Download Protein synthesis: methionly-tRNAi recognizes the AUG start codon

Protein synthesis: methionly-tRNAi recognizes the AUG start codon
Initiation factor
Translation initiation usually occurs
near the first AUG closest to the 5’ end
of an mRNA
eIF4A (helicase activity) →uses
energy→ unwind RNA→complex
move
Kozak sequence: ACCAUGG
UTR
Initiator factor: IF
•
Initiation
The inactive 40S and 60S subunits will bind to each other with high affinity to form
inactive complex
This is achieved (達到) by eIF3, which bind to the 40S subunit mRNA forms an
preinitiation complex (also bound eIF1A, Met-tRNAi Met, eIF, GTP) with a
ribosome; when eIF 2 phosphorylated → GDP/GTP exchange X→ translation
X
A number of initiation factors participate in the process.
Cap sequence present at the 5’ end of the mRNA is recognized by eIF4 complex,
eIF4 interaction with preinitation complex.
Subsequently eIF3 is bound and cause the binding of small 40S subunit in the
complexes (initiation complex) step 2→ slide or scan; eIF4A plus mRNA →
helicase→open RNA secondary structure (ATP dependent)
The 18S RNA present in the 40 S subunit is involved in binding the cap sequence
eIF2 binds GTP and initiation tRNA, which recognize the start codon AUG → eIF2
GTP hydrolysis → irreversible step for further scanning.
eIF4A unwind the RNA secondary structure by hydrolysis of ATP, 40S complex
migrate down stream until it finds AUG start codon
eIF5 hydorlysis a GTP →The large 60S subunit is then bound to the 40S subunit
It is accompanied by the dissociation of several initiation factor and GDP
The formation of the initiation complex is now completed
Ribosome complex is able to translate
During chain elongation each incoming aminoacyltRNA move through three ribosomal sites
Elongation factor (EFs): help ribosome move
and tRNA move
Translocation: ribosome move
correct
Conformational
change
Peptidyltransf
erase reaction
by large
rRNA
Help move
ribosome contains two sites where the
tRNAs can bind to the mRNA.
P (peptidyl) site allows the binding of the
initiation tRNA to the AUG start codon.
The A (aminoacyl) site covers the second
codon of the gene and the first is
unoccupied
On the other side of the P site is the exit (E)
site where empty tRNA is released
The elongation begins after the
corresponding aminoacyl-tRNA occupies
the A site by forming base pairs with the
second codon
Two elongation factors (eEF) play an
important role. EF1 and EF2
eEF1α binds GTP and guides the
corresponding aminoacyl-tRNA to the A
site, during which GTP is hydrolized to
GDP and P.
1
correctly base-pair → hydrolysis GTP →
conformational change → tight bind
aminoacyl-tRNA in A site and release
EF1-GDP
Peptide bond formation by large rRNA (peptidlytransferase reaction)
The cleavage of the energy-rich anhydride bond
in GTP enables the aminoacyl-tRNA to
bind to codon at the A site
Afterward the GDP still bound to eEF1α, is
exchange for GTP as mediated by the
eEF1βγ, can recycle
The eEF1 α-GTP is now ready for the next cycle
Subsequently a peptide linkage is form between
the carboxyl group of methionine and the
amino group of amino acid of the tRNA
bound to A site
Peptidyl transferase catalyzing the reaction. It
facilitates the N-nucleophilic attack on the
carboxyl group, whereby the peptide bond
is formed with the released of water.
Accompanied by the hydrolysis of one molecule GTP to form GDP and Pi, the eEF2
facilitates the translocation of the ribosome along the mRNA to three bases
downstream
Free tRNA arrives at site E is released, and tRNA loaded with the peptide now
occupies the P Site
The third aminoacyl-tRNA binds to the vacant A site and a further elongation
An RNA-RNA hybrid of only three base pairs is not
stable for normal physiological condition.
