Download Chapter 10

Chapter 10
Molecular Biology of the
Gene
PowerPoint Lectures for
Biology: Concepts and Connections, Fifth Edition
– Campbell, Reece, Taylor, and Simon
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Sabotage Inside Our Cells
• A saboteur
– Lies low waiting for the right moment to strike
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Viruses are biological saboteurs
– Hijacking the genetic material of host cells in order
to reproduce themselves
• Viruses provided some of the earliest evidence
– That genes are made of DNA
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
• The Hershey-Chase experiment showed that
certain viruses reprogram host cells
– To produce more viruses by injecting their DNA
Head
Tail
DNA
Tail fiber
Figure 10.1A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• The Hershey-Chase experiment
Phage
Radioactive
protein
Bacterium
Empty
protein shell
Radioactivity
in liquid
Phage
DNA
DNA
Batch 1
Radioactive
protein
Centrifuge
Pellet
1 Mix radioactively
labeled phages with
bacteria. The phages
infect the bacterial
cells.
2 Agitate in a blender to
separate phages outside
the bacteria from the cells
and their contents.
3 Centrifuge the mixture
so bacteria form a pellet
at the bottom of the test
tube.
4 Measure the
radioactivity in
the pellet and
the liquid.
Radioactive
DNA
Batch 2
Radioactive
DNA
Centrifuge
Pellet
Figure 10.1B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Radioactivity
in pellet
• Phage reproductive cycle
Phage attaches
to bacterial cell.
Phage injects DNA.
Phage DNA directs host
cell to make more phage
DNA and protein parts.
New phages assemble.
Cell lyses and releases
new phages.
Figure 10.1C
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
10.2 DNA and RNA are polymers of nucleotides
• DNA is a nucleic acid
– Made of long chains of nucleotide monomers
Sugar-phosphate backbone
Phosphate group
A
C
Nitrogenous base
A
Sugar
DNA nucleotide
C
Nitrogenous base
(A, G, C, or T)
Phosphate
group
O
H3C
O
T
T
O P
O
CH2
O–
G
C
HC
O
C
N
N
C
H
O
Thymine (T)
O
C H
G
H
C
HC
CH
H
Sugar
(deoxyribose)
T
T
DNA nucleotide
Figure 10.2A
DNA polynucleotide
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• DNA has four kinds of nitrogenous bases
– A, T, C, and G
H
O
C
H3C
H
C
C
H
H
N
C
H
N
C
N
C
C
C
N
H
O
N
H
N
H
O
H
H
Thymine (T)
Cytosine (C)
Pyrimidines
Figure 10.2B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
N
H
O
C
C
N
C
C
N
H
C
N
N
H
N
C
C
N
H
Adenine (A)
Guanine (G)
Purines
H
C
N
H
C
C
N
H
H
• RNA is also a nucleic acid
– But has a slightly different sugar
– And has U instead of T
Nitrogenous base
(A, G, C, or U)
O
Phosphate
group
H
O
O
P
N
C
C
H
O CH2
N
H
O
Uracil (U)
O–
O
C H
H C
H C
C H
O
Figure 10.2C, D
C
C
OH
Sugar
(ribose)
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Key
Hydrogen atom
Carbon atom
Nitrogen atom
Oxygen atom
Phosphorus atom
10.3 DNA is a double-stranded helix
• James Watson and Francis Crick
– Worked out the three-dimensional structure of
DNA, based on work by Rosalind Franklin
Figure 10.3A, B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• The structure of DNA
– Consists of two polynucleotide strands
wrapped around each other in a double helix
Figure 10.3C
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Twist
• Hydrogen bonds between bases
– Hold the strands together
• Each base pairs with a complementary partner
– A with T, and G with C
G
C
T
A
A
Base
pair
T
C
G
C
C
G
A
T
T
O
O
P
–O
O
H2C
O
O
P
–O
O
H2C
G
T
O
OH
P
O
O
H2C
–O
A
O
O
–O P
O
H2C
A
A
T
A
Hydrogen bond
OH
O
O
A
T
O
CH2
O O–
O P
O
O
CH2
O
O–
P
O
O
O
CH2
O
O–
P
HO O
G
C
A
T
T
OH
G
A
Figure 10.3D
CH2
O O–
P
O
O
C
G
O
O
Ribbon model
C
T
Partial chemical structure
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Computer model
DNA REPLICATION
10.4 DNA replication depends on specific base
pairing
• DNA replication
– Starts with the separation of DNA strands
• Then enzymes use each strand as a template
