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Chapter 12
Molecular Genetics
12.1 Vocabulary
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DNA – molecule that contains genetic information
that is used in the development and functioning of all
living organisms. Found in the nucleus and is the
“code of life”.
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Nucleotide
Double Helix
Chromosome
Nucleosome
Section 1
DNA: The Genetic Material
Standards: 2.3, 4.1-4.3, 4.7
Objectives:
• Summarize the experiments leading to the
discovery of DNA as the genetic material.
• Diagram and label the basic structure of DNA.
• Describe the basic structure of the Eukaryotic
chromosome.
Discovery of DNA
• Fredrick Griffith (1928) – studied bacteria that
causes pneumonia.
– Strains can be transformed into other forms.
• Oswald Avery (1944) – identified the specific
molecule that transformed the R strain  S strain.
Discovery of DNA
• Alfred Hershey & Martha
Chase (1952) – studied a
bacteriophage (virus that
attacks bacteria) by using
radioactive labeling.
– DNA & not protein is the
transforming factor.
DNA
• Deoxyribonucleic Acid
• Genetic material found in nucleus
– Also in mitochondria & chloroplasts
• Instructions written in a chemical code
– Determines all cellular functions (appearance,
develop, etc.)
Structure of DNA
• Building blocks of DNA  nucleotides
– Nucleotide contains:
• 1 nitrogen base (A, T, G, C)
• 5 carbon sugar  deoxyribose
• 1 phosphate group
How Many Nucleotides?
Structure of DNA
• James Watson & Francis Crick
(1953) – discovered that DNA is
a double helix (two strands of
twisted nucleotides  twisted
ladder or spiral staircase)
Structure of DNA
• “Railing”  alternating phosphates & sugars
on outside (backbone)
• “Steps”  nitrogen base pairs on inside
“Railing” 
4 Base Pairs
–Adenine (A)
–Guanine (G)
–Thymine (T)
–Cytosine (C)
Structure of DNA
• “Complementary base pairing”
– Bases pair up in middle of DNA molecule and
held together by H bonds following the BASEPAIR RULE:
• A always pairs with T by 2 H bonds
• G always pairs with C by 3 H bonds
• Purines ALWAYS bind to Pyrimidines
There are 4 different nitrogen bases
They are categorized into 2 groups:
Purines:
Double-carbon ring
• Adenine
• Guanine
Pyrimidines:
Single-carbon ring
• Cytosine
• Thymine
Base-Pairing Practice
• A = _____
• T = _____
• C = _____
• G = _____
• T = _____
• C = _____
DNA Structure
• The two sides of DNA are oriented in opposite
directions  Antiparallel
– Like a two-lane road
Chromosome Structure
• DNA coils around histones (proteins) to form
chromatin fibers  creating a Nucleosome 
supercoils for form chromosome  X shape
DNA packaging HHMI
12.2 Vocabulary
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Semiconservative
Replication
DNA Polymerase
Okazaki Fragment
Section 2
Replication of DNA
Standards: 2.3, 4.3
Objectives:
• Summarize the role of the enzymes involved in
the replication of DNA.
• Explain how leading and lagging strands are
synthesized differently.
DNA Replication
• Semiconservative Replication – parental strands
of DNA separate, serve as templates, and
produce DNA molecules that have one strand of
parental DNA and one strand of new DNA.
– Process of copying DNA
– Occurs in 3 Steps
Replication - basic
Replication - schematic
Semiconservative Replication
Step 1: Unwinding the Double Helix
– DNA Helicase (an enzyme) unwinds and unzips
DNA into two separate strands  H bonds break 
leaving single strands of DNA.
– Single-stranded binding proteins keeps strands
separate during replication.
– Primase (an enzyme) adds primer (short segments)
on each DNA strand.
Step 1: Unwinding the Double Helix
Semiconservative Replication
Step 2: Add New Base Pairs
– DNA Polymerase (an enzyme) – catalyzes the addition
of appropriate nucleotides to the new DNA strand.
• Adds new nucleotides to the old DNA molecule.
