Life Cycle of Viruses

Function of the viral core

1. control the viral heredity

2. control viral variation

3. responsible for the infectivity

Function of the protein shell

1. Protect the viral nucleic acid

2. Participate in the viral infection

3. Share the antigenicity

susceptible

cell has a functional receptor for a given virus - the cell may or may not be able to support viral replication

resistant

cell has no receptor - it may or may not be competent to support viral replication

permissive

cell has the capacity to replicate virus - it may or may not be susceptible

susceptible and permissive

cell is only cell that can take up virus particle and replicate it

a virus is an organism with two phases

virion and infect living cell

life cycle of viruses

virologists divide the infectious cycle into steps to facilitate their study, but no such artificial boundaries occur

how does the viral genome enter the host cell

T even phages

phage life cycle

1. Attachment and Adsorption

2. Penetration

3. Uncoating

4. Synthesis of viral genomes and proteins

5. Release

when does transcription and translation take place

after uncoating

attachment

plasma membrane proteins and glycoproteins

entry

capsid enters by endocytosis or fusion

uncoating

enzymatic removal of capsid proteins

biosynthesis

in nucleus or cytoplasm

chronic infection

latency: slow viral infections; cancer

release

enveloped viruses bud out nonenveloped viruses rupture plasma membrane

One-step growth cycle (phage growth cycle)

a method to study virus reproduction in cells

studying the infectious cycle in cells

Ender, Weller, Robbins, propagate poliovirus in human cell culture in 1954

Bacteriophage

the viruses that infect bacteria

Ellis and Delbruck, 1939

studies on E.coli bacteriophages

Bacteriophage structure

1. Polyhedral head

2. Helical tail

Polyhedral head

consists of 2000 capsomeres with double-stranded DNA enclosed within

Helical tailt

inner hollow tube which is surrounded by a contractile sheath with 24 annular rings. the distal end consists of basal plate with tail fibers at each corner. the distal end consists of a basal plate with tail fibers at each corner. the bacteriophage attaches to the bacteria with the help of these tail fibers

two major lifecycle

1. lytic cycle

2. lysogenic cycle

Lytic Cycle

Infects a bacteria and kills it to release progeny virus

Lytic cycle steps

1. Attach and entry by injection of viral DNA

2. Synthesis of core and capsid

3. Assembly

4. Lysis of the host

Lysogenic Cycle

the phage becomes integrated with the chromosome of the host cell

Bacteriophage Cycle steps

1. Attach and entry by injection

2. Integrate with bacterial chromosome

3. Binary fission

4. Renters lytic phase

5. Synthesis of core and capsid

6. Assembly

7. Lysis of the host cell

Replication of Eukaryotic Viruses

1. Attachment

2. Entry

3. Direct Penetration for viruses

4. Membrane fusion for enveloped viruses

5. Endocytosis for naked viruses

6. Uncoating

7. Synthesis

8. Assembly

9. Release by bursting for naked viruses and by budding for enveloped viruses

Attachment (in Eukaryotic Viruses)

comes in contact with receptor sites on the surface of host cells and the glycoprotein spikes mediate attachment

Penetration (in Eukaryotic Viruses)

1. Direct

2. Fusion

3. Endocytosis

Uncoating and NA synthesis (in Eukaryotic Viruses)

the usual replication for most is (dsDNA)

Parvoviridae

DNA SS , cellular enzyme transcribes viral DNA in Nucleus

Heroesviridae, Papovaviridae

DNA DS, cellular enzyme transcribes viral DNA in nucleus

Poxviridae

DNA DS, Enzyme transcribes viral DNA in virion, in cytoplasm

Hepodnaviridae

DNA Reverse transcriptase, cellular enzyme transcribes viral DNA in nucleus: reverse tronscriptase copies mRNA to make viral DNA

Picornaviridae, Togaviridae

RNA + strand, viral RNA functions as a templates for synthesis of RNA polymerase which copies - strand RNA to make mRNA in the cytoplasm

Rhabdoviridae

RNA strand, Viral enzyme copies viral RNA to make mRNA in cytoplasm

Reoviridae

RNA DS, viral enzyme copies strands RNA to make mRNA in cytoplasm

Retroviridae

RNA reverse transcriptase, viral enzyme copies viral RNA to make DNA in cytoplasm: DNA moves to nucleus

Attachment

Penetration

Uncoating

Synthesis of viral genomes and proteins

Assembly

Release

Phage assembly

Life cycle of bacteriophage

Phage attaches to host cell and injects DNA

Phage DNA circularizes and enters lytic or lysogenic cycle

Phage DNA integrates within the bacterial chromosomes by recombination

the prophage may excise from the bacterial chromosome by another recombination event

Chorioallantoic membrane inoculation

Shell membrane

Amniotic inoculation

Yolk sac inoculation

Allantoic inoculation

Albumin

Primary Human foreskin fibroblasts

Mouse fibroblast cell line (3T3)

Human epithelial cell line (HeLa)

head

DNA /RNA

Collar

Sheaths

Base Plates

Pins

Lytic Cycle

Bacteriopghage Cycle

Lysogenic Cycle

Direct Penetration

Membrane fusion

Endocytosis

Cell Lysis

Budding