Download 2- Viruses

CHAPTER 18
2- Viruses
Page 328
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1- Viruses are much smaller than bacteria
2- Viruse is about 20nm in diameter
3- Viruses are not cells
4- A virus is a genome
enclosed in a
protective coat
Fig. 18.1, Page 329
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Virus is a genome enclosed in a protective coat
Genome (DNA)
Protein coat (capsid)
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Fig. 18.2a & b, Page 330
Viral Capsid and Envelope
A- Capsid
• A protein shell that encloses the viral genome.
• It is rode-shaped, helical, polyhedral or more complex.
• Capsomeres: Are the protein units that form capsid.
Capsomeres
(proteins)
Capsid
Membranous envelop
(viral envelope)
Sometimes further rapped ‫ يُغَلف‬in a membranous
envelope (Viral envelope), ex. Influenza virus.
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Page 329, 330
• Some viruses have viral
envelopes, membranes
cloaking their capsids.
• These envelopes are derived
from the membrane of the host
cell.
• They also have some viral
proteins and glycoproteins.
Fig. 18.2c, Page 330
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Viral Genome: (Hereditary material ‫)المادة الوراثية‬
– Viral genomes may consist of double-stranded
DNA, single-stranded DNA, double-stranded RNA,
or single-stranded RNA, depending on the specific
type of virus.
– The viral genome is usually organized as a single
linear or circular molecule of nucleic acid.
– The smallest viruses have only four genes, while
the largest have several hundred.
Page 328
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Key Characters Viruses
- DNA enclosed in a protein coat (membranous envelop)
- Can be crystallised
- Each type of virus infects a limited range of host cells
(host range)
- They lack enzymes for metabolism
- Have no ribosomes for making their own proteins
- Reproduced only within a living host cell (obligate parasitism).
Most viruses of eukaryotes attack specific tissues. Eg. Human cold
viruses infect only the cells lining the upper respiratory tract, and
The AIDS virus binds only to certain white blood cells.
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Virus-Reproduction within a living host cell
Living
Cell
obligate parasitism ‫تطفل إجبارى‬
1-Inter the cell and releases DNA
2- Replication using host
nucleotides and enzymes
Viral RNA
3- Production of new capsids
proteins using host cell resources.
4- The new viral DNA and proteins
assemble to form new viruses
Page 331 (Fig. 18.3)
Capsid
Proteins
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• A viral infection begins when
the genome of the virus enters
the host cell.
• Once inside, the viral genome
commandeers its host,
reprogramming the cell to copy
viral nucleic acid and
manufacture proteins from the
viral genome.
• The nucleic acid molecules and
capsomeres then self-assemble
into viral particles and exit the
cell.
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Fig. 18.3, Page 331
Bacteria infecting Viruses
• Viruses that infect bacteria,
called
bacteriophages
or
phages.
• It has a 20-sided capsid-head
that encloses their DNA and
protein tail piece that attaches
the phage to the host and
injects the phage DNA inside.
• Phages reproduce by Lytic
(‫ ) تحـللية‬Cycle and/or Lysogenic
(‫ )غير تحـللية‬cycle.
Fig. 18.2d Page 330
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Bacteriophages (on E. coli)
Phages reproductive cycles within bacteria:
Virus
Bacteria
1)- lytic cycle (‫ )الدورة التحـللية‬,
the phage reproductive cycle results in the death of the host.
– In the last stage, the bacterium lyses (breaks open) and releases
the phages produced within the cell to infect others.
• Virulent phages reproduce only by a lytic cycle.
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Phage T4
(virulent viruses)
‫فيروس مميت‬
Fig. 18.4, Page 332
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2)- lysogenic cycle (‫)الدورة غير التحـللية‬
• The phage genome replicates without
destroying the host cell.
• Temperate phages, like phage lambda (‫)ג‬,
may use both lytic and lysogenic cycles.
Temperate
virus
‫فـيــروس‬
‫غير مميت‬
• Within the host, the virus’ circular
DNA engages in either the lytic or lysogenic cycle.
• During a lytic cycle, the viral gene immediately turn the
host cell into a virus-producing factory, and the cell
soon lyses and releases its viral products.
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2)- Lysogenic cycle (‫)الدورة غير التحـللية‬
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Fig. 18.5, Page 333
Phage T4
Many tail fibre
(virulent viruses)
Phage lambda (‫)ג‬
Only 1 tail fibre
Temperate virus
‫فيروس مميت‬
‫فيروس غير مميت‬
lytic cycle
(‫)الدورة التحـللية‬
Lysogenic cycle
(‫)الدورة غير التحـللية‬
&
CD: Chapter 18
lytic cycle
(‫)الدورة التحـللية‬
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B- Eukaryotic Cell
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Internal membranes ‫ أغشية داخلية‬compartmentalize
‫ تحدد أعضاء وظيفية‬the functions of a eukaryotic cell
•
An eukaryotic cell has internal membranes, which partition ‫ تـُقـَســم‬the cell
into compartments ‫أعضاء وظيفية‬.
•
These membranes also participate in metabolism as many enzymes are
built into membranes.
•
The general structure of a biological membrane is a double layer ‫ثنائى الطبقات‬
of phospholipids and diverse proteins ‫بروتينات متنوعة‬.
•
Each type of membrane has a unique combination ‫ تركيب مـُمـَيـٍز‬of lipids and
proteins for its specific functions.
– For example, those in the membranes of mitochondria function in
cellular respiration.
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Page 114
B- Eukaryotic Cell
Eu: True
Karyon: Nucleus
Animal Cell
Plant Cell
What are the functions of cell organelles ?
Compare between Animal and Plant cell?
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Page 114 - 115
‫المادة الوراثية‬
‫النواة‬
‫الشبكة اإلندوبالزمية‬
‫نوية‬
‫الجدار النووى‬
‫سوط حركى‬
‫جسم مركزى‬
‫ريبوسوم‬
‫حهاز جولـﭽـى‬
‫غشاء بالزمى‬
‫حلمات دقيقة‬
‫ميتوكوندريا‬
‫جسