Download Chapter 5. Viruses (at the threshold of life)

Chapter 5. Viruses (at the threshold of life)
1. Discovery and Structure of viruses
What is virus ?
- virus : living things or not ? -> “organules” “molechisms”
- Golden age of microbiology (1880 - 1915)
-> increased understanding of infectious disease (tuberculosis, typhoid fever..)
-> industrial utilization of microbes
-> importance of microbes in environments
Yet unknown causation of some disease (measles, chickenpox, polio, hepatitis)
“ i ” was used
“virus”
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f tto unseen or unknown
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id
identity
tit ffor iinfectious
f ti
di
disease
- bacterium for tuberculosis
- protozoan for malaria
caused
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by virus
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?
The true nature of virus remained a mystery
Chapter 5. Viruses (at the threshold of life)
1) Development of virology
- 1892, Dmitri Iwanowski :
-> attempting
p g to isolate bacterial cause of tobacco mosaic disease
-> use filter to trap smallest known bacteria
-> filtered juice contained infectious agent -> called filterable virus
- 1898,, Martinus Beijerinck
j
-> diluted filtered juice -> still infectious -> called contagious living fluid
- 1930s, Wendell M. Stanley : crystalization of tobacco mosaic virus
->
> implies that TMV is chemical molecule
-> viral disease caused fever, a response by immune system or other symptom
- 1933, electron microscope 발명 -> TMV 전자사진
Chapter 5. Viruses (at the threshold of life)
2) Structure of viruses
- size : 27nm(poliovirus) ~ 250nm(smallpox virus)
- 500 or more viruses fit inside a single
g bacterial cell
Chapter 5. Viruses (at the threshold of life)
2) Structure of viruses
- shape : helical, icosahedral, complex type
HIV
Chapter 5. Viruses (at the threshold of life)
3) Component of viruses
- genome : core of nucleic acid
-> DNA or RNA, not both
-> long and helical, or folded
-> single molecule, occasionally segmented
- caps
capsid : coat of p
protein -> consisted of capsom
capsomeres
r s
- nucleocapsid : combination of genome and capsid
- envelope : membrane-like envelope enclosing nucleocapsid
- no cytoplasm
☞ spikes : projections on the envelope
-> contacting virus to its host
-> assist penetration into host cell
Chapter 5. Viruses (at the threshold of life)
Chapter 5. Viruses (at the threshold of life)
☞ 생물체로서의 미생물의 특징
- 성장을 위한 영양분의 흡수, 소화
- 소화산물의 배설
- 독립적 생식능력
생물계 구성원의 공통특징
- 환경변화에 대한 적응능력
- 자극에 대한 반응능력
☞ virus의 생화학적 특징
- no chemistry going on within a virus
- no intake of nutrient
- no production of waste
- no increase or decrease in size
- no metabolism
- but, they replicate
-> p
programs
g
a host cell biochemicallyy to p
produce hundreds of copies
p
Chapter 5. Viruses (at the threshold of life)
2 Viral replication
2.
- virus : inert particle outside a host cell
-> becomes highly efficient replicating machine in the host cell
-> utilize metabolism of the cell
-> produces multiple copies of itself in several stages
1) Stages of replication
(1) attachment stage
- high degree of specificity for a certain host
- 1. protein molecule in envelope keys off receptor sites(lock) in the host cell
- 2. or viral tail attaches to receptor sites
- 3. or blending of viral envelope with host cell membrane
- 4. or by phagocytosis
X 200,000
Viral attachmnet
X 70,000
Chapter 5. Viruses (at the threshold of life)
(2) penetration stage
- nonenveloped virus is left outside the host cell, the genome passes into the cell
(3) uncoating stage
- within the cell, cellular protein removes the protein capsid from the viral genome
(4) synthesis stage
- viral proteins are synthesized from viral genome using cellular machinery
(5) assembly stage
- viral parts are combined to form new viral particles after synthesis is completed
(6) release stage
- viral envelope is built from cellular membrane
-> disrupt cell membrane => lytic cycle
Replication
p
of Enveloped
p
virus
Clickk to view
Cli
i
animation.
animation
Unenveloped virus
Chapter 5. Viruses (at the threshold of life)
2) L
Lytic
tic and llysogenic
s enic c
cycle
cle
- some times, virus enters a host cell but do not replicate immediately =>lysogeny
- lysogenic
y g
cycle
y
:
-> virus incorporates its genes into the host genes and becomes part of it
-> multiply together with host genes => becomes provirus(oncogene)
Chapter 5. Viruses (at the threshold of life)
Lytic and lysogenic cycle
Chapter 5. Viruses (at the threshold of life)
Lysogenic and lytic cycle
Clickk to view
Cli
i
animation.
Clickk to view
Cli
i
animation.
animation
Chapter 5. Viruses (at the threshold of life)
3) Viriods and prions
- Viriods : single-stranded molecules of RNA without protein coat
-> infect plants causing disease
- Prions : proteineous infectious particle
-> composed solely of protein, extremely stable
-> transformed from prion protein(PrP, harmless protein in itself in brain cell)
-> PrP : protect cells by helping rid of dangerous chemicals
-> prion : clump together -> block molecular traffic
3 Viruses and Cancer
3.
- Cancer : results from uncontrolled reproduction of cells
-> grows into abnormal, functionless mass of cells => tumor
- body’s response to the tumor
-> surrounding it with a capsule of connective tissue => benign
-> outgrowth and break out of the capsule and spread => malignant
- Properties of cancer cell differing from normal cells
-> more rapid growth than normal cell
-> stick together less firmly than...
-> undergo dedifferentiation (reverting to an early stage to divide indefinitely
-> fail to exhibit contact inhibition -> not stop growing when contacted
Chapter 5. Viruses (at the threshold of life)
Contact inhibition
Chapter 5. Viruses (at the threshold of life)
☞ How viruses transform cells
- oncogene : transforming genes normally reside in the chromosomal DNA of cells
-> not viral in origin but part of the genetic endowment
->
> over 60 oncogenes were identified
- proto-oncogene : fore-runners of oncogenes
-> can be converted to oncogenes by carcinogens including viruses