During chain elongation each incoming aminoacyl- tRNA move through three ribosomal sites

Translation initiation usually occurs near the first AUG closest to the 5’ end of an mRNA

Initiation factor

eIF4A (helicase activity) →uses energy→ unwind RNA→complex move

Kozak sequence: ACCAUGG

Protein synthesis: methionly-tRNAi recognizes the AUG start codon

Initiator factor: IF

UTR

• 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-tRNA_i^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’ endof 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 aminoacyl- tRNA 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.

correctly base-pair → hydrolysis GTP → conformational change → tight bind aminoacyl-tRNA in A site and release EF1-GDP

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.

Peptide bond formation by large rRNA (peptidlytransferase reaction)

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

Translation is terminated by release factors when a stop codon is reached

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

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

Protein synthesis-4 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.

Protein-protein and protein –mRNA interactions form a bridge.

Purified poly(A)-binding protein I, eIF4E and eIF4G , mRNA formed a circular structure

The Synthesis of Protein

Polyribosomes = A cluster of ribosomes simultaneously translating an mRNA molecule

Polyribosomes are found in both prokaryotes and eukaryotes

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

Life cycle of an mRNA-2

DNA-directed DNA polymerases Semi-conservative

5’ – 3’

Replication forks

Uni- or Bi- directional

Semi-discontinuous

Primers

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

TopoisomeraseI: 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

removes RNA primer, which are replace with

DNA nucleotides by DNA polymerase

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

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-polσ complex process each

Okazaki fragment Replication protein A

Homotrimeric Heterotrimeric

protein, maintain the template in a uniform conformation for DNA polymerase

Proliferating cell nuclear antigen: prevent loss complex dissociating from DNA

Maintains the template in a uniform conformatio

starting nascent DNA chains

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.

Primases DNA helicases - Separation of the Watson/Crick helix

DNA helicases utilize energy of ATPhydrolysis to cause disruption of hydrogen bondsin 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

SSBsbind 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

DNA ligasesform phosphodiester bonds; join strands of DNA

Replication fork → move →supercoil

Role of Topoisomerases

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).

Type I- change L by multiples of 1 by causing a transient ssDNAbreak.

Type II- change L by multiples to 2 by causing a transient dsDNAbreak.

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.

Topoisomerases

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).

The Okazaki fragment

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)

In prokaryotes, the leading and lagging strand DNA replication machines are associated.

DNA replication generally occurs bi-directionally form each origin

– 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.

1. Using energy, ATP hydrolysis → single DNA and bound RPA

2. Primase and polα complex synthesis short primer

3. PCNA-Rfc-Polσ complex replace the Primase and polα complex → generate leading strand

4. helicase unwind the parental strands, and RPA bind to newly single strand 5. PCNA-Rfc-Polσ complex synthesis

DNA

6. Primase and polα complex synthesis short primer for lagging strand 7. PCNA-Rfc-Polσ complex replace the

Primase and polα complex → extend the lagging strand –Okazaki fragments.

It eventually ligated to the 5’ends of the leading strands.

Rfc: replication factor c

Coordination of the leading and lagging strand synthesis-5

DNA repair and recombination

Mutation

1. Spontaneous errors in DNA replication (10

)

2. A consequence of the damaging effects of physical or chemical mutagens on DNA

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

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 1/10

happen, however, only about 1/10

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.

Proofreading by DNA polymerase

Bind to single strand template

Polymerase catalytic activity

Uncorrected base-pairing in 3’ end → melting of newly formed end of the duplex→ polymerase stop → transfer to exonuclease site (exo)

3nm I

Chemical and radiation damage to DNA can lead to mutations

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

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 Point mutation: a single base change

> Genetic polymorphism

Depurination and deamination

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.

Base excision repair of a T.G mismatch

Apurinic exdonuclease I RECOGNIZED

Human cells contain a battery of glycosylases, is specific for a different set of chemically modified DNA base.

DNA glycosylases

cleaves N-glycosylic bond

AP endonuclease

DNA polymerase

& polymerase activity II

AP endonuclease cleaves 5’ to AP site AP lyase cleaves 3’ to AP site

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

Base excision repair system

Mismatch excision repairs other mismatches and

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.

