Chapter 10
Molecular Biology of the
Gene
Sabotage Inside Our Cells
•
Viruses are biological saboteurs
– Hijack the genetic material of host cells in order to
reproduce themselves
– May remain permanently dormant in the body
•
Viruses share some characteristics of living organisms
but are not generally considered alive
– Genetic material composed of nucleic acid
PowerPoint Lectures for
Biology: Concepts and Connections, Fifth Edition
– Campbell, Reece, Taylor, and Simon
– Not cellular
– Cannot reproduce on their own
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
THE STRUCTURE OF THE GENETIC MATERIAL
10.1 Experiments showed that DNA is the
genetic material
• "Transforming factor" postulated in 1928 by
Frederick Griffith
• Hershey-Chase experiments in 1952
determined that the heredity material was DNA
not protein
•
First understanding of DNA based on viruses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-1b
Phage
Bacterium
DNA
Batch 1
Radioactive
protein
Mix radioactively
labeled phages with
bacteria. The phages
infect the bacterial cells.
– Studied the simple bacteriophage T2
– Showed that the virus injects its DNA into
host cells and reprograms them to produce
more viruses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Radioactive
protein
Batch 2
Radioactive
DNA
Empty
protein shell
Radioactivity
in liquid
Phage
DNA
Centrifuge
Agitate in a blender to
separate phages outside
the bacteria from the
cells and their contents.
Pellet
Centrifuge the mixture Measure the
so bacteria form a
radioactivity in
pellet at the bottom of
the pellet and
the test tube.
the liquid.
Radioactive
DNA
Centrifuge
Pellet
Radioactivity
in pellet
LE 10-1c
10.2 DNA and RNA are polymers of nucleotides
• Nucleic acids are polynucleotides made of long
chains of nucleotide monomers
– Nitrogenous bases
Phage attaches
to bacterial cell.
Phage injects DNA.
Phage DNA directs host
cell to make more phage
DNA and protein parts.
New phages assemble.
• Single-ring pyrimidines: thymine (T),
cytosine ( C)
Cell lyses and
releases
new phages.
• Double-ring purines: adenine (A), guanine
(G)
– Sugar-phosphate backbone
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-2a
• DNA and RNA are identical except for two
things
Sugar-phosphate backbone
Phosphate group
A
– Nitrogenous bases
• DNA: A, C, G, T
• RNA: A, G, C, U
C
Nitrogenous base
Sugar
DNA nucleotide
Animation: DNA and RNA Structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Phosphate
group
Nitrogenous base
(A, G, C, or T)
Thymine (T)
G
• RNA: ribose
C
T
T
– Sugars
• DNA: deoxyribose
A
G
T
T
Sugar
(deoxyribose)
DNA nucleotide
DNA polynucleotide
LE 10-2b
LE 10-2c
Nitrogenous base
(A, G, C, or U)
Phosphate
group
Uracil (U)
Cytosine (C)
Thymine (T)
Adenine (A)
Guanine (G)
Purines
Pyrimidines
Sugar
(ribose)
10.3 DNA is a double-stranded helix
• James Watson and Francis Crick worked out
the three-dimensional structure of DNA, based
on X-ray crystallography by Rosalind Franklin
• DNA consists of two polynucleotide strands
wrapped around each other in a double helix
– Sugar-phosphate backbones are on the
outside and nitrogenous bases on the inside
– Each base pairs with a complementary
partner
• A with T, and G with C
– Hydrogen bonds between the bases hold
the strands together
• The Watson-Crick model of DNA suggested a
molecular explanation for genetic inheritance
Animation: DNA Double Helix
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-3c
LE 10-3d
C
G
T
A
Base
pair
C
Hydrogen bond
T
A
T
G
C
G
A
T
A
C
G
C
G
T
C
A
G
A
T
A
T
G
A
T
A
T
A
C
T
Ribbon model
Computer model
Partial chemical structure
Twist
DNA REPLICATION
LE 10-4a
10.4 DNA replication depends on specific base pairing
•
The Watson-Crick model of DNA structure suggested a
mechanism for its replication
– DNA strands separate
– Enzymes use each strand as a template to
assemble new nucleotides into complementary
strands
•
The mechanism of DNA replication is
semiconservative