• Follows the base-pair rule: (A-T, G-C)
Semiconservative Replication
Step 2: Add New Base Pairs (cont’d)
• Two new strands produced in different ways:
– Leading Strand  new nucleotides added in a
smooth, continuous motion.
– Lagging Strand  new nucleotides added in small
chunks called Okazaki Fragments and in a
discontinuous motion.
• DNA Ligase (an enzyme) adds more bases to fill
in the gaps between the Okazaki Fragments to
make a continuous new DNA strand.
Step 2: Add New Base Pairs
Step 2: Add New Base Pairs
Semiconservative Replication
Step 3: Joining Base Pairs
– DNA polymerase removes primer and fills
in the place with nucleotides.
– DNA ligase joins the sections to make each
strand continuous.
– At the end of replication  2 new strands
of daughter DNA are produced.
• Each is made of ½ old DNA and ½ new DNA
DNA Replication
Prokaryotes/Eukaryotes
Prokaryotes
• Circular DNA strand  replicated in one section
• 2 directions
• DNA is shorter.
Eukaryotes
• Replicated in several sections
• 2 directions.
• DNA is longer.
12.3 Vocabulary
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RNA
Messenger RNA
Ribosomal RNA
Transfer RNA
Transcription
RNA Polymerase
Codon
Translation
Section 3
DNA, RNA, and Protein
Standards: 2.2, 4.1, 4.3-4.4
Objectives:
• Explain how messenger RNA, ribosomal RNA, and
transfer RNA are involved in the transcription and
translation of genes.
• Summarize the role of RNA polymerase in the
synthesis of messenger RNA.
• Describe how the code of DNA is translated into
messenger RNA and is utilized to synthesize a
particular protein.
Why Proteins are Important
• DNA  “code of life” or “genetic code” because
it contains the code for each protein that
organisms need.
• Proteins (or protein messages) determine how
an organism looks & functions.
Why Proteins are Important
• Gene – segment of DNA that contains instructions
for making a protein.
– Specific location on a chromosome
– Controls inherited trait expression that is passed on
for generations.
• Ribosomes make proteins
DNA  RNA
• Problem: DNA contains instructions for making
proteins but DNA can’t leave the nucleus.
• Solution: RNA will take DNA’s instructions to the
ribosomes for protein synthesis. RNA just
speaks a different language than DNA.
RNA
• RNA is a nucleic acid called Ribonucleic Acid
• Single stranded
• Composed of nucleotides:
– Sugar  Ribose
– Phosphate Group
– Nitrogen Bases:
• G bonds with C
• A bonds with (U) uracil
• NO (T)
RNA
3 Types of RNA:
1. mRNA (messenger RNA) – long strands of RNA
formed complementary to one strand of DNA
2. rRNA (ribosomal RNA) – associated with proteins to
form ribosomes in the cytoplasm
3. tRNA (transfer RNA) – small segments of RNA that
transport amino acids to the ribosome
RNA
How Proteins are Made
Part 1: Transcription (DNA  mRNA)
– occurs in nucleus
– gene for a specific protein is turned ON and that
gene is copied into mRNA
Example: (DNA) T A C G G T A
(mRNA) A U G C C A U
• RNA Polymerase (an enzyme) – regulates RNA
synthesis as DNA strand unwinds and unzips.
– mRNA detaches and leaves nucleus and enters
cytoplasm. TWO DNA strands rejoin.
Transcription
Transcription Practice
1. DNA
mRNA
CGTTAGCAACTG
2. DNA
mRNA
ACGTCAACGTTA
Genetic Code
• DNA codes for protein synthesis.
• DNA varies among organisms 
N base sequence is different.
• Amino acids make up proteins
• 20 amino acids total
• Codon – 3 base code (N base)
1 codon = 1 amino acid
DNA
CODON
T
A
C
This section of DNA
represents a gene.
C
C
G
T
A
T
• How many codons
do you see in this
gene?
C
A
T
C
G
A
A
T
T
• How many amino
acids total make
this protein?
