Chapter 12:
DNA- The Molecule
of Heredity
• DNA determines the structure of
proteins
–All living things contain proteins
–Provide complete instructions for
making proteins
–Made up of nucleotides
History of DNA
Griffith (1928)
• Tried to figure out how bacteria causes
pneumonia
• Experiment:
– 1st: injected mice with disease-causing bacteria
(all died) and again with harmless bacteria (no
sickness)
• He heated the disease-causing bacteria to kill
them and injected it into mice (mice lived)
– 2nd: mixed heat-killed bacteria with live,
harmless bacteria and injected into mice (all
died)
• Conclusion: transformation  1 strain
changed into another
– Genes control changes to organisms
History of DNA
Avery (1944)
• Repeated Griffith’s work
– Experiment: Made an extract from heat-killed
bacteria and treated it with enzymes (destroyed
organic compounds)
• Transformation still occurred
• Repeated using enzymes to break down DNA
– Transformation did not occur
• Conclusion: DNA stores and transmits
genetic info
Hershey and Chase (1952)
– Did experiments using radioactive viruses to infect
bacteria (bacteriophages)
• Used radioactive markers to determine what actually entered a
bacterial cell
– Conclusion: Discovered DNA was the genetic material of
all living things
Franklin and Wilkins (1950’s)
– Experiment: Used X-ray diffraction on DNA
– Conclusion: strands in DNA are twisted around
each other (helix)
Watson and Crick (1953)
– No experiment
– Conclusion: Discovered the structure of DNA
• Made up of 2 chains of nucleotides held together by
nitrogen bases
• Double helix (twisted ladder)
Chapter 12 Scientist Review:
Match the scientist with the description of his
or their conclusions:
Griffith
Avery
Hershey &Chase
_____ concluded that the genetic material of a
bacteriophage is DNA
_____ concluded that DNA was the factor that
caused one bacterium to transform into
another
_____ concluded that bacteria could be transformed
from harmless to disease-causing by an
unknown factor
DNA in Cells
• Located in the nucleus
of cells as
chromosomes
• Packed tightly
• Consists of more than
30 million base pairs
• Complimentary DNA
strands
– Can use 1 strand to make
a copy of the other strand
using base pairing
Nucleotides
• Make up DNA
• 3 parts to a
nucleotide:
– A simple sugar
called Deoxyribose
– A phosphate group
– A nitrogen base
Nitrogen Bases
• 4 possible
nitrogen bases:
–Adenine (A)
–Guanine (G)
–Cytosine (C)
–Thymine (T)
Adenine (A) and Guanine (G)
• Double-ringed nitrogen bases
• Called purines
Thymine (T) and Cytosine (C)
• Single-ringed nitrogen bases
• Called pyrimidines
Chargaff
• % of Guanine and Cytosine are equal
• % of Adenine and Thymine are equal
• Nucleotides join together to form long
chains of complimentary base pairs
– Adenine always pairs with Thymine (A-T
or T-A)
– Guanine always pairs with Cytosine (G-C
or C-G)
Structure of DNA
• Nitrogen bases of
the nucleotides
hold 2 strands of
DNA together with
weak hydrogen
bonds
• Twisted DNA 
double helix
Sides of the ladder:
• alternating phosphate
groups and sugar
molecules
Rungs of the ladder:
• pairs of nitrogen bases
• joined by weak
hydrogen bonds
DNA Replication
• Making a copy of DNA
• DNA is copied before cell division
– Takes 6 hours in humans
– During the S phase of interphase
• DNA will separate into 2 strands
– Carried out by the enzyme DNA
polymerase
• Unzips DNA by breaking
hydrogen bonds to unwind the
double helix
– Each strand acts as a template or
model to make new DNA strands
• Makes new complimentary
strands through base-pairing
• Example:
– TACGTT – Old DNA
strand
ATGCAA – New DNA
strand
• After DNA is replicated, DNA
will have 1 old strand and 1
new strand
Chapter 12 Bell Ringer #1:
1. The structure of a DNA molecule can be
described as a _____.
2. During DNA replication, the DNA molecule
______ into two strands.
3. DNA looks like a twisted ladder. Which
parts of a twisted ladder represent the
hydrogen bonds and the sugar-phosphate
backbones?
The Genetic Code
• DNA controls protein synthesis
• Proteins have chains of amino acids
• A code is needed to convert messenger RNA
(mRNA) into a protein
• 20 amino acids
– Codon: a group of 3 Nitrogen bases that code
for a specific amino acid
• 64 possible combinations of codons
• Some code for amino acids
• Some code for making proteins
• More than 1 codon can code for the same
amino acid
• There is 1 start codon (amino acid
methionine)
– DNA  TAC
– RNA  AUG
• There are 3 stop codons
– Code for no amino acids
• The sequence of nucleotides (N-bases)
is the code for what controls the
production of all proteins
Transcription
•
•
•
•
Occurs in the nucleus
Making an RNA copy of a part of DNA
Makes messenger RNA (mRNA)
Requires RNA polymerase
– Binds to and separates DNA
– Strands of DNA used as a template
– Binds to DNA regions called promoters
• 4 Steps:
– RNA polymerase unzips the DNA
– Free RNA nucleotides floating in the
cytoplasm base pair with nucleotides on
DNA strand (makes mRNA)
– mRNA strand breaks away and DNA
strands go back together
– mRNA leaves nucleus and goes out to the
cytoplasm
• Result of transcription: formation of 1 singlestranded RNA molecule
Example:
DNA
A
G
C
T
G
A
C
T
G
mRNA
Example:
DNA
mRNA
A
U
G
C
C
G
T
A
G
C
A
U
C
G
T
A
G
C
Name of amino acid:
Two Types of Nucleic
Acids: RNA and DNA
DNA
RNA
(3 types)
Sugar
Deoxyribose
Ribose
Bases
G, C, A, T
G, C, A, U
(uracil)
Structure
Doublestranded
Single-stranded
Location in a
Cell
Base Pairing
Only in the
nucleus
C-G and A-T
In nucleus and
cytoplasm
C-G and A-U
Messenger RNA (mRNA)
• Brings instructions from DNA out of
the nucleus and into the cytoplasm
• Moves toward the ribosomes
Ribosomal RNA (rRNA)
• Makes up
ribosomes
• Binds to
messenger RNA
• Uses the
instructions from
DNA to put
amino acids in
the correct order
Transfer RNA (tRNA)
• Delivers the
amino acids to
the ribosomes to
be made into a
protein
RNA Editing
• DNA has introns (sequences of nucleotides)
– Edited out before they become functional
– Not involved in coding for proteins
• Exons: code for proteins
– Remaining pieces of DNA  put together with
cap and tail = final RNA molecule
DNA Controls Protein Synthesis
• What are proteins?
