Download Chapter 10

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
– Not cellular
– Cannot reproduce on their own
•
First understanding of DNA based on viruses
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
– Studied the simple bacteriophage T2
– Showed that the virus injects its DNA into
host cells and reprograms them to produce
more viruses
LE 10-1a
Head
DNA
Tail
300,000
Tail fiber
LE 10-1b
Phage
Radioactive
protein
Bacterium
DNA
Batch 1
Radioactive
protein
Mix radioactively
labeled phages with
bacteria. The phages
infect the bacterial cells.
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
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.
Animation: Phage T2 Reproductive Cycle
Animation: Hershey-Chase Experiment
10.2 DNA and RNA are polymers of nucleotides
• Nucleic acids are polynucleotides made of long
chains of nucleotide monomers
– Nitrogenous bases
• Single-ring pyrimidines: thymine (T), cytosine
(C)
• Double-ring purines: adenine (A), guanine (G)
– Sugar-phosphate backbone
• DNA and RNA are identical except for two
things
– Nitrogenous bases
• DNA: A, C, G, T
• RNA: A, G, C, U
– Sugars
• DNA: deoxyribose
• RNA: ribose
Animation: DNA and RNA Structure
LE 10-2a
Sugar-phosphate backbone
Phosphate group
A
Nitrogenous base
Sugar
A
C
C
DNA nucleotide
Nitrogenous base
(A, G, C, or T)
Phosphate
group
T
T
Thymine (T)
G
G
Sugar
(deoxyribose)
T
T
DNA nucleotide
DNA polynucleotide
LE 10-2b
Cytosine (C)
Thymine (T)
Pyrimidines
Adenine (A)
Guanine (G)
Purines
Nitrogenous base
(A, G, C, or U)
Phosphate
group
Uracil (U)
Sugar
(ribose)
LE 10-2d
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 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
Animation: DNA Double Helix
– 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
LE 10-3d
C
G
T
A
T
A
Base
pair
C
Hydrogen bond
T
G
C
G
A
T
A
C
G
C
G
T
T
C
A
G
A
A
T
A
T
A
G
A
Ribbon model
T
C
T
Partial chemical structure
Computer model
DNA REPLICATION
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
LE 10-4a
A
T
A
T
C
G
C
G
G
C
G
A
T
A
T
A
T
Parental
molecule
of DNA
A
C
C
Nucleotides
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
Both parental
strands serve
as templates
Animation: DNA Replication Overview
Two identical
daughter molecules
of DNA
LE 10-4b
G C
A T
G
C
C
G
A T
C
10.5 DNA replication: A closer look
• DNA replication begins at specific sites (origins
of replication) on the double helix
– Proteins attach and separate the strands
– Replication proceeds in both directions,
creating replication bubbles
• Parent strands open, daughter strands
elongate
– Replication occurs simultaneously at many
sites
LE 10-5a
Origin of replication
Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
• DNA's sugar-phosphate backbones are
oriented in opposite directions
– The enzyme DNA polymerase adds
nucleotides at only the 3’ end
• One daughter strand is synthesized as a
continuous piece
• The other strand is synthesized as a series
of short pieces
• The two strands are connected by the
enzyme DNA ligase
LE 10-5b
3 end
5 end
P
HO
5
2
4
3
1
A
T
2
P
C
G
P
P
G
C
P
P
T
3 end
4
5
P
OH
3
1
A
P
5 end
LE 10-5c
DNA polymerase
molecule
3
5
5
Daughter strand
synthesized
continuously
Parental DNA
3
3
5
5
3
Daughter
strand
synthesized
In pieces
Animation: Origins of Replication
Animation: Leading Strand
DNA ligase
Overall direction of replication
Animation: Lagging Strand
Animation: DNA Replication Review
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 DNA
bases
• A particular gene—a linear sequence of many
nucleotides—specifies a particular polypeptide
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
– A gene dictates production of a specific enzyme
–
The hypothesis has been restated to one geneone polypeptide
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
• Three-letter words (codons)
• Each word codes for one amino acid in a
polypeptide
LE 10-7a
DNA molecule
Gene 1
Gene 2
Gene 3
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
Codon
Translation
Polypeptide
Amino acid
10.8 The genetic code is the Rosetta stone of life
