Chapter 12: DNA & RNA
Section 12.1 – Structure of DNA
DNA – Deoxyribonucleic Acid; traits are
determined by your genes, genes code for
proteins, and genes are coded for in your
DNA
*Was discovered in the 1940’s to be
the genetic material responsible for
traits.
I. Scientists Discovering DNA
A. Frederick Griffith (1928)
1. Worked with two strains of a pneumonia
bacterium, one pathogenic and one harmless
2. When he mixed heat-killed remains of the
pathogenic strain with living cells of the
harmless strain, some living cells became
pathogenic
3. Transformation – one strain of bacteria is
changed into another strain; some “factor”
was transferred from one strain to the other
Griffith’s Experiment
B. Oswald Avery (1944)
1. Repeated Griffith’s experiment, but added
enzymes to heat-killed bacteria that would
destroy carbs, proteins, lipids, & RNA.
2. When mixed with harmless bacteria,
transformation still occurred (mice died)
3. Repeated and added enzymes that destroyed
DNA, and when mixed with harmless bacteria,
no transformation occurred (mice lived!)
4. Proved DNA was the transformation “factor”,
meaning DNA stores & transmits genetic info.
Avery’s Experiment
C. Alfred Hershey & Martha Chase (1952)
1. Provided further evidence that DNA is genetic
material by using bacteriophages (phages),
viruses that infect bacteria
2. Used a phage known as T2 which infects E. coli
3. Radioactive Isotopes – used to tag DNA and
protein
• Sulfur to tag protein
• Phosphorus to tab DNA
4. DNA was proven to be the genetic material of
viruses versus protein
Phage infecting Bacterium
Figure 16.4-1
EXPERIMENT
Phage
Radioactive
protein
Bacterial cell
Batch 1:
Radioactive
sulfur
(35S)
DNA
Radioactive
DNA
Batch 2:
Radioactive
phosphorus
(32P)
Figure 16.4-2
EXPERIMENT
Phage
Radioactive
protein
Empty
protein
shell
Bacterial cell
Batch 1:
Radioactive
sulfur
(35S)
DNA
Phage
DNA
Radioactive
DNA
Batch 2:
Radioactive
phosphorus
(32P)
Figure 16.4-3
EXPERIMENT
Phage
Radioactive
protein
Empty
protein
shell
Radioactivity
(phage protein)
in liquid
Bacterial cell
Batch 1:
Radioactive
sulfur
(35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive
DNA
Batch 2:
Radioactive
phosphorus
(32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
I. The Shape of DNA
A. A Winding Staircase
1. DNA is made of 2 parallel strands linked
together and shaped like a spiral staircase.
2. The spiral shape is called a double helix.
3. Each strand is made of linked subunits called
nucleotides.
B. The Parts of the Nucleotide
1. Phosphate group – a phosphorus atom
surrounded by oxygen atoms, PO3
2. Deoxyribose – a 5-carbon sugar molecule
3. Nitrogen-base – molecule containing nitrogen
• There are four possible nitrogen bases in DNA
C. Putting it together
1. The phosphates and deoxyribose sugars link
together to form the “backbone” or outer
strands.
2. The nitrogen bases pair up to form the “rungs”
holding the 2 strands together.
III. The Information in DNA
A. Nitrogen Bases
1. The four nitrogen bases are adenine (A),
guanine (G), thymine (T), and cytosine (C).
2. Purines – made of 2 rings of carbon &
nitrogen; adenine & guanine
3. Pyrimidines – made of 1 ring of carbon &
nitrogen; thymine and cytosine
B. Base-Pairing Rules (Chargaff’s Rules)
1. A Purine on one strand is always paired with a
Pyrimidine on the other strand.
2. More specifically:
• Adenine with Thymine (A with T)
• Cytosine with Guanine (C with G)
3. The base pairs are held together by weak
hydrogen bonds and are called complementary
base pairs.
C. Complementary Strands
1. The two strands are complementary because
they fit together like a puzzle and their bases are
identical.
Ex: One strand has the following nucleotides:
ATTCGGTACCCC
The other strand would be what?
TAAGCCATGGGG
IV. Discovering DNA’s Structure
A. Edwin Chargaff – in all organisms, the
amount of adenine almost equaled the
amount of thymine, and the amount of
cytosine equaled the amount of guanine.
• Base-Pairing Rule!!
B. Rosalind Franklin – produced an X-ray
(photo 51!!) that showed the shape of DNA
1.
2.
3.
4.
X-ray Diffraction
X-shape = helix
Angle of X = 2 strands
Other clues suggest N-bases
are in the middle
C. James Watson and Francis Crick
1. Used the findings of Chargaff and Franklin, and
their knowledge of chemical bonding to build an
accurate, three-dimensional model of DNA
2. They were also able to propose a method of DNA
replication based on its structure.
Recap of DNA Structure
Section 12.2 – DNA Replication
* In DNA Replication, the DNA molecule
unwinds, and the 2 sides split. Then, new
nucleotides are added to each side (using the
base-pairing rules) resulting in 2 identical
molecules.
I. Steps of DNA Replication
A. The 2 strands unwind & separate from
each other.
1. The point where the 2 chains separate is
called the replication fork.
2. DNA Helicase – enzyme that breaks the Hbonds between nitrogen-bases and separates
the 2 strands.
B. At the replication fork, new nucleotides are
added to each strand, using the base-pairing
rules.
1. DNA Polymerase – enzyme that binds to a strand
and adds complementary nucleotides to
complete the double helix.
2. DNA Polymerase also “proofreads” by back
tracking to a mismatched nucleotide, remove it,
and add the correct one.
C. Simulating DNA Replication
1. If one strand of original DNA is:
A-T-T-C-C-G
Then the new strand added by DNA Polymerase
is:
T-A-A-G-G-C
II. Prokaryotic vs. Eukaryotic DNA Replication
A. Prokaryotic
1. Have 1, circular DNA molecule
2. Replication begins at one point, 2 replication
forks form, and replication occurs in opposite
directions until the 2 forks meet on opposite
sides of the DNA loop.
B. Eukaryotic
1. Have several, linear DNA molecules
2. On each DNA molecule, there are many
replication sites. At each site, 2 forks form and
replication occurs in opposite directions.
3. This forms replication “bubbles”.
4. This allows eukaryotes to copy their DNA much
faster than prokaryotes.
III. Mutations
A. Change in at least one nucleotide.
B. Caused by:
1. Wrong nucleotide is not corrected
2. Chemical exposure
3. UV Radiation
Section 13.3 – RNA & Gene Expression
I. Gene Expression
A. Gene Expression – the display of genes
into specific traits
1. Gene expression produces proteins by
transcription and translation.
2. Both processes require RNA.
B. Transcription: DNA to RNA
1. Copying DNA into RNA to move the genetic
information from the nucleus to the cytoplasm.
C. Translation: RNA to proteins
1. Using RNA to make a specific protein.
***The central dogma is the concept that all cells
are governed by a cellular chain of command:
DNA RNA protein
II. Ribonucleic Acid – RNA
*moves genetic information from DNA (in
nucleus) to the cytoplasm to make
proteins.
A. DNA
vs.
Phosphate
Sugar is Deoxyribose
Nitrogen Bases: C,G,A,T
Double Helix
RNA
Phosphate
Sugar is Ribose
Nitrogen Bases: C,G,A,U
(uracil replaces thymine
in RNA; A-U, C-G)
Single Strand
B. Types of RNA
1. Messenger RNA (mRNA) – one, uncoiled chain
carries genetic information from DNA (nucleus)
to the cytoplasm
2. Transfer RNA (tRNA) – single chain of about 80
nucleotides, binds to amino acids
3. Ribosomal RNA (rRNA) – RNA in globs, these join
with proteins to make ribosomes
III. Transcription: genetic information is copied
from DNA to make RNA (in the nucleus)
1. An enzyme called RNA Polymerase binds
to DNA at a specific spot called the
promotor.
a) Promotor – special sequence of nitrogen
bases that marks the beginning of a gene or
section of DNA to be copied.
2. The double helix of DNA separates at the
promotor.
3. RNA Polymerase begins binding RNA
nucleotides together as it “reads” the DNA
strand
a) RNA uses uracil (U) instead of thymine (T).
Ex: DNA: CCTGCTAGA
mRNA: GGACGUCU
4. Transcription continues until RNA Polymerase
reaches another specific section of DNA
called the termination signal.
a) The newly formed mRNA is released into the
cytoplasm.
b) The RNA Polymerase releases and the double
helix reforms (thus DNA is unchanged in the this
process!)
IV. How to Make Proteins:
*DNA goes through Transcription to make
mRNA. The mRNA works with tRNA and
rRNA to make proteins in Translation
A. Protein Structure
1. Proteins are made of amino acids, linked
together in chains. There are 20 amino acids.
2. The sequence of amino acids determines how
the protein will twist and fold into a 3D shape.
3. A chain of amino acids is a polypeptide.
B. Genetic Code – a sequence of mRNA
nucleotides (bases) can be translated into a
sequence of amino acids
1. 3 mRNA nucleotides is called a codon.
