNA as the genetic material
Hershey-Chase Results
The genetic material in bacteriophages was the _______________
(not the____________________________)!!!
Chargaff’s Rule & Rosalind Franklin
• Edwin Chargaff discovered that in almost any DNA
sample, the % G nearly equals the % C and the % A
nearly equals the % T
• Rosalind Franklin used x-ray diffraction to get
information about the structure of DNA.
• She aimed an X-ray beam at concentrated DNA
samples and recorded the scattering pattern of the Xrays on film.
Watson & Crick
• Using clues from Franklin’s pattern, James Watson
and Francis Crick built a model that explained how
DNA carried information and could be copied.
• Their model was the previously referenced “doublehelix”, in which two strands of DNA were wound
around each other to form a “twisted-ladder” type
structure.
DNA Structure
• Made of monomers called nucleotides
• Nucleotide structure:
A nucleotide can have one of four bases:
Types of bases:
 Adenine
 Guanine
 Cytosine
 Thymine
A & G are bigger and are called purines
C & T are smaller and are called pyrimidines
Base-Pairing
• Watson & Crick discovered that bonds can only
form between certain base pairs, Adenine &
Thymine and Cytosine & Guanine.
• The base-pairing rule means that purines only
pair with pyrimidines, making the rungs equally
spaced like a ladder.
• The nitrogenous bases are held together by
hydrogen bonds.
– A & T are held together by TWO hydrogen bonds
– C & G are held together by THREE hydrogen bonds
DNA is a “double-helix” or twisted ladder:
oThe “backbone” or sides of the DNA molecule are made up of
alternating sugars and phosphates and the “rungs” are made up
of interlocking nitrogen bases.
oThe sugars and the phosphates are held together by covalent
bonds and the nitrogen bases are held together by hydrogen
bonds.
Identify the parts of the DNA double-helix
Chromosome Structure
• Eukaryotic chromosomes contain DNA and protein
tightly packed together to from chromatin.
• Chromatin consists of DNA tightly coiled around
proteins called histones.
• DNA & histone molecules form nucleosomes, which
pack together to form thick fibers of chromosomes.
DNA Replication
• Why would a cell need to replicate (copy) it’s
DNA?
• When in the cell cycle does this process occur?
• In order to maintain the integrity of the cell,
what must be accomplished during DNA
replication?
DNA Replication
• Before a cell can divide, it’s DNA must be replicated or
copied in the S-phase of the cell cycle.
• In most prokaryotes, replication begins at a single point
and continues in two directions.
• In eukaryotes, replication occurs in hundreds of places
simultaneously and proceeds in two directions until
complete.
• Sites of replication are called replication forks.
How does the process occur?
1. Helicase untwist DNA molecules.
2. Restriction enzymes unzip the molecule.
3. DNA polymerase brings in complementary base
pairs for each strand
4. Ligase “glues” together the nucleotides
Process is semi-conservative.
 Each “new” strand of DNA consists of one original
template strand and one newly made strand.
 This allows for proofreading, using the template
strand as the “master”.
Visual Summary of DNA replication
ANIMATION
Meselson & Stahl Experiment
“Central Dogma”
Protein synthesis occurs in 2 steps:
transcription (DNA  RNA) & translation (RNA  protein)
RNA & Protein Synthesis
• Genes are coded DNA instructions for the
construction of proteins.
• DNA is located in the nucleus, but proteins are made
in ribosomes
• To avoid damage to the DNA molecules, they are first
decoded into RNA which is sent to the ribosome to be
the instructions for protein synthesis.
DNA v. RNA
DNA
1. Sugar is deoxyribose
2. Double-stranded
3. A, T, C & G bases
RNA
1. Sugar is ribose
2. Single-stranded
3. Uracil instead of thymine
Three types of RNA
• mRNA (messenger) – carries copies of instructions for
assembling amino acids into proteins
• rRNA (ribosomal) –
makes up part of the ribosome
• tRNA (transfer) – carries each AA
needed to build the protein to the ribosome
Transcription
• RNA is produced when RNA polymerase copies a
sequence of DNA (gene) into a complementary RNA
strand.
• Note: Gene sequence is only one side of DN A
molecule
RNA Processing/Editing
Translation
• Decoding of an mRNA message
into a polypeptide chain (protein)
• mRNA molecules are “read” in
three base segments called codons
• Each codon specifies a particular
amino acid
• Some AA are specified by more
than one codon.
The Genetic Code
Steps of translation
1. After RNA is transcribed in the nucleus, it enters the
cytoplasm and attaches to a ribosome
2. Codons are read at the “A site” and the appropriate
AA is brought in by tRNA, which has a
complimentary anticodon
3. The tRNA and amino acid shift over to the P site and
“wait” for the next AA to arrive at the A site.
4. A peptide bond forms between the two AA and the
molecule shifts over again
5. Empty tRNA can now be reused
6. Process continues until a STOP codon is reached.
Animation
Mutations
• Mutations are changes in the genetic material
– Gene Mutations: change is within a single gene
– Chromosomal mutations: change in an entire chromosome
• Point mutations are gene mutations involving a change in
one or a few nucleotides
– Substitution: usually changes only one AA
– Frameshift: addition or deletion of a nucleotide shifts the
grouping of codons
Significance of Mutations
• Many mutations have little or no effect on gene
expression
• Some mutations are the cause of genetic disorders
• Some mutations may be beneficial and lead to
production of proteins with new or altered activities
• These beneficial mutations have an important role in
the evolutionary process of natural selection
Substitution (missense)Mutations
Frameshift (nonsense) Mutations