– To assemble new nucleotides into
complementary strands
Figure 10.4A
A
T
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
C
G
G
C
G
C
G
C
G
C
A
T
A
T
A
T
A
T
T
A
T
A
T
A
T
A
Parental molecule
of DNA
C
A
Nucleotides
Both parental strands serve
as templates
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Two identical daughter
molecules of DNA
• DNA replication is a complex process
– Due in part to the fact that some of the helical
DNA molecule must untwist
G C
A
T
G
C
C
G
A
T
T
C
A
A
G
T
C
G
C
C
T
A
T
A
A
G
G
C
G
T
A
G
G
C
T
C
G
C
T
A
C
T
Figure 10.4B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
A
A
G
T
T
A
T
A
T
A
10.5 DNA replication: A closer look
• DNA replication
– Begins at specific sites on the double helix
Origin of replication
Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
Figure 10.5A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Each strand of the double helix
– Is oriented in the opposite direction
5 end
P
3 end
HO
5
4
3
2
2
1
A
T 1
5
P
3
4
P
C
G
P
P
G
C
P
P
T
OH
3 end
Figure 10.5B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
A
P
5 end
• Using the enzyme DNA polymerase
– The cell synthesizes one daughter strand as a
continuous piece
• The other strand is synthesized as a series of short pieces
– Which are then connected by the enzyme DNA ligase
DNA polymerase
molecule
5
3
3
5
Daughter strand
synthesized
continuously
Parental DNA
3
5
5
3
DNA ligase
Figure 10.5C
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Overall direction of replication
Daughter
strand
synthesized
in pieces
THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
• 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 its DNA bases
• A particular gene, a linear sequence of many
nucleotides
– Specifies a polypeptide
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• The DNA of the gene is transcribed into RNA
– Which is translated into the polypeptide
DNA
Transcription
RNA
Translation
Protein
Figure 10.6A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Studies of inherited metabolic disorders in mold
– First suggested that phenotype is expressed
through proteins
Figure 10.6B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
10.7 Genetic information written in codons is
translated into amino acid sequences
• The “words” of the DNA “language”
– Are triplets of bases called codons
• The codons in a gene
– Specify the amino acid sequence of a
polypeptide
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
A A A C C G G C A A A A
Transcription
RNA
U U U G G C C G U U U U
Codon
Translation
Polypeptide
Figure 10.7
Amino acid
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
10.8 The genetic code is the Rosetta stone of life
• Nearly all organisms
– Use exactly the same genetic code
Second base
U
U
C
UUU
Phe
UUC
UCU
UCC
UUA
UCA
UUG
Leu
CUU
C
CUC
First base
CUA
CUG
A
Leu
UGA Stop A
UGG Trp
CCU
CAU
His
CAC
CGU
CGC
CAA
CAG Gln
CGA
CGG
AAU
Asn
AAC
U
AGU
Ser
AGC
C
AAA
AGA
A
AGG Arg
G
U
GGU
C
GGC
Gly
GGA
A
CCC
CCA
CCG
ACA
ACC
GUA
Pro
Thr
AAG
GAU
GCU
GCC
Val
GUG
Figure 10.8A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
UAC
UGU Cys U
UGC
C
UAA Stop
AUA
GUC
G
Ser
Tyr
UAG Stop
ACU
ACC
GUU
UAU
G
UCG
AUU
AUC Ile
Met or
AUG
start
A
GCA
GCG
GAC
Ala
Lys
Asp
GAA
GAG
Glu
GGG
G
U
C
Arg
A
G
G
• An exercise in translating the genetic code
Strand to be transcribed
T
A
C
T
T
C
A
A
A
A
T
C
A
T
G
A
A
G
T
T
T
T
A
G
U
A
G
DNA
Transcription
A
U
G
A
A
G
U
U
U
RNA
Start
condon
Stop
condon
Translation
Figure 10.8B
Polypeptide
Met
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Lys
Phe
10.9 Transcription produces genetic messages in
the form of RNA
• A close-up view of transcription
RNA nucleotides
RNA
polymerase
T C C A
A U
T
T
A
C C A
T A G G T
Direction of
transcription
Figure 10.9A
Newly made RNA
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
A
Template
Strand of DNA
• In the nucleus, the DNA helix unzips
– And RNA nucleotides line up along one
strand of the DNA, following the base