Multiple interactions between the large and small
rRNAs and general domains of tRNAs can
stabilized the tRNA in the A and P site.
E. Coli 70S ribosomes
2
Translation is terminated by release
factors when a stop codon is reached
Release factor RF:
In eukaryote
eRF1 like tRNA, can bind to A site of ribosome
eRF3 is GTP binding protein
Promote cleavage of the peptidyl-tRNA, and
releasing the protein chain
In bacterial
RF1 and RF2 like eRF1
RF 3 GTP bind factor
Chaperone protect protein and help folding
GTPase
When A site finally binds to a stop codon (UGA, UAG,
UAA)
Stop codons bind eRF accompanied by hydrolysis GTP
to form GDP and P
Binding of eRF to the stop codon alters the specificity
the peptidyl transferase
Water instead amino acid is now the acceptor for the
peptide chain
Protein released from the tRNA
Protein synthesis-4
Termination of translation
When the ribosome reaches a stop codon in
the A site, one of three releasing factors and
initiate hydrolysis of the peptide chain from
the tRNA in the P site
– RF-1 recognizes UAA and UAG
– RF-2 recognizes UAA and UGA
– RF-3 binds GTP and enhances the effects of
RF-1 and RF-2
Polysomes and rapid ribosome recycling increase the efficiency of
translation
Polysomes and recycling increase translation
Eukarotic mRNA in circular form stabilized by interactions between protein bound at
the 3’ and 5’ ends
Poly A binding protein (PABPI), interact with both mRNA poly A and eIF4g
Two ends is very close together, then ribosome subunit easy to bind.
3
The Synthesis of Protein
Polyribosomes = A cluster of ribosomes simultaneously translating an mRNA
molecule
Polyribosomes are found in both prokaryotes and eukaryotes
Protein-protein and protein –mRNA interactions form a bridge.
Purified poly(A)-binding protein I, eIF4E and eIF4G , mRNA formed a circular structure
Life cycle of an mRNA-2
The biological activity of proteins depends on a precise folding of the polypeptide
chain into 3-D conformation
Some proteins must undergo post-translational modification before they become
fully functional
DNA replication:
DNA-directed DNA polymerases
Semi-conservative
5’ – 3’
Replication forks
Uni- or Bi- directional
Semi-discontinuous
Primers
4
DNA replication is semiconservative mechanism
DNA polymerase require a primer to initiate replication
DNA replication direction: 5’ to 3’
DNA polymerase need a primer to initiation.
Action of DNA polymerase. DNA is elongated in its 5′ → 3′ direction.
Helicase : open DNA double strand
Replication origin
DNA polymerase
Primase: provide a short primer
Replication fork
Topoisomerase I: release the local unwinding of DNA produces
torsional stress (扭力; supercoil)
DNA ligase
Single-stranded binding proteins
Leading strand
Lagging strand
Helicase: separates the two DNA strands, starting at
replication origins (rich in A-T base pairs)
RNA primase: inserts a starter of RNA nucleotides at the
initiation point
DNA polymerase binds a complementary leading strand of
DNA nucleotides starting at the 3’end of the RNA primer
Exonuclease removes RNA primer, which are replace with
DNA nucleotides by DNA polymerase
5
Two strands are anti-parallel & DNA, polymerase synthesizes 5’ TO
3’
DNA synthesis is discontinuous on the lagging strand but continuous
on the leading strand (Okazaki et al 1968).
The short DNA fragments on lagging strand are called Okazaki
fragments.
DNA polymerase requires a primers so each Okazaki fragment must
begin with a primer.
How are primers synthesized? First primer (starts strand
synthesis) and primers for each Okazaki fragment
Primases
starting nascent DNA chains
Primases synthesize short RNA (or
RNA/DNA) oligonucleotides that act
as primers for DNA polymerase.
Can initiate synthesis on ssDNA de novo
(no 3’=OH needed).
Usually part of protein complex or need
specific interactions with other
replication proteins for efficient
primer synthesis.
Most primases start synthesis at a
random sites; do not synthesize
primers with a specific sequence.
Homotrimeric
Heterotrimeric
protein, maintain
the template in a
uniform
conformation for
DNA polymerase
1.Like Helicase open DNA (unwind)
2.RPA (heterotrimeric protein) bind single
DNA, Single-stranded binding proteins
3.Leading strand synthesis by DNA
polymerase s, PCNA and Rfc
(replication factor) complex
4.Lagging strand synthesis by pol
σ→Okazaki fragment
5. PCNA-Rfc-pol σ complex process each
Okazaki fragment
Proliferating cell nuclear antigen: prevent loss
complex dissociating from DNA
Maintains the template in a uniform conformatio
Replication protein A
DNA helicases - Separation of the Watson/Crick helix
DNA helicases utilize energy of ATP hydrolysis to
cause disruption of hydrogen bonds in the double
helix.
Helicases are necessary for movement of a
replication fork. In E. coli, primary replicative
helicase is dnaB
Helicases function by moving along ssDNA in one
direction disrupting hydrogen bonds as they
move.
Both 5’ to 3’ and 3’ to 5’ helicases exist.
Nomenclature - direction of helicase movement is
defined on the strand the helicase binds. (A 5’ to
3’ helicase isshown at right).
Single-stranded DNA binding proteins (SSBs)
bind tightly to ssDNA.
SSBs prevent formation of secondary structure,
renaturation of ssDNA and non-specific
interactions on ssDNA.
– SSBs usually bind cooperatively.
– SSBs usually interact with other replication
proteins; these interactions promote efficient
replication
6
Role of Topoisomerases
SSBs bind tightly to ssDNA.
– SSBs prevent formation of
secondary structure, renaturation of
ssDNA and non-specific interactions on
ssDNA.
– SSBs usually bind cooperatively.
– SSBs usually interact with other
replication proteins; these interactions
promote efficient replication
Replication fork → move →supercoil
During DNA replication Topoisomerases
act to release the links between the
parental DNA strands both during
replication (swiveling轉環) or after
replication (decatenation去除連銷).
DNA ligases form phosphodiester bonds; join strands of DNA
Topoisomerases
Type I - change L by multiples of 1 by causing a transient ssDNA break.
Type II - change L by multiples to 2 by causing a transient dsDNA break.
Topoisomerases function by forming a covalent intermediate with the transiently
broken end(s) of the DNA.
Almost all topoisomerases relax both positively and negatively supercoiled
DNA.
Topoisomerase I
lagging strand is used in discontinuous synthesis forms Okazaki
fragments
fragments joined by DNA ligase
Must supply a primer (i.e.
3’-OH) to start DNA
synthesis
This is the function of
primase which makes
RNA primers
Must ‘seal’ the DNA
fragments made on the
lagging strand template
This is the function of
DNA ligase
After DNA is synthesized, RNA primer is being degraded and replaced
by DNA (strand replacement synthesis).
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The Okazaki fragment
In prokaryotes, the leading and lagging strand DNA
replication machines are associated.
Okazaki fragments are the short DNA fragments produced
during lagging strand DNA synthesis. They will be ligated
together by ligase shortly after completion.
Prokaryotes like E. coli has Okazaki fragment of 1000~2000
nucleotides long while eukaryotes like us has shorter
Okazaki fragments (100~200 nucleotides)
DNA replication generally occurs bi-directionally form each origin
Mapping Using Electron-microscope
– Isolate partially replicated DNA
(replication intermediates). Enrich
using di-deoxynucleotides or density
labeling.
– Compare location of replication
bubble for a number of molecules (many)
– Orientation of DNA! Need reference
point. Usually a restriction site.
– Very small bubbles identify location
of origin.
– Movement of ends indicates number
of active forks.
Eukaryotic chromosomal DNA contain
mutiple replication origins separated by
tens to hundreds of kilobases.
ORC: origin recognition complex (6
subunits) combine with other factor (such
as hexameric helicases) to start replication.
8
1.
2.
3.
4.
5.
6.
7.
Using energy, ATP hydrolysis → single
DNA and bound RPA
Primase and pol α complex synthesis
short primer
PCNA-Rfc-Pol σ complex replace the
Primase and pol α complex → generate
leading strand
helicase unwind the parental strands,
and RPA bind to newly single strand
Coordination of the leading and lagging strand synthesis-5
PCNA-Rfc-Pol σ complex synthesis
DNA
Primase and pol α complex synthesis
short primer for lagging strand
PCNA-Rfc-Pol σ complex replace the
Primase and pol α complex → extend
the lagging strand –Okazaki fragments.
It eventually ligated to the 5’ends of the
leading strands.
Rfc: replication factor c
Mutation
1. Spontaneous errors in DNA replication (10 -7)
2. A consequence of the damaging effects of physical or
chemical mutagens on DNA
DNA repair and recombination
Several mechanisms can prevent it (repair system).
1. DNA polymerase proof reading
2. Base excision repair; T-G mismatch repair (one base)
3. Mismatch excision repairs (several base)
4. Nucleotide excision repair (transcription-coupled repair)
5. Recombination to repair double strand breaks in DNA
9
DNA polymerase introduce copying errors and also
correct them
DNA polymerase is the first line of defense in preventing mutation. It
can proofreading.
In E. coli about 1/104 happen, however, only about 1/109 nucleotides
incorporated into growing strand.
Proofreading depends on 3’-5’ exonuclease activity of some DNA
polymerase.
Uncorrected base-paring → polymerase stop → transfer 3’ end of
growing strand → to its exonuclease site → remove
E. Coli only has one type DNA polyermase, in eukaryotic DNA δ and
ε, used for proofreading activity.
I
Proofreading by DNA polymerase
Bind to single strand
template
3nm
Uncorrected base-pairing in 3’ end → melting of newly
formed end of the duplex→ polymerase stop →
transfer to exonuclease site (exo)
Polymerase
catalytic activity
Chemical and radiation damage to DNA can lead to mutations
In normal cell, many chemical reaction (lipid oxidation, peroxisome,
mitochondria…..) always work. Environmental factor always damage to DNA.
Many spontaneous mutations are point mutations.
Most frequent point mutation are deaminiation: cytosine (C) base convent to
uracil (U) base. Common modified is 5-methycytosine convent to thymine via
deamination.
Other environmental factor: UV, ionizing radiation…….
DNA undergoes damage
spontaneously from hydrolysis and
deamination > unnatural sites
Deamination:
C > U (pairing with A)
A > hypoxanthine (with C)
G > xanthine (with C)
5-mC > T
Depurination: common
Depyrimidination (脱嘧啶作用) : rare
10
Point mutation: a single base change
Depurination and deamination
Transition : Purine or pyrimidine is
replaced by the other
A↔G
T ↔C
Transversion : a purine is replaced by a
pyrimidine or vice versa
A ↔T or C
T ↔ A or G
G ↔T or C
C ↔ A or G
> Genetic polymorphism
Spontaneous Alterations of nucleotides
Red: oxidative damage; blue: hydrolytic attack; green:
uncontrolled methylation
High-Fidelity DNA excision-repair systems recognized and
repair damage
Excision-repair systems: high homologs of key bacteria protein exist in
eukaryotes; similar manner process: segment of the damaged DNA is
excised → gap → filled by DNA polymerase → ligase → repair ok
In normal, most common point mutation is C to T. using base excision
repair system can repair it. Other mutation such as C to U or 5-methyl
C to T also using the same system.
11
II
Base excision repair of a T.G mismatch
RECOGNIZED
Human cells contain a battery of
glycosylases, is specific for a
different set of chemically
modified DNA base.