Homolog to bacteria MutS 2 and 6

Homolog to bacteria MutL III

Bacteria Eukaryotes

MutS MSH1-6

MutL MLH1, PMS1-2

Mismatch Repair in Human Cells

Defective mismatch repair is the primary cause of certain types of human cancers MSH2 and MSH6 bind to mismatch- containing DNA and distinguish between the template and newly synthesized strand

from Lodish et al., Molecular Cell Biology, 6^thed. Fig 4-37

MLH1 nicks the newly synthesized DNA and an exonuclease removes the mismatched base

The gap is filled in by DNA polymerase and DNA ligase

Nucleotide excision repairs chemical adducts that distort normal DNA shape

VI

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 in human cells

Nucleotide Excision repair enzymes cleave damaged DNA on either side of the lesion

Transcription factor endonuclease

XP-G + RAP → unwind and distabilize

24-32 base

endonuclease 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

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

Nucleotide excision repair also

called transcription-coupled repair

Two systems utilize recombination to repair double- strand breaks in DNA

Ionizating radiation or cancer drugs → double strand break →

nonhomologous end joining or homologous recombination → repair

Emergency DNA Repair for Double helix break 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

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

DNA recombination: exchange of strands between separated DNA molecules

Meiotic recombination

Recombination has DNA repair mechanism and generated genetic diversity.

Homologous recombination

Repair of a collapsed replication fork

倒塌

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

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

Double strand DNA break repair by homologous

recombination

heteroduplex:

is any region of double- stranded nucleic acid (DNA, RNA), where the two strands come from two different original molecules.

Gene Conversion

Mismatch repair system

If very complementary

Non-crossover Crossover event

Holliday Model of recombination: resolution 回覆

(1964)

Enzyme vs. DNA replication DNA replication is bi-direction

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

Animal virus or plant virus

Physical Characteristics Genetic Material

Nucleic acid

RNA (ssRNA, dsRNA, segmented) DNA (ssDNA, dsDNA)

Protein coat (subunit structure) Nucleoprotein Capsid

Capsomeres, Geometrical constraints Envelope (some)

Figure 18.4d

80 × 225 nm

50 nm (d) Bacteriophage T4

DNA Head

Tail fiber Tail sheath

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

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

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

There are two major structures of viruses called the naked nucleocapsid virus and the enveloped virus.

Figure 18.4a, b

18 × 250 mm 70–90 nm (diameter)

20 nm 50 nm

(a) Tobacco mosaic virus (b) Adenoviruses RNA

DNA Capsomere

Glycoprotein Capsomere

of capsid

Virus Structure –

1. Helical: single coat protein, tobacco mosaic virus 2. Icosahedron: 20 faces

Helical symmetry

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 energy- independent fashion Self-assembly also occurs in the

absence of RNA

Coat protein RNA

TMV

小兒麻痺

二十面體

Function of the capsid/envelope

Protect nucleic acid from the host’s acid- and protein- digesting enzymes Assist in binding and penetrating host cell Stimulate the host’s immune

system

Animal RNA Plant RNA

Monkey DNA

SV40 structure SV40 structure

Viral capsids are regular arrays of one or a few types of protein

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

Plaque assay

Clone: all the progeny birions in a plaque are derived from a single parental virus

Plate → seeding host cell → virus add → infect host cell → host cell lysis → plaque

Lytic viral growth cycles lead to death of host cell

1.Adsorption 2.Penetration 3.Replication 4.Assembly 5.Release

dd DNA

Capsid &

assembly protein Degrade the host cell

DNA → provide nucleotides for synthesis viral DNA

E coil phage

Lytic virus

ssRNA

H+

Induced viral glycoprotein conformational change Fusion of viral envelope with endosomal lipid bilayer membrane and release of the nucleocapsid into cytosol

Viral RNA polymerase replicated RNA 子代

Has envelope

Viruses vs. life cycle

Viral Reproduction I Bacteriophages

are viruses that infect bacteria. They reproduce by:

a) Lytic cycle

b) Lysogenic cycle

Viral DNA is integrated into host cell genome in some nonlytic viral growth cycles

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

Progeny (後代) virions of enveloped viruses are released by budding from infected cells

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

Figure 18.9 Reverse transcriptase

Viral envelope

Capsid Glycoprotein

RNA (two identical strands)

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

Retroviral life cycle

1. Viral glycoprotein in two envelop interact with specific hose cell membrane → entry nucleocapsid into cytoplasm 2. Viral reverse

transcriptase and protein → copy the ssRNA to ds DNA→

3. ds DNA → transport into the nucleus → integrated HOST chromosomal DNA → leading to provirus

& translation

Figure 18.10

mRNA RNA genome

for the next viral generation

Viral RNA

RNA-DNA hybrid

DNA

Chromosomal DNA

NUCLEUS Provirus HOST CELL

Reverse transcriptase

New HIV leaving a cell HIV entering a cell

0.25 µm HIV Membrane of white blood cell

The virus fuses with the cell’s plasma membrane.

The capsid proteins are removed, releasing the viral proteins and RNA.

1

Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA.

2

Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first.

3

The double-stranded DNA is incorporated as a provirus into the cell’s DNA.

4

Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.

5

The viral proteins include cap proteins and reverse transcriptas (made in the cytosol) and envelo glycoproteins (made in the ER).

6

Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane.

Capsids are 7 assembled around viral genomes and reverse transcriptase molecules.

8

New viruses bud off from the host cell.

9

HIV

end