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Chapter 10
Molecular Biology of the Gene
PowerPoint Lectures for
Biology: Concepts & Connections, Sixth Edition
Campbell, Reece, Taylor, Simon, and Dickey
Copyright © 2009 Pearson Education, Inc.
What do you know?
1. What are the two types of nucleic acids?
2. What is the role of each nucleic acid in the cell?
3. What is the structure of a nucleic acid – be
specific.
4. What are 3 examples of proteins found in the
body? What are their functions?
5. How are proteins made in the body?
THE STRUCTURE OF THE
GENETIC MATERIAL
DNA
DNA vs. RNA
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DNA and RNA are polymers of nucleotides
 The monomer unit of DNA and RNA is the
nucleotide, containing:
1. Nitrogenous base
2. 5-carbon sugar
3. Phosphate group
 Called polynucleotides – many nucleotides joined
together
 Sugar-phosphate backbone – covalent bonds
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Sugar-phosphate backbone
Phosphate group
Nitrogenous base
Sugar
Nitrogenous base
(A, G, C, or T)
DNA nucleotide
Phosphate
group
Thymine (T)
Sugar
(deoxyribose)
DNA nucleotide
DNA polynucleotide
In DNA, there are four different nitrogen bases:
Thymine (T)
Cytosine (C)
Pyrimidines
Adenine (A)
Guanine (G)
Purines
Nitrogenous base
(A, G, C, or U)
Phosphate
group
In RNA,
Uracil (U)
Instead of
thymine, they
have the base
uracil
Sugar
(ribose)
DNA is a double-stranded helix
 Watson and Crick deduced the secondary structure of
DNA, with data from Rosalind Franklin and Maurice
Wilkins
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DNA Structure
 DNA is 2 polynucleotide strands bonded
together through hydrogen bonds between
nitrogen bases
 Twisted into a helical shape
 Sugar-phosphate on outside
 Nitrogen bases on inside
 Base pair rules
- adenine always pairs with thymine 2 H+
bonds
- Guanine always pairs with cytosine  3 H+
bonds
Twist
Hydrogen bond
Base
pair
Ribbon model
Partial chemical structure
Computer model
DNA REPLICATION
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DNA replication depends on specific base pairing
 DNA replication follows a semiconservative
model
– The two DNA strands separate
– Each strand is used as a pattern to produce a
complementary strand, using specific base pairing
– Each new DNA helix has one old strand with one new
strand
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Parental
molecule
of DNA
Nucleotides
Parental
molecule
of DNA
Both parental
strands serve
as templates
Nucleotides
Parental
molecule
of DNA
Both parental
strands serve
as templates
Two identical
daughter
molecules of DNA
DNA replication proceeds in two directions at
many sites simultaneously
 DNA replication begins at the origins of replication
1. Helicase (enzyme) unwinds DNA at the origin to produce a “bubble”
by breaking hydrogen bonds between nitrogen bases
2. DNA polymerase brings in nucleotides to the existing DNA strands
(using complimentary base pairing) and replication proceeds in both
directions from the origin
- one chain is continuous
- one chain is brought in pieces
3. DNA ligase joins pieces on non-continuous strand
4. Replication ends when products from the bubbles merge with each
other
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Origin of replication
Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
5 end
P
5
4
3
2
1
P
3 end
2
3
1
4
5
P
P
P
P
P
P
3 end
5 end
DNA polymerase
molecule
5
3
3
5
Daughter strand
synthesized
continuously
Parental DNA
3
5
5
3
DNA ligase
Overall direction of replication
Daughter
strand
synthesized
in pieces
THE FLOW OF GENETIC
INFORMATION FROM DNA TO
RNA TO PROTEIN
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The DNA genotype is expressed as proteins,
which provide the molecular basis for
phenotypic traits
 A gene - sequence of DNA that directs the synthesis
of a specific protein
– DNA is transcribed into RNA
– RNA is translated into protein
 The presence and action of proteins determine the
phenotype of an organism
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DNA
Transcription
RNA
Nucleus
Cytoplasm
Translation
Protein
Genetic information written in codons is
translated into amino acid sequences
 The sequence of nucleotides in DNA provides a code
for constructing a protein
– Protein construction requires a conversion of a nucleotide
sequence  amino acid sequence
– Transcription rewrites DNA code into RNA
– Three nucleotides on RNA is a codon
– Translation takes codons and makes them into amino
acids
– Each amino acid is specified by a codon
– 64 codons are possible
– Some amino acids have more than one possible codon
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DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
DNA strand
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
The genetic code is the Rosetta stone of life
 Characteristics of the genetic code
– Triplet (codon): Three nucleotides specify one amino acid
– 61 codons correspond to amino acids
– AUG codes for methionine and signals the start of transcription
– 3 “stop” codons signal the end of translation
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Third base
First base
Second base
Strand to be transcribed
DNA
Transcription
RNA
Start
codon
Polypeptide
Translation
Stop
codon
Transcription produces genetic messages in the
form of RNA

Transcription – making DNA into RNA; occurs in nucleus
1. Initiation – RNA polymerase binds to region called promoter
2. Elongation – RNA polymerase brings in RNA nucleotides
complimentary to DNA strand
- One strand is used as a pattern to produce an RNA chain, using