pairing rules
• As the single-stranded messenger RNA
(mRNA) peels away from the gene
– The DNA strands rejoin
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Transcription of a gene
RNA polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
1 Initiation
2 Elongation
3 Termination
Completed RNA
Figure 10.9B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Area shown
In Figure 10.9A
Growing
RNA
RNA
polymerase
10.10 Eukaryotic RNA is processed before leaving
the nucleus
• Noncoding segments called introns are spliced out
– And a cap and a tail are added to the ends
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
Figure 10.10
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
10.11 Transfer RNA molecules serve as
interpreters during translation
• Translation
– Takes place in the cytoplasm
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• A ribosome attaches to the mRNA
– And translates its message into a specific
polypeptide aided by transfer RNAs
(tRNAs)
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Figure 10.11A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Anticodon
• Each tRNA molecule
– Is a folded molecule bearing a base triplet
called an anticodon on one end
• A specific amino acid
– Is attached to the other end
Amino acid
attachment site
Figure 10.11B, C
Anticodon
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
10.12 Ribosomes build polypeptides
• A ribosome consists of two subunits
– Each made up of proteins and a kind of
RNA called ribosomal RNA
tRNA
molecules
Growing
polypeptide
Large
subunit
mRNA
Figure 10.12A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Small
subunit
• The subunits of a ribosome
– Hold the tRNA and mRNA close together
during translation
tRNA-binding sites
Large
subunit
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
mRNAbinding site
mRNA
Small
subunit
Codons
Figure 10.12B, C
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
10.13 An initiation codon marks the start of an
mRNA message
Start of genetic message
End
Figure 10.13A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• mRNA, a specific tRNA, and the ribosome
subunits
– Assemble during initiation
Met
Met
Large
ribosomal
subunit
Initiator tRNA
P site
U
A C
A U G
U
A C
A U G
Start
codon
1
mRNA
A site
Small ribosomal
subunit
Figure 10.13B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
2
10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon terminates
translation
• Once initiation is complete
– Amino acids are added one by one to the
first amino acid
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Each addition of an amino acid
– Occurs in a three-step elongation process
Amino
acid
Polypeptide
P site
A site
Anticodon
mRNA
Codons
1 Codon recognition
mRNA
movement
Stop
codon
2 Peptide bond
formation
New
Peptide
bond
Figure 10.14
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
3 Translocation
• The mRNA moves a codon at a time
– And a tRNA with a complementary
anticodon pairs with each codon, adding its
amino acid to the peptide chain
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Elongation continues
– Until a stop codon reaches the ribosome’s
A site, terminating translation
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
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
– Spells out the primary structure of a
polypeptide
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Summary of transcription and translation
DNA
Transcription
1 mRNA is transcribed
from a DNA template.
mRNA
RNA
polymerase
Amino acid
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
Start Codon
mRNA
3 Initiation of
polypeptide synthesis
The mRNA, the first tRNA,
and the ribosomal
subunits come together.
Small
ribosomal
subunit
New peptide
bond forming
Growing
polypeptide
4 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.
Codons
mRNA
Polypeptide
5 Termination
The ribosome recognizes a
stop codon. The poly-peptide
is terminated and released.
Figure 10.15
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Stop codon
10.16 Mutations can change the meaning of
genes
• Mutations are changes in the DNA base
sequence
– Caused by errors in DNA replication or
recombination, or by mutagens
Normal hemoglobin DNA
C
T
T
mRNA
A
T
G U
A
C
mRNA
G
Figure 10.16A
Mutant hemoglobin DNA
A
A
Normal hemoglobin
Glu
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Sickle-cell hemoglobin
Val
• Substituting, inserting, or deleting nucleotides
alters a gene
– With varying effects on the organism
Normal gene
A U G A A G U U U G G C G C A
mRNA
Met
Protein
Lys
Phe
Gly
Ala
Base substitution
A U G A A G U U U A G C G C A
Met
Lys
Phe
Ser
Ala
U Missing
Base deletion
A U G A A G U U G G C G C A U
Figure 10.16B
Met
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Lys
Leu
Ala
His
MICROBIAL GENETICS
10.17 Viral DNA may become part of the host
chromosome
• Viruses
– Can be regarded as genes packaged in
protein
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• When phage DNA enters a lytic cycle inside a
bacterium
– It is replicated, transcribed, and translated
• The new viral DNA and protein molecules
– Then assemble into new phages, which
burst from the host cell
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• In the lysogenic cycle
– Phage DNA inserts into the host
chromosome and is passed on to
generations of daughter cells
• Much later
– It may initiate phage production
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Phage reproductive cycles
Phage
1
Attaches
to cell
Bacterial
chromosome
Phage DNA
Cell lyses,
releasing phages
Phage injects DNA
7
2
Many cell
divisions
4
Lytic cycle
Lysogenic cycle
Phages assemble
Phage DNA
circularizes
3
Lysogenic bacterium reproduces
normally, replicating the prophage
at each cell division
Prophage
5
6
OR
New phage DNA and
proteins are synthesized
Figure 10.17
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Phage DNA inserts into the bacterial
chromosome by recombination
CONNECTION
10.18 Many viruses cause disease in animals
• Many viruses cause disease
– When they invade animal or plant cells
• Many, such as flu viruses
– Have RNA, rather than DNA, as their
Membranous
genetic material
envelope
RNA
Protein
coat
Figure 10.18A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Glycoprotein spike
• Some animal viruses
– Steal a bit of host cell membrane as a
protective envelope
– Can remain latent in the host’s body for
long periods
Glycoprotein spike
VIRUS
Protein coat
Envelope
Viral RNA
(genome)
Plasma membrane 1
of host cell
2
Viral RNA
(genome)
3
Entry
Uncoating
RNA synthesis
by viral enzyme
4 Protein
mRNA synthesis
New
viral proteins
5
RNA synthesis
(other strand)
Template
6 Assembly
Exit
Figure 10.18B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
7
New viral
genome
CONNECTION
10.19 Plant viruses are serious agricultural pests
• Most plant viruses
– Have RNA genomes
– Enter their hosts via wounds in the plant’s
outer layers
Protein RNA
Figure 10.19
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
CONNECTION
Figure 10.20A, B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Colorized TEM 370,000
Colorized TEM 50,000
10.20 Emerging viruses threaten human health
10.21 The AIDS virus makes DNA on an RNA
template
• HIV, the AIDS virus
– Is a retrovirus
Envelope
Glycoprotein
Protein coat
RNA (two
identical strands)
Reverse transcriptase
Figure 10.21A
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Inside a cell, HIV uses its RNA as a template
for making DNA
– To insert into a host chromosome
Viral RNA
CYTOPLASM
1
RNA
strand
NUCLEUS
Chromosomal
DNA
2
Doublestranded
DNA
3
Provirus
DNA
4
Viral
RNA
and
proteins
5
RNA
6
Figure 10.21B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
10.22 Bacteria can transfer DNA in three ways
• Bacteria can transfer genes from cell to cell by
one of three processes
– Transformation, transduction, or
conjugation
Mating bridge
DNA enters
cell
Fragment of DNA
from another
bacterial cell
Bacterial
chromosome
(DNA)
Phage
Phage
Fragment of
DNA from
another
bacterial cell
(former phage
host)
Figure 10.22A–C
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Sex pili
Donor cell
(“male”)
Recipient cell
(“female”)
• Once new DNA gets into a bacterial cell
– Part of it may then integrate into the
recipient’s chromosome
Donated DNA
Figure 10.22D
Recipient cell’s
chromosome
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Crossovers
Degraded DNA
Recombinant
chromosome
10.23 Bacterial plasmids can serve as carriers for
gene transfer
• Plasmids
– Are small circular DNA molecules separate
from the bacterial chromosome
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Plasmids can serve as carriers
– For the transfer of genes
F factor (plasmid)
F factor
(integrated)
Male (donor) cell
Origin of F
replication
Bacterial
chromosome
F factor starts replication
and transfer of chromosome
Male (donor) cell
Bacterial chromosome
F factor starts replication
and transfer
Only part of the chromosome
transfers
Plasmid completes transfer
and circularizes
Plasmids
Recombination
can occur
Cell now male
Figure 10.23A–C
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Colorized TEM 2,000
Recipient cell