DNA glycosylases
Base excision repair system
Base excision repair system works primarily on
modifications caused by endogenous agents
At least 8 DNA glcosylases are present in mammalian cells
DNA glycosylases remove mismatched or abnormal bases
cleaves N-glycosylic bond
AP endonuclease
Apurinic exdonuclease I
cleaves apurinic or
apyrimidinic site
AP endonuclease cleaves 5’ to AP site
AP lyase cleaves 3’ to AP site
DNA polymerase
3’→5’ exonuclease activity
& polymerase activity
Mismatch excision repairs other mismatches and
small insertions and deletions
III
Another DNA repair systems, is also
conserved from bacteria to human
Eliminates base-pair mismatches,
insertions or deletions or few
nucleotides that are accidentally
produced by DNA polymerase.
Mismatch excision repair: determining
normal and mutant DNA → repair
latter.
Bacteria
Eukaryotes
MutS
MSH1-6
MutL
MLH1, PMS1-2
Homolog to bacteria
MutS 2 and 6
Mismatch Repair in Human Cells
MSH2 and MSH6 bind to mismatchcontaining DNA and distinguish between
the template and newly synthesized strand
Homolog to
bacteria MutL
MLH1 nicks the newly synthesized
DNA and an exonuclease removes
the mismatched base
The gap is filled in by DNA polymerase
and DNA ligase
Defective mismatch repair is the primary
cause of certain types of human cancers
from Lodish et al., Molecular Cell Biology, 6th ed. Fig 4-37
12
VI
Nucleotide excision repairs chemical adducts that distort
normal DNA shape
When chemical modified bases →
nucleotide excision repair system
six core factors encompassing 15 to
18 polypeptide chains for excision,
plus repair synthesis and ligation
Certain protein →slide along double
stranded DNA → search bulges or
irregularities shape →
endonucleases activity → repair
Nucleotide excision repair also
called transcription-coupled repair
Nucleotide excision repair in
human cells
Shared subunits in transcription and
DNA repair at the same time.
DNA damage in higher eukaryotes is
repaired at a much faster rate in
regions of the genome being actively
transcribed than in nontranscribed
regions
Nucleotide Excision repair
enzymes cleave damaged DNA
on either side of the lesion
XP-G + RAP →
unwind and
distabilize
Transcription factor
endonuclease
24-32 base
endonuclease
Transcription-coupled repair:
nucleotide excision repair (NER)
system is capable of rescuing RNA
polymerase that has been arrested
by the presence of lesions in the
DNA template
13
Two systems utilize recombination to repair doublestrand breaks in DNA Emergency DNA Repair for Double helix break
Ionizating radiation or cancer drugs → double strand break →
nonhomologous end joining or homologous recombination → repair
Nonhomologous end joining
Ku and DNA-dependent protein kinase
1. Complex bind to break DNA end;
2. removal of a few base by
nucleases activity
3. ligate
Homologous recombination can repair DNA damage and
generate genetic diversity
Homologous recombination
DNA recombination: exchange of strands between separated
DNA molecules
Meiotic recombination
Recombination has DNA repair mechanism and generated
genetic diversity.
Generate genetic diversity
among the individuals of
a species by causing the
exchange of large regions
of chromosomes between
the maternal and paternal
pair of homologous
chromosomes during the
cellular division the
generates germ cells
Meiotic recombination
14
Repair of a collapsed replication fork
倒塌
RecA/Rad 51 catalyzed invasion of a duplex DNA by
a single stranded complement of one of the strands
is key to the recombination process. Moreover, no
base air are lost or gained in this process, called
strand invasion
If do not repair, generally
death to at least one
daughter cell
Formed double strand break
RecA and Rad51 are
homologous protein; It bind to
single strand DNA → →
created fork collapse →
invading 入侵 another single
strand → formed perfectly
complementary hybridization
Dark red is invading strand
ATP
Double strand DNA break repair by homologous
recombination
15
heteroduplex:
is any region of doublestranded nucleic acid (DNA,
RNA), where the two
strands come from two
different original molecules.
Mismatch repair system
If very complementary
Gene Conversion
16
Holliday Model of recombination: resolution 回覆
(1964)
Non-crossover
Crossover event
Enzyme vs. DNA replication
DNA replication is bi-direction
17
Prokaryote-Eukaryote Differences
Viruses: parasites of the cellular genetic system
Most viral host ranges are narrow
Viruses cant not reproduced by themselves (no life without host)
RNA virus: replicate in the host cell cytoplasm
DNA virus: replicate in the host cell nucleus
Viral genomes has single or double stranded
Virion: entire infectious virus particle, consists nuclei acid and shell of protein
Bacteriophage (phage): infect only bacteria
Head
Animal virus or plant virus
Tail
DNA
sheath
Tail
fiber
Physical Characteristics
Genetic Material
Nucleic acid
RNA (ssRNA, dsRNA, segmented)
DNA (ssDNA, dsDNA)
Protein coat (subunit structure)
Nucleoprotein
Capsid
Capsomeres, Geometrical constraints
Envelope (some)
80 × 225 nm
Figure 18.4d
VIRUS STRUCTURE
Basic rules of virus architecture, structure, and assembly are
the same for all families
Some structures are much more complex than others, and
require complex assembly and dissassembly
The capsid (coat) protein is the basic unit of structure;
functions that may be fulfilled by the capsid protein are to:
–
–
–
–
–
–
Protect viral nucleic acid
Interact specifically with the viral nucleic acid for packaging
Interact with vector for specific transmission
Interact with host receptors for entry to cell
Allow for release of nucleic acid upon entry into new cell
Assist in processes of viral and/or host gene regulation
50 nm
(d) Bacteriophage T4
Nucleoprotein must be stable but dissociatable
Capsid is held together by non-covalent, reversible bonds:
hydrophobic, salt, hydrogen bonds
Capsid is a polymer of identical subunits
Terms:
– Capsid = protein coat
– Structural unit = protein subunit
– Nucleocapsid = nucleic acid + protein
– Virion = virus particle
Capsid proteins are compactly folded proteins which:
– Fold only one way, and robustly
– Vary in size, generally 50-350 aa residues
– Have identifiable domains
– Can be described topologically; similar topological features do
not imply evolutionary relationships
18
Virus Structure –
1. Helical: single coat protein, tobacco mosaic virus
2. Icosahedron: 20 faces
There are two major structures of viruses called the naked
nucleocapsid virus and the enveloped virus.
Helical symmetry
A NAKED virus. The red balls represent the
protein subunits that make up the protective
covering around the viral genome (DNA in
the case). These subunits are called
CAPSOMERES and the entire protein coat is
called the CAPSID
An ENVELOPED virus. Enveloped viruses
have a lipid-based membrane surrounding
the protein capsid. This envelope is partly
composed of the cell membrane within which
the virus replicated, and it contains proteins
and carbohydrates. Some of the proteins are
from the host cell and some are from the virus
Coat protein
Tobacco mosaic virus is typical,
well-studied example
Each particle contains only a
single molecule of RNA
(6395 nucleotide residues)
and 2130 copies of the coat
protein subunit (158 amino
acid residues)
TMV protein subunits + nucleic
acid will self-assemble in
vitro in an energyindependent fashion
Self-assembly also occurs in the
absence of RNA Figure 18.4a, b
Capsomere
of capsid
RNA
Capsomere
DNA
Glycoprotein
18 × 250 mm
70–90 nm (diameter)
20 nm
(a) Tobacco mosaic virus
50 nm
(b) Adenoviruses
RNA
Function of the
capsid/envelope
Protect nucleic acid from the
host’s acid- and proteindigesting enzymes
Assist in binding and
penetrating host cell
Stimulate the host’s immune
system
小兒麻痺
Monkey DNA
Animal RNA
Plant RNA
二十面體
TMV
19
SV40 structure
Viral capsids are regular arrays of one or a few types of protein
Capsid (protein coat): nucleic acid of a virion is enclosed, composed of
multiple copies of one protein or few different protein.