• The genetic code specifies the
correspondence between RNA codons and
amino acids in proteins
– Includes start and stop codons
– Redundant but not ambiguous
• Nearly all organisms use exactly the same
genetic code
Second base
C
U
A
UAU
UCU
UUU
Phe
U
UAC
UCC
UUC
G
U
UGU
Tyr
Cys
UGC
C
Ser
UUA
UCA
UAA Stop
UGA
Stop A
UUG
UCG
UAG Stop
UGG
Trp
CUU
CCU
CAU
CGU
Leu
U
His
CUC
C
G
CCC
Leu
CUA
CGU
CAC
Pro
CCA
C
Arg
CAA
CGA
A
CGG
G
Gln
CUG
CCG
CAG
AUU
ACU
AAU
AUC lle
A
ACC
U
AGU
Ser
Asn
AAC
C
AGC
Thr
AUA
ACA
Met or
A
AGA
AAA
Arg
Lys
AUG start
ACG
AAG
AGG
G
GUU
GCU
GAU
GGU
U
GUC
GCC
GAC
G
Val
GUA
GCA
Asp
C
GGC
Gly
Ala
GAA
GGA
A
GGG
G
Glu
GUG
GCG
GAG
LE 10-8b
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
Stop
codon
Start
codon
Translation
Polypeptide
Met
Lys
Phe
10.9 Transcription produces genetic messages in
the form of RNA
• 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
• Synthesis starts
• 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
LE 10-9a
RNA nucleotides
RNA
polymerase
C
C
A
A
U
C
C
A
T
A
G
G
T
Direction of
transcription
Newly made RNA
A
T
Template
strand of DNA
LE 10-9b
RNA polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
Initiation
Elongation
Termination
Completed RNA
Area shown
In Figure 10.9A
Growing
RNA
RNA
polymerase
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
• In eukaryotes, RNA transcribed in the nucleus
is processed before moving to the cytoplasm
for translation
• 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
LE 10-10
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
10.11 Transfer RNA molecules serve as interpreters
during translation
•
Transfer RNA (tRNA) molecules match the right amino
acid to the correct codon
•
tRNA is a twisted and folded single strand of RNA
– Anticodon loop at one end recognizes a particular
mRNA codon by base pairing
– Amino acid attachment site is at the other end
•
Each amino acid is joined to the correct tRNA by a
specific enzyme
LE 10-11a
Amino acid attachment site
Hydrogen bond
Amino acid
attachment site
RNA polynucleotide chain
Anticodon
Anticodon
10.12 Ribosomes build polypeptides
• A ribosome consists of two subunits
– 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
tRNA
molecules
Growing
polypeptide
Large
subunit
tRNA-binding sites
mRNA
Small
subunit
Large
subunit
mRNA
binding
site
Small
subunit
LE 10-12c
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
mRNA
Codons
10.13 An initiation codon marks the start of an
mRNA message
• The initiation phase of translation
– Brings together mRNA, a specific tRNA,
and the two subunits of a ribosome
– Establishes exactly where translation will
begin
• Ensures that mRNA codes are translated in
the correct sequence
• Initiation is a two-step process
– Step 1
• mRNA binds to a small ribosomal subunit
• Initiator tRNA, carrying the amino acid Met,
binds to the start codon
– Step 2
• A large ribosomal subunit binds to the small
one, forming a functional ribosome
• Initiator tRNA fits into one binding site; the
other is vacant for the next tRNA
LE 10-13b
Start of genetic message
End
Large
Ribosomal
subunit
Initiator tRNA
P site
U A C
A U G
U A C
A UG
Start codon
mRNA
A site
Small ribosomal
subunit
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 in a three-step elongation
process
1. Codon recognition
2. Peptide bond formation
3. Translocation
•
Elongation continues until a stop codon reaches
the ribosome's A site, terminating translation
LE 10-14
Amino
acid
Polypeptide
A site
P site
Anticodon
mRNA
Codons
Condon recognition
mRNA
movement
Stop
codon
Peptide bond
formation
New
peptide
bond
Translocation
Animation: Translation
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
1. Transcription of mRNA from a DNA
template
2. Attachment of amino acid to tRNA
3. Initiation of polypeptide synthesis
4. Elongation
5. Termination
10.16 Mutations can change the meaning of
genes
• Mutation: any change in the nucleotide
sequence of DNA
– Caused by errors in DNA replication or
recombination, or by mutagens
– Can involve large regions of a chromosome