– Each new double helix consists of one old and one
new strand
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A
T
A
C
G
C
G
C
G
A
T
A
T
A
T
Parental
molecule
of DNA
T
G
C
A
C
A
Nucleotides
Both parental
strands serve
as templates
T
A
T
A
T
G
C
G
C
G
C
G
C
G
C
T
A
T
A
T
A
T
A
T
A
Two identical
daughter molecules
of DNA
LE 10-4b
G C
• DNA replication is a complex process
A T
A
T
C
T
G
C
A
T
T
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-5a
Origin of replication
Parental strand
Daughter strand
• DNA replication begins at specific sites (origins
of replication) on the double helix
– Proteins attach and separate the strands
Bubble
– Replication proceeds in both directions,
creating replication bubbles
• Parent strands open, daughter strands
elongate
– Replication occurs simultaneously at many
sites
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
T
C
A
T
Animation: DNA Replication Overview
10.5 DNA replication: A closer look
A
T
A
A
A
A
T
T
T
T
G
A
C
T
G
C
G
G
G
C
G
C
G
G
C
C
A
C
G
A T
A
G
C
A
– Some of the helical DNA molecule must
untwist
Two daughter DNA molecules
LE 10-5b
3¢ end
5¢ end
• DNA's sugar-phosphate backbones are
oriented in opposite directions
P
4¢
3¢
– The enzyme DNA polymerase adds
nucleotides at only the 3’ end
HO
5¢
2¢
1¢
2¢
A
P
P
C
P
P
T
• The two strands are connected by the
enzyme DNA ligase
4¢
G
P
G
• The other strand is synthesized as a series
of short pieces
3¢
1¢
5¢
P
C
• One daughter strand is synthesized as a
continuous piece
T
OH
3¢ end
A
P
5¢ end
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-5c
DNA polymerase
molecule
THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
3¢
5¢
5¢
Daughter strand
synthesized
continuously
Parental DNA
3¢
3¢
5¢
Daughter
strand
synthesized
In pieces
10.6 The DNA genotype is expressed as
proteins, which provide the molecular basis for
phenotypic traits
• The information constituting an organism's
genotype is carried in its sequence of DNA
bases
• A particular gene—a linear sequence of many
nucleotides—specifies a particular polypeptide
5¢
3¢
DNA ligase
Overall direction of replication
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
The flow of genetic information
1. Transcription of the genetic information in DNA
into RNA
2. Translation of RNA into the polypeptide
•
Beadle-Tatum one gene-one enzyme hypothesis
–
Studies of inherited metabolic disorders in mold
suggested that phenotype is expressed through
proteins
10.7 Genetic information written in codons is
translated into amino acid sequences
• Genetic information flows from DNA to RNA to
protein
• Nucleotide monomers represent letters in an
alphabet that can form words in a language
– Triplet code
–
A gene dictates production of a specific enzyme
• Three-letter words (codons)
–
The hypothesis has been restated to one geneone polypeptide
• Each word codes for one amino acid in a
polypeptide
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-7a
DNA molecule
Gene 1
• The genetic code specifies the
correspondence between RNA codons and
amino acids in proteins
Gene 2
Gene 3
– Includes start and stop codons
DNA strand
A
A
A C
C
G
G
C
A
A
A
A
U
U
U G
G
C
C
G
U
U
U
U
Transcription
RNA
10.8 The genetic code is the Rosetta stone of life
– Redundant but not ambiguous
• Nearly all organisms use exactly the same
genetic code
Codon
Translation
Polypeptide
Amino acid
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-8a
LE 10-8b
Strand to be transcribed
Second base
UUU
UUC
U
UUA
First base
UAC
U
UGU
Tyr
UGC
Cys
Ser
C
UAA
Stop
UGA
Stop
A
UAG
Stop
UGG
Trp
G
CUU
CCU
CAU
CUC
CCC
CAC
CUA
Leu
AUU
AUC
lle
AUA
AUG
GUC
GUA
GUG
CCA
Val
Pro
CAA
CCG
CAG
ACU
AAU
ACC
AAC
ACA
Met or
start
GUU
G
UAU
UCC
UCA
Leu
CUG
A
UCU
G
UCG
UUG
C
Phe
A
His
Gln
U
CGU
C
CGA
Arg
U
AGU
Asn
Lys
AGC
AGA
Ser
Arg
A
AAG
AGG
G
GCU
GAU
GGU
U
GCC
GAC
GCA
Ala
GCG
GAA
GGA
GGG
Gly
• One DNA strand serves as a template for the
new RNA strand
• RNA polymerase constructs the RNA strand in