Nucleocapsid: a capsid plus the enclosed nucleic acid, protect functions;
Two structure:
Envelope: some vriuses, symmetrically arranged nucleocapsid is covered
by an external membrane (envelope), which consists mainly of a
phospholipid but also contains one or two types of virus-encoded
glycoproteins.
Enable pleomorphic (多形性) shape of the virus
– Spherical (球形)
– Filamentous (絲形)
Viral protein spikes protrube
Influenza
20
Lytic viral growth cycles lead to death of host cell
E coil phage
dd DNA
1.Adsorption
2.Penetration
3.Replication
4.Assembly
5.Release
Plaque assay
Clone: all the progeny birions in a plaque are
derived from a single parental virus
Degrade the host cell
DNA → provide
nucleotides for
synthesis viral DNA
Plate → seeding host cell → virus add → infect host
cell → host cell lysis → plaque
Capsid &
assembly
protein
Lytic virus
Viruses vs. life cycle
Has envelope
Viral
Reproduction I
ssRNA
Bacteriophages
are viruses
that infect
bacteria. They
reproduce by:
子代
a) Lytic cycle
Viral RNA
polymerase
replicated
RNA
H+
Induced viral
glycoprotein
conformational
change
b) Lysogenic
cycle
Fusion of viral envelope
with endosomal lipid
bilayer membrane and
release of the nucleocapsid
into cytosol
21
Viral DNA is integrated into host cell genome in some nonlytic viral
growth cycles
Progeny (後代) virions of enveloped viruses are released by budding
from infected cells
Some viruses, nonlytic association with host cell (not kill) is called temperate phages
Prophage: integrated into the host cell chromosomes rather than being replicated
Lysogeny: Instead of destroying host to produce virus progeny, the
viral genome remains within the host cell and replicates
with the bacterial chromosome.
This relationship between phage and host is called
lysogeny
viral DNA is integrated into the hose cell genome in some nonlytic
viral growth cycles
Retroviruses
• Such as HIV, use the enzyme reverse transcriptase
– To copy their RNA genome into DNA, which can then be
integrated into the host genome as a provirus
Glycoprotein
The lysogenic cycle
– Replicates the phage genome without destroying the host
Temperate phages
– Are capable of using both the lytic and lysogenic cycles of reproduction
Prophage: integrated viral DNA
1.
Viral envelope
Capsid
2.
3.
Reverse
transcriptase
Figure 18.9
RNA
(two identical
strands)
Viral glycoprotein in
envelop interact with
specific hose cell
membrane → entry
nucleocapsid into
cytoplasm
Viral reverse
transcriptase and
protein → copy the
ssRNA to ds DNA→
ds DNA → transport
into the nucleus →
integrated HOST
chromosomal DNA →
leading to provirus
two
& translation
Retroviral life cycle
22
HIV
HIV
Membrane of
white blood cell
1 The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the
viral proteins and RNA.
2 Reverse transcriptase
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA.
HOST CELL
3 Reverse transcriptase
catalyzes the synthesis of
a second DNA strand
complementary to the first.
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
4 The double-stranded
DNA is incorporated
as a provirus into the cell’s
DNA.
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Chromosomal
DNA
end
Provirus
5
Proviral genes are
transcribed into RNA
molecules, which serve as
genomes for the next viral
generation and as mRNAs for
translation into viral proteins.
RNA genome
for the next
viral generation
mRNA
6
The viral proteins include cap
proteins and reverse transcriptas
(made in the cytosol) and envelo
glycoproteins (made in the ER).
Figure 18.10
9 New viruses bud
New HIV leaving a cell
off from the host cell.
8 Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules.
7 Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
23