or a single base pair
– Can cause many genetic diseases, such as
sickle-cell disease
LE 10-16a
Normal hemoglobin DNA
C
T
Mutant hemoglobin DNA
T
mRNA
C
A
T
G
U
A
mRNA
G
A
A
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
• 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
LE 10-16b
Normal gene
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
10.17 Viral DNA may become part of the host
chromosome
• Viruses are infectious particles consisting of
nucleic acid enclosed in a protein capsid
• 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
– Lytic cycle
• Host produces more viruses
• Host cell lyses (breaks open) to release new
viruses
– Lysogenic cycle
• 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
Phage
Attaches
to cell
Phage DNA
Cell lyses,
releasing phages
Bacterial
chromosome
Phage injects DNA
Many cell
divisions
Lytic cycle
Lysogenic cycle
Phages assemble
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
Animation: Simplified Viral Reproductive Cycle
Animation: Phage T4 Lytic Cycle
Animation: Phage Lambda Lysogenic and Lytic Cycles
CONNECTION
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)
– Protein coat
– Sometimes a membranous envelope with
glycoprotein spikes
• The envelope helps the virus enter and leave
the host cell during its reproductive cycle
Membranous
envelope
VIRUS
Viral RNA
(genome)
Plasma membrane
of host cell
Glycoprotein spike
Protein coat
Envelope
Entry
Uncoating
Viral RNA
(genome)
RNA synthesis
by viral enzyme
RNA
Protein
synthesis
Protein
coat
RNA synthesis
(other strand)
mRNA
Template
New viral
genome
New
viral proteins
Assembly
Glycoprotein spike
Exit
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
– May spread throughout the plant through
plasmodesmata
• There is no cure for most plant viruses
LE 10-19
Protein
RNA
CONNECTION
10.20 Emerging viruses threaten human health
• Emerging viruses have appeared suddenly or
have recently come to the attention of
scientists
– Examples: HIV, SARS, Ebola, West Nile
• Processes contributing to emergence
– Mutation
– Contact between species
– Spread from isolated populations
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
– Inside a cell, HIV uses its RNA as a
template for making DNA
– The enzyme reverse transcriptase catalyzes
reverse transcription
Animation: HIV Reproductive Cycle
LE 10-21a
Envelope
Glycoprotein
Protein
coat
RNA
(two identical
strands)
Reverse
transcriptase
LE 10-21b
Viral RNA
CYTOPLASM
NUCLEUS
DNA
strand
Chromosomal
DNA
Doublestranded
DNA
Viral
RNA
and
proteins
Provirus
DNA
RNA
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
– Transduction: transfer of bacterial genes by
a phage
– Conjugation: union of two bacterial cells and
the transfer of DNA between them
LE 10-22a
DNA enters
cell
Fragment of
DNA from
another
bacterial cell
Bacterial chromosome
(DNA)
LE 10-22b
Phage
Fragment of DNA from
another bacterial cell
(former phage host)
LE 10-22c
Mating bridge
Sex pili
Donor cell
(“male”)
Recipient cell
(“female”)
• Once new DNA is in a bacterial cell, part of it
may integrate into the recipient's chromosome
– Occurs by crossing over between the two
molecules
– Leaves the recipient with a recombinant
chromosome
LE 10-22d
Donated DNA
Recipient cell’s
chromosome
Crossovers
Degraded DNA
Recombinant
chromosome
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
– Contains an origin of replication
– Can transfer chromosomal DNA by
integrating into the donor bacterium's
chromosome or entering the cell as a
plasmid
F factor (integrated)
Male (donor)
cell
Origin of F
replication
Bacterial
chromosome
F factor starts replication
and transfer of chromosome
F factor (plasmid)
Male (donor)
cell
Bacterial
chromosome
F factor starts replication
and transfer
Recipient cell
Only part of the
chromosome transfers
Recombination can occur
Plasmid completes
transfer and circularizes
Cell now male
• Plasmids
– Small circular DNA molecules separate from the
bacterial chromosome
– Can serve as carriers for the transfer of genes
Colorized TEM 2,000
Plasmids