Genetic Code
Genetic Code
Practice Converting
mRNA  Amino Acids
P. 338
1.
2.
3.
4.
5.
6.
7.
8.
AUG =
CUC =
AAG =
GGU =
UAC =
CAC =
CAA =
UGA =
How Proteins are Made
Part 2: Translation (mRNA  Protein)
– Occurs in cytoplasm at the ribosome
– Interprets genetic message and builds proteins
– mRNA attaches to a ribosome (rRNA) and is read 3
bases at a time (codon)
How Proteins are Made
Part 2: Translation (continued)
• tRNA is activated by an enzyme and carries amino
acids to the ribosome & drops them off.
– 20 different types of tRNA molecules
– tRNA structure:
• Anticodon site – 3 nucleotide base complementary to
the codon of mRNA; end of tRNA molecule
• Amino acid attached on other end
How Proteins are Made
Part 2: Translation (continued)
– Amino acids joins together in a chain by peptide
bonds  forming a protein.
– Continues until STOP codon is read on the mRNA
 last amino acid is added  protein breaks away
from ribosome  protein synthesis ends.
Translation
DNA
mRNA
Amino acid
T
A
A
U
C
G
C
G
C
G
G
C
T
A
A
U
T
A
C
G
A
U
T
A
C
G
G
C
A
U
A
U
T
A
T
A
Methionine
(start)
• The section of DNA
you see here is a gene;
glycine
isoleucine
valine
– Use the mRNA you see
here and the chart in
your book to figure out
which amino acid will
go into this protein.
• Pg 338
alanine
stop
12.4 Vocabulary
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Gene Regulation
Mutation
Mutagen
Section 4
Gene Regulation and Mutation
Standards: 4.8
Objectives:
• Summarize the various types of mutations.
Gene Regulation
• Gene Regulation – ability of an organism to
control which genes are transcribed.
– Transcription factors  controls what and when
genes are expressed to make proteins.
– 2 Transcription Factors:
1. Guide & stabilize the binding of RNA polymerase
2. Controls rate of transcription
Mutation
• Mutation – permanent change (or alteration) in
DNA.
– Changes vary from…
• One base pair (gene mutations)  large segments
of DNA (chromosomal mutations)
• Beneficial  harmful (maybe lethal)
• Unnoticeable  disorders or death
Mutation
• If mutant cell is a body cell (somatic cell) then
daughter cells can be affected but mutation will
not be passed to offspring  aging and/or cancer.
• If mutant cell is a gamete (sex cell) then mutation
will be passed to offspring.
Point Mutation
• Point Mutation – change in one base pair (one
nucleotide).
– Missense Mutations (substitution) – codes for the
wrong amino acid.
Normal: AUG CAU UAC (histidine)
Mutated: AUG GAU UAC (aspartate)
– Nonsense Mutations (substitution) – change amino
acid codon to a stop codon; terminates translation
early  proteins can’t function normally
Normal: AUG CAU UAC
Mutated: AUG UGA
Frameshift Mutation
• Frameshift Mutations – change the “frame” of the
amino acid sequence by adding or deleting
nucleotides  changes the multiples of 3 codons.
– Deletion Mutation – loss of a nucleotide
Normal: AUG CAU UAC GUA
Mutated: AUG AUU ACG UAU
– Insertion Mutation – additions of a nucleotide
Normal: AUG CAU UAC GUA
Mutated: AUG CCA UUA CGU A
Duplication Mutation
• Duplication Mutation – entire codon(s) repeat;
changes the number of amino acids used.
Normal: AUG CAU UAC GUA
Mutated: AUG CAU CAU CAU CAU UAC GUA
How Mutations Occur
• Can occur spontaneously during (meiosis)
replication  DNA polymerase may add the
wrong nucleotide
• Mutagens – substances which cause mutations;
certain chemicals and radiation.
• Most mutations repaired  no effect
Effects of Mutation
• The shape of a protein controls how it works.
– Shape is determined by amino acids.
• Incorrect amino acids change protein’s
shape  protein may not work properly.