Chapter 11
DNA
and the
Language of Life
11.1 Genes are made of DNA
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Frederick Griffith studying two strains of bacteria
One strain fatal to mice, while other strain harmless
Griffith’s experiment
 Strain 1 injected = mouse dies
 Strain 2 injected = mouse healthy
 Heat treated Strain 1 = mouse healthy
 Mixture of Strain 2 and Heat treated Strain 1 =
mouse dies
Harmless bacteria had been “transformed”
becoming deadly
Griffith Experiment
Avery Experiment
 Destroyed
proteins
 Mice still died with
mix
Avery shows DNA is the transforming factor
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Attention was focused on two types of chemicals:
protein and DNA
 Scientists already knew that chromosomes consist
of DNA and protein
Avery took Griffith’s experiment one step farther.
Treated mixture of heat treated deadly strain and live
harmless strain with protein destroying enzymes
 Bacterial strains still transformed
 Conclusion was protein cannot be transforming
factor
 Next, treated mixture with DNA destroying enzymes
 This time colonies failed to transform
 Avery concluded DNA is genetic material of the
cell
Virus experiments provide more evidence
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Many scientists still skeptical after Avery’s findings
 Protein has more complexity (20 different amino acid
building blocks)
 DNA less complexity (4 nucleotide building blocks) “too
simple”
Hershey and Chase experiment using viruses
 Virus- package of nucleic acid wrapped in protein coat
 Bacteriophage- (AKA-phage) virus that infects bacteria
 Virus (phage) they worked with had 2 basic components:
DNA on inside and coat made of protein on outside
 Use of radioactive sulfur for protein coat and radioactive
phosphorus for DNA for the phages
 Radioactivity was detected outside the infected bacteria
for protein but inside the infected bacteria for the DNA
 Conclusion: DNA must carry genetic information
responsible for producing new phages
Hershey Chase Experiment
11.2 Nucleic Acids store information
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A. People involved with discovering DNA’s
structure
1. Rosalind Franklin & Maurice Wilkins –1950,
photographs of the DNA molecule using X-ray
crystallography which showed the shape to be
a helix
2.Erwin Chargaff – 1951, proved that the % of A =
T and % of G = C
3. James Watson & Francis Crick – 1953, used
data from the other scientists and built models
to finally figure out the exact structure of DNA….
1962 won the Nobel prize in
Medicine/Physiology
DNA Structure
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a. DNA is a double helix (twisted ladder) made of
subunits called nucleotides
b. 4 different nucleotides Adenine, Thymine,
Guanine, Cytosine
*Nucleotides are made of 3 parts:
5-carbon sugar (deoxyribose)
phosphate group
nitrogenous base
c. The sides of the double helix are made of
alternating sugars and phosphate groups, the rungs
of the double helix are made of nitrogenous bases
Complementary Base Pairs in making the double
helix
A pairs with T
G pairs with C
Nucleotides
Sugar-phosphate “backbone”
 DNA
strand
11.3 DNA REPLICATION
 DNA
replication is the process of copying the
DNA molecule.
 During DNA copying, the two strands of the
double helix separate.
 Each single strand acts as a “negative” or
“template” for producing a new,
complementary strand.
Easy steps for DNA Replication.
 Enzymes
are protein molecules that catalyze
chemical reactions in a cell – usually any protein
ending in “ase” is an enzyme
 1. DNA replication begins at specific sites called
the origins
 2. DNA helicase unwinds and separates the two
strands of original DNA molecule
 3. DNA polymerase 3 adds complementary
nucleotides to each separated strand
 4. DNA polymerase 1 checks for correct pairing
of nucleotides (looks for mutations) and fixes any
mispaired nucleotides
11.4 and 11.5 Protein Synthesis
 A.
Transcription is the making of single
stranded mRNA from a DNA strand
within the nucleus of a cell. During
transcription, RNA nucleotides basepair one-by-one with DNA nucleotides
on one of the DNA strands (called the
template strand). RNA polymerase links
the RNA nucleotides together.
TRANSCRIPTION (in the nucleus of the cell)
 B.
This takes place before the RNA leaves the
nucleus. First, the introns are removed and the
coding regions of the RNA transcript and the
exon are joined together, thus, producing the
mRNA molecule with a continuous coding
sequence. This process is called RNA splicing.
With it complete, the “final draft” of mRNA is
ready for translation.
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 C.
Base pairing is the same as DNA
replication, except that RNA has uracil instead
of thymine: the U in RNA pairs with A in DNA
Transcription
TRANSLATION
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Translation is converting the nucleic language into
amino acid language; much like when a reporter
transcribes a speech. The language remains the
same, however, the form of the message changes.
This is the transfer RNA (tRNA), or interpreter. An amino
acid is attached to the tRNA and will follow the threebase word to sequence for the proper “sentence”.
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A codon is the three-base “word” that codes for one
amino acid. Several codons form a “sentence” that
translates into a polypeptide. The three-base word,
AUG, dictates the start of the “sentence”. The process
continues until the ribosome reaches a stop codon,
UAA, UAG or UGA.
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G. The same genetic coding system is shared by
almost all organisms
Mutations can change the
meaning of genes
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1. A mutation is any change in the nucleotide sequence
of DNA.
2.Can be divided into two categories of mutations
 base substitutions – replacement of one nucleotide
with another
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Occasionally, a base substitution causes no change to a
protein, but sometimes it results in a change that affects the
function of a protein, sometimes drastically (ex: sickle cell
disease)
base insertions or base deletions – addition of an
extra nucleotide or subtracting a nucleotide usually
results in a more disastrous affect
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mRNA is read in triplets, by adding or subtracting nucleotides
may alter the triplet groupings of the genetic message.
Therefore, all the nucleotides that follow the mutation will be
regrouped into different codons. The result will be a
different, and probably a nonworking protein.
What causes mutations?
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Mistakes during DNA replication can cause
mutations
Mutagens – physical or chemical agents that
cause mutations
 Physical mutagens – high energy radiation, Xrays, Ultraviolet light
 Chemical mutagens – chemicals that are
similar to DNA bases and cause incorrect basepairing
Mutations are often harmful but may alter a
protein in a way that is beneficial
1. if a mutation is present in the gametes of an
organism it is passed on the offspring