specific base pairing
–
For A in DNA, U is placed in RNA
3. Termination – RNA polymerase reaches terminator (bases on DNA
say “stop”)
4. Polymerase detaches, DNA is joined back together
5. RNA leaves the nucleus (after processing)
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RNA nucleotides
RNA
polymerase
Direction of
transcription
Newly made RNA
Template
strand of DNA
RNA polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
1
Initiation
2
Elongation
3
Termination
Completed
RNA
Area shown
in Figure 10.9A
Growing
RNA
RNA
polymerase
Eukaryotic RNA is processed before leaving the
nucleus
 Messenger RNA (mRNA) - contains codons for
protein sequences
 Eukaryotic mRNA has interrupting sequences called
introns, separating the coding regions called exons
 Eukaryotic mRNA undergoes processing before
leaving the nucleus
– Cap added to one end: single guanine nucleotide
– Tail added to other end
– RNA splicing: removal of introns and joining of exons to
produce a continuous coding sequence
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Exon Intron
Exon
Intron Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
Transfer RNA molecules serve as interpreters
during translation
Amino acid attachment site
 Transfer RNA (tRNA) match an amino acid to its
corresponding mRNA codon
– structure allows it to convert
one language to the other
– An amino acid attachment site
allows each tRNA to carry a
specific amino acid
– An anticodon allows the
tRNA to bind to a specific
mRNA codon, complementary
in sequence
– A pairs with U, G pairs
with C
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Anticodon
Translation - Ribosomes build polypeptides
 Translation - occurs on the surface of the
ribosome
- mRNA used to make amino acid
chains/polypeptides
– Ribosomes have two subunits: small and large
– Each subunit is composed of ribosomal RNAs and
proteins
– subunits come together during translation
– have binding sites for mRNA and tRNAs
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tRNA
molecules
Growing
polypeptide
Large
subunit
mRNA
Small
subunit
tRNA-binding sites
Large
subunit
mRNA
binding
site
Small
subunit
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
mRNA
Codons
Translation occurs in steps
1. Initiation occurs in two steps
1. mRNA binds to a small ribosomal subunit, and the first
tRNA binds to mRNA at the start codon
– The start codon reads AUG
– The first tRNA has the anticodon UAC, brings in methionine
2. A large ribosomal subunit joins the small subunit,
allowing the ribosome to function
Start
End
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Large
ribosomal
subunit
Initiator tRNA
1
mRNA
Start
codon
Small ribosomal
subunit
2
Translation occurs in steps
2. Elongation - addition of amino acids to the
polypeptide chain
 Each cycle of elongation has three steps
1. Codon recognition: next tRNA binds to the mRNA
2. Peptide bond formation: joining of the new amino acid to
the chain
3. Translocation: tRNA is released from the ribosome and
moves down the length of the mRNA molecule
- continues until ribosomes reacher STOP codon on
mRNA
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Translation occurs in steps
3. Termination
– The completed polypeptide is released
– The ribosomal subunits separate
– mRNA is released and can be translated again
Copyright © 2009 Pearson Education, Inc.
Amino
acid
Polypeptide
A site
P site
Anticodon
mRNA
Codons
1 Codon recognition
mRNA
movement
Stop
codon
2 Peptide bond
formation
New
peptide
bond
3 Translocation
Review: The flow of genetic information in the
cell is DNA  RNA  protein
 Does translation represent:
– DNA  RNA or RNA  protein?
 Where does the information for producing a
protein originate:
– DNA or RNA?
 Which one has a linear sequence of codons:
– rRNA, mRNA, or tRNA?
 Which one directly influences the phenotype:
– DNA, RNA, or protein?
Copyright © 2009 Pearson Education, Inc.
Transcription
DNA
mRNA
RNA
polymerase
Amino acid
1 mRNA is transcribed
from a DNA template.
Translation
2 Each amino acid
attaches to its proper
tRNA with the help of a
specific enzyme and
ATP.
Enzyme
ATP
tRNA
Anticodon
Large
ribosomal
subunit
Initiator
tRNA
3 Initiation of
polypeptide synthesis
The mRNA, the first
tRNA, and the ribosomal
sub-units come together.
Start Codon
mRNA
Small
ribosomal
subunit
New peptide
bond forming
Growing
polypeptide
4 Elongation
Codons
A succession of tRNAs
add their amino acids
to the polypeptide chain
as the mRNA is moved
through the ribosome,
one codon at a time.
mRNA
Polypeptide
5 Termination
Stop codon
The ribosome
recognizes a stop
codon. The polypeptide
is terminated and
released.
Mutations can change the meaning of genes
 A mutation is a change in the nucleotide sequence
of DNA
– Base substitutions: replacement of one nucleotide with
another
– Effect depends on whether there is an amino acid change that
alters the function of the protein
– Deletions or insertions
– Alter the reading frame of the mRNA, so that nucleotides are
grouped into different codons
– Lead to significant changes in amino acid sequence
downstream of mutation
– Cause a nonfunctional polypeptide to be produced
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10.16 Mutations can change the meaning of genes
 Mutations can be
– Spontaneous: due to errors in DNA replication or
recombination
– Induced by mutagens
– High-energy radiation
– Chemicals
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Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
Normal gene
mRNA
Protein
Met
Lys
Phe
Gly
Ala
Lys
Phe
Ser
Ala
Base substitution
Met
Base deletion
Met
Missing
Lys
Leu
Ala
His