2. Each codon codes for 1 specific amino acid.
3. There are two special codons:
a) Start Codon – 3 mRNA nucleotides with the bases
AUG. This codon triggers a ribosome to begin
translation. AUG codes for the amino acid
methionine, so this is the first amino acid of every
protein.
b) Stop Codon – 3 mRNA nucleotides with bases UAA,
UAG, or UGA. This codon tells a ribosome to stop
translating mRNA.
4. tRNA: one end holds an amino acid & the
other end is an anticodon. The anticodon is
three bases complementary to the bases on
the mRNA.
Ex:
mRNA: CAU GGA
tRNA anticodon: GUA CCU
V. Translation – taking mRNA, “reading” it to bring
the correct amino acids and connect them to
make a protein.
1. mRNA is made from a single strand of DNA in the
nucleus (transcription).
2. mRNA is carried from the nucleus to the cytoplasm.
3. mRNA goes to a ribosome (rRNA). The ribosome
attaches to the start codon.
4. A tRNA attaches to an amino acid in the cytoplasm
and carries it to the mRNA and ribosome.
5. Each tRNA lines up its anticodon with the codon on
the mRNA (base-pairing rules).
*For example, if the mRNA codon is CCU, the
anticodon on the tRNA will be GGA. Now the
amino acid at the other end of the tRNA is in the
right position to form an amino acid chain.
6. Amino acids are connected to make a protein.
7. When a stop codon is reached, no amino acid is
added. The mRNA, ribosome, tRNA’s, and protein
are released into cytoplasm.
Mutations - changes in DNA sequence
I. Gene Mutations: result from changes in a
single gene
A. Point Mutations: affect 1 DNA nucleotide
1. Also called substitutions, as 1 DNA nucleotide
is substituted for another
B. Frameshift Mutations: shift the “reading
frame” of the genetic message
Insertion
Deletion
DNA:
TAC
GCA
TGG
AAT
DNA:
TAC
GCA
TGG
AAT
mRNA:
AUG
CGU
ACC
UUA
mRNA:
AUG
CGU
ACC
UUA
Leu
Amino
Acids:
Met
Arg
Thr
Leu
AT
UA
Amino
Acids:
Met
DNA:
↓ Insertion
TAT
CGC
ATG
GAA T
DNA:
↓ Deletion
GGA
CAT
TAG
mRNA:
AUA
CUU A
mRNA:
AUC
GUA
CCU
Leu
Amino
Acids:
Tyr
Val
Pro
Amino
Acids:
Ile
Arg
GCG
Ala
Thr
UAC
Tyr
II. Chromosomal Mutations: changes in the # or
structure of chromosomes
A. Deletion: loss of all or part of a chromosome
B. Duplication: segment of a chromosome is
repeated
C. Inversion: part of a chromosome becomes
oriented in reverse directions
D. Translocation: part of 1 chromosome breaks
off and attaches to another chromosome
Chromosomal Mutations
DNA Replication
1. The 2 chains of the DNA double helix separate.
a) The point of separation is called the replication
fork.
b) The 2 chains are separated by an enzyme called
helicase.
2. Another enzyme called DNA Polymerase binds
to a chain and adds the complementary bases to
complete the double helix.
3. You end up with 2 identical copies of original
DNA, and the cell can then undergo cell division.
Transcription – genetic information is copied
from DNA to make RNA
1. An enzyme called RNA Polymerase binds to
DNA at a specific spot called the promotor.
a) Promotor – special sequence of DNA bases that
marks the beginning of a gene or section of DNA
to be copied.
2. The double helix of DNA separates at the
promotor.
3. RNA Polymerase begins binding RNA
nucleotides together.
a) RNA uses Uracil (U) instead of Thymine (T).
Example: DNA: CCTGCTAGTT
RNA:
4. Transcription continues until RNA polymerase
reaches another specific section of DNA called
the termination signal. The newly form mRNA is
released into the cytoplasm. The RNA
Polymerase releases and the double helix
reforms.
Translation - translating the N-base sequence of
an mRNA into an amino acid sequence (protein)
1. rRNA and protein combine to form
ribosomes.
2. The ribosome attaches to the start codon of
an mRNA. The mRNA then moves through
the ribosome.
3. A tRNA attaches to an amino acid in the
cytoplasm and transfers it to the mRNA and
ribosome.
4. Each tRNA lines up its anticodon on the
mRNA. Now the amino acid at the end of the
tRNA is in the right position to be added to
the amino acid chain.
5. To add an amino acid, the growing amino
acid chain is added to the new amino acid.
6. When the ribosome comes to a stop codon,
the ribosome releases the mRNA, and
everything is released to the cytoplasm.
• DNA Replication: DNA is copied to make more
DNA
• Transcription: DNA is transcribed into mRNA
• Translation: mRNA is translated into an amino
acid sequence (a.k.a protein!)