a multistep process
– Initiation
• RNA polymerase attaches to the promotor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
T
C
A
A
A
A
T
C
A
T
G
A
A
G
T
T
T
T
A
G
U
A
G
A
U
G
A
A
G
U
U
U
RNA
Stop
codon
Start
codon
Translation
A
G
10.9 Transcription produces genetic messages in
the form of RNA
• Synthesis starts
T
C
GGC
Glu
GAG
C
Transcription
C
ACG
Asp
A
A
G
CGG
Thr
AAA
CGU
T
DNA
Third base
C
U
Polypeptide
Met
Lys
Phe
• Elongation:
– RNA synthesis continues
– RNA strand peels away from DNA template
– DNA strands come back together in
transcribed region
• Termination
– RNA polymerase reaches a terminator
sequence at the end of the gene
– Polymerase detaches
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-9a
LE 10-9b
RNA polymerase
RNA nucleotides
RNA
polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
Initiation
A
C
T
C
A
A
T
T
T
C
U
G
A
C
G
A
T
G
G
A
G
U
C
U
T
C
A
C
T
A
A
Termination
Direction of
transcription
Area shown
In Figure 10.9A
Elongation
C
Template
strand of DNA
Growing
RNA
Completed RNA
Newly made RNA
10.10 Eukaryotic RNA is processed before
leaving the nucleus
• The RNA that encodes an amino acid
sequence is messenger RNA (mRNA)
• In prokaryotes, transcription and translation
both occur in the cytoplasm
• RNA Splicing
– Noncoding segments called introns are cut
out
– Remaining exons are joined to form a
continuous coding sequence
– A cap and a tail are added to the ends
• In eukaryotes, RNA transcribed in the nucleus
is processed before moving to the cytoplasm
for translation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
RNA
polymerase
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-10
Exon Intron
Exon
Intron
Exon
DNA
10.11 Transfer RNA molecules serve as interpreters
during translation
Transcription
Addition of cap and tail
Cap
RNA
transcript
Introns removed
with cap
and tail
Tail
•
Transfer RNA (tRNA) molecules match the right amino
acid to the correct codon
•
tRNA is a twisted and folded single strand of RNA
Exons spliced together
– Anticodon loop at one end recognizes a particular
mRNA codon by base pairing
mRNA
Coding sequence
Nucleus
– Amino acid attachment site is at the other end
•
Each amino acid is joined to the correct tRNA by a
specific enzyme
Cytoplasm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-11a
LE 10-11b
Amino acid
attachment site
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Anticodon
Anticodon
LE 10-12a
Growing
polypeptide
tRNA
molecules
10.12 Ribosomes build polypeptides
• A ribosome consists of two subunits
Large
subunit
– Each is made up of proteins and ribosomal
RNA (rRNA)
• The subunits of a ribosome
– Hold the tRNA and mRNA close together in
binding sites during translation
– Allow amino acids to be connected into a
polypeptide chain
mRNA
Small
subunit
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-12b
tRNA-binding sites
LE 10-12c
Next amino acid
to be added to
polypeptide
Large
subunit
Growing
polypeptide
tRNA
mRNA
binding
site
mRNA
Codons
Small
subunit
10.13 An initiation codon marks the start of an
mRNA message
• Initiation is a two-step process
– Step 1
• The initiation phase of translation
• mRNA binds to a small ribosomal subunit
– Brings together mRNA, a specific tRNA,
and the two subunits of a ribosome
• Initiator tRNA, carrying the amino acid Met,
binds to the start codon
– Establishes exactly where translation will
begin
– Step 2
• A large ribosomal subunit binds to the small
one, forming a functional ribosome
• Ensures that mRNA codes are translated in
the correct sequence
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Initiator tRNA fits into one binding site; the
other is vacant for the next tRNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-13a
LE 10-13b
Start of genetic message
Met
Met
Large
Ribosomal
subunit
Initiator tRNA
P site
U A C
AUG
Start codon
mRNA
End
A site
U A C
AUG
Small ribosomal
subunit
LE 10-14
Amino
acid
Polypeptide