– Long chains of amino
acids (polypeptides)
– Key structures and
regulators of cell
functions
• Help with structural
parts
• Enzymes  chemical
reactions
• Help in transport
through cell
membrane
Making Proteins
• Protein production is similar to
building car
–DNA provides workers with
instructions for making proteins
–Workers build proteins (RNA)
–Other workers bring parts (amino
acids) to the assembly line
Translation
• Process of building proteins from mRNA
• Takes place in the ribosomes
• Transfer RNA (tRNA) brings amino acids to the
ribosomes
– Attaches to only 1 type of amino acid
– Amino acid will become bonded to 1 side of the tRNA
– The other side of the tRNA has 3 nitrogen bases called
an anticodon
• Pairs up with mRNA codon
• Amino acids are joined by
peptide bonds
• Anticodon bind to the codon
of mRNA through base
pairing
– Example: Codon:
CGA
Anticodon: GCU
• A chain of amino acids form
until a stop codon is reached
– Translation will end
– Amino acid strand is
released from the
ribosome to become
proteins
Chapter 12 Bell Ringer #2:
1. The 3 main types of RNA are ___, ___, & ___.
2. Copying part of a nucleotide sequence of DNA
into a complementary sequence in RNA is called
____.
3. During the process of _____, the information
carried by mRNA is used to produce proteins.
4. Each tRNA molecule contains 3 unpaired bases,
called the _____, which ensure that amino acids
are added in the correct sequence.
Mutations
• Any change in the sequence of
DNA
• Can be caused by errors in:
–DNA replication
–Transcription
–Cell division
–External agents
Mutations in Reproductive
Cells: Birth Defects
• Within the egg or sperm cells
• Can produce new traits
• Can result in proteins that do
not work (can kill organism)
• Could have positive effects
– Faster
– Stronger
– Important in the evolution
of a species
Mutations in Body
Cells
• Not passed on to
offspring
• May impair cell
function
• Can affect genes
that control cell
division (cancer)
Point Mutation (substitution)
• Change in 1 N-base in
DNA
• Example: CGATTACGC
(normal DNA)
CGATTTCGC
(mutated DNA)
• Albinism
– Inability to produce
pigments
– Lethal to plants
Frameshift Mutation
• 1 N-base is added or deleted
• Changes all codons from that
point on
• Example: CGATTACGC
CGAATTACGC (N-base added)
• Example: CGATTACGC
CGTTACGC (N-base deleted)
• May cause no problems or can be
severe
• More dangerous than point
mutations
Chromosomal Mutations
• Involve many genes
• Usually very bad
– Can change location of genes or number of copies
• Involve changes in number or structure of chromosomes
• 4 types:
– Deletions  taking away
– Insertions  adding
– Inversions  switching parts
(ex: ab ba)
– Translocations  breaking
off
• Many occur from improper
separation during meiosis
Causes of Mutations
• Spontaneous or random
mutations
• Mutagens (things that
cause mutations)
– Radiation, X-Rays, UV
light, chemicals
– Carcinogens
• Source of genetic
variation
Gene Regulation
• Certain DNA sequences serve as promoters
(binding sites for RNA polymerase)
• Ex: E. coli
– Group of 3 genes that are turned on and off
together (called an operon)
– E. coli uses lactose as food
• Genes must be expressed  called lac operon
• Lac genes turned off by repressors (binds to
operator)
– Prevents transcription of its genes
• Lac genes turned on by presence of lactose
– Binds to repressor, allowing RNA polymerase to
transcribe genes
• Operons generally not found in eukaryotic
cells
• Eukaryotic cells (more complex)
– Has short region of DNA (TATA box)
• 30 base pairs long
• Helps RNA polymerase position itself
– Hox genes
• Series of genes that controls organs and tissues that
develop in embryos
• Determine basic body plan
• Mutations can change organs
– Ex: fruit fly
• Expressed genes are transcribed into RNA
– Genes are expressed with help from DNAbinding proteins
Chapter 12 Bell Ringer #3:
1. Genetic information is altered when changes in
the DNA sequence, called ____ occur.
2. Changes in the DNA sequence of a single gene
are called _____.
3. What causes the lac genes in E. Coli to turn off?
4. What causes the Lac genes in E. Coli to turn on?