10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon terminates
translation
•
A site
P site
Anticodon
mRNA
Codons
Condon recognition
Once initiation is complete, amino acids are
added one by one in a three-step elongation
process
mRNA
movement
Stop
codon
1. Codon recognition
Peptide bond
formation
2. Peptide bond formation
New
peptide
bond
3. Translocation
•
Elongation continues until a stop codon reaches
the ribosome's A site, terminating translation
Translocation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-15
Transcription
DNA
10.15 Review: The flow of genetic information in
the cell is DNA ® RNA ® protein
•
The sequence of codons in DNA, via the
sequence of codons in RNA, spells out the
primary structure of a polypeptide
mRNA
RNA
polymerase
Translation
Amino acid
Enzyme
ATP
Anticodon
Initiator
tRNA
Start Codon
mRNA
Large
Initiation of
ribosomal
polypeptide synthesis
subunit
The mRNA, the first
tRNA, and the ribosomal
Sub units come together.
Small
ribosomal
subunit
New peptide
bond forming
Growing
polypeptide
2. Attachment of amino acid to tRNA
3. Initiation of polypeptide synthesis
Each amino acid
attaches to its proper
tRNA with the help of a
specific enzyme and ATP.
tRNA
U AC
AU G
1. Transcription of mRNA from a DNA
template
mRNA is
transcribed from a
DNA template.
Codons
mRNA
Elongation
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.
Polypeptide
4. Elongation
5. Termination
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Termination
Stop codon
The ribosome recognizes
a stop codon. The polypeptide is terminated
and released.
LE 10-16a
10.16 Mutations can change the meaning of
genes
Normal hemoglobin DNA
• Mutation: any change in the nucleotide
sequence of DNA
– Caused by errors in DNA replication or
recombination, or by mutagens
T
C
Mutant hemoglobin DNA
T
mRNA
A
T
G
U
A
mRNA
A
G
– Can involve large regions of a chromosome
or a single base pair
C
A
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
– Can cause many genetic diseases, such as
sickle-cell disease
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LE 10-16b
Normal gene
• Two general categories of genetic mutations
– Base substitutions replace one base with
another
• Most are harmful but may occasionally have
no effect or be beneficial
– Base insertions or deletions alter the
reading frame
• Result is most likely a nonfunctioning
polypeptide
• Mutagenesis caused by spontaneous error or
a physical or chemical mutagen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A
U
G
A
A
G
U
U
U
G
G
C
G
C
A
mRNA
Protein
Met
Lys
Phe
Gly
Ala
Base substitution
A
U
G
A
Met
A
G
U
Lys
G
A
Phe
Base deletion
A
U
U
C
G
Ser
C
A
Ala
U Missing
U
Met
G
A
A
Lys
G
U
U
Leu
G
G
C
Ala
G
C
A
His
U
MICROBIAL GENETICS
– Lytic cycle
10.17 Viral DNA may become part of the host
chromosome
• Host produces more viruses
• Viruses are infectious particles consisting of
nucleic acid enclosed in a protein capsid
• Host cell lyses (breaks open) to release new
viruses
• Viruses depend on their host cells for the
replication, transcription, and translation of
their nucleic acid
– DNA enters host bacterium, circularizes,
and enters one of two pathways
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-17
– Lysogenic cycle
Phage
• Phage DNA inserted by recombination into
the host chromosome; is now a prophage
• Prophages replicated each time host cell
divides; passed on to generations of
daughter cells
• Does not destroy host
• Environmental signal may trigger switch
from lysogenic to lytic cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Attaches
to cell
Phage DNA
Cell lyses,
releasing phages
Bacterial
chromosome
Phage injects DNA
Many cell
divisions
Lytic cycle
Phages assemble
Lysogenic cycle
Phage DNA
circularizes
Prophage
Lysogenic bacterium reproduces normally, replicating the
prophage at each cell division
OR
New phage DNA and
proteins are synthesized
Phage DNA inserts into the bacterial
chromosome by recombination
LE 10-18a
CONNECTION
Membranous
envelope
10.18 Many viruses cause disease in animals
• Structure of a virus that invades animal cells
– Genetic material may be RNA (examples:
flu, HIV) or DNA (examples: hepatitis,
herpes)
RNA
– Protein coat
Protein
coat
– Sometimes a membranous envelope with
glycoprotein spikes
• The envelope helps the virus enter and leave
the host cell during its reproductive cycle
Glycoprotein spike
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-18b
VIRUS
Viral RNA
(genome)
Plasma membrane
of host cell
Glycoprotein spike
Protein coat
Envelope
Entry
CONNECTION
10.19 Plant viruses are serious agricultural pests
• Most plant viruses
Uncoating
Viral RNA
(genome)
Protein
synthesis
RNA synthesis
by viral enzyme
RNA synthesis
(other strand)
mRNA
New
viral proteins
Template
Assembly
New viral
genome
– Have RNA genomes
– Enter their hosts via wounds in the plant's
outer layers
– May spread throughout the plant through
plasmodesmata
• There is no cure for most plant viruses
Exit
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
CONNECTION
10.20 Emerging viruses threaten human health
• Emerging viruses have appeared suddenly or
have recently come to the attention of
scientists
10.21 The AIDS virus makes DNA on an RNA
template
• HIV, the AIDS virus, is a retrovirus
– Flow of genetic information is RNA _ DNA
– Examples: HIV, SARS, Ebola, West Nile
– Inside a cell, HIV uses its RNA as a
template for making DNA
• Processes contributing to emergence
– Mutation
– The enzyme reverse transcriptase catalyzes
reverse transcription
– Contact between species
– Spread from isolated populations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-21a
Animation: HIV Reproductive Cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-21b
Envelope
Glycoprotein
Protein
coat
RNA
(two identical
strands)
Reverse
transcriptase
Viral RNA
CYTOPLASM
NUCLEUS
DNA
strand
Chromosomal
DNA
Doublestranded
DNA
Viral
RNA
and
proteins
Provirus
DNA
RNA
LE 10-22a
DNA enters
cell
10.22 Bacteria can transfer DNA in three ways
• Bacteria can transfer genes from cell to cell by
one of three processes
– Transformation: the uptake of foreign DNA
from the surrounding environment
Fragment of
DNA from
another
bacterial cell
– Transduction: transfer of bacterial genes by
a phage
– Conjugation: union of two bacterial cells and
the transfer of DNA between them
Bacterial chromosome
(DNA)
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LE 10-22b
Phage
LE 10-22c
Mating bridge
Sex pili
Fragment of DNA from
another bacterial cell
(former phage host)
Donor cell
(“male”)
Recipient cell
(“female”)
LE 10-22d
• Once new DNA is in a bacterial cell, part of it
may integrate into the recipient's chromosome
Donated DNA
Crossovers
Degraded DNA
– Occurs by crossing over between the two
molecules
– Leaves the recipient with a recombinant
chromosome
Recipient cell’s
chromosome
Recombinant
chromosome
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-23a
10.23 Bacterial plasmids can serve as carriers for
gene transfer
• The F factor is a piece of bacterial DNA
– Carries genes for things needed for
conjugation
F factor (integrated)
Male (donor)
cell
Origin of F
replication
Bacterial
chromosome
F factor starts replication
and transfer of chromosome
Recipient cell
– Contains an origin of replication
– Can transfer chromosomal DNA by
integrating into the donor bacterium's
chromosome or entering the cell as a
plasmid
Only part of the
chromosome transfers
Recombination can occur
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-23b
F factor (plasmid)
Male (donor)
cell
Bacterial
chromosome
F factor starts replication
and transfer
• Plasmids
– Small circular DNA molecules separate from
the bacterial chromosome
– Can serve as carriers for the transfer of
genes
Plasmid completes
transfer and circularizes
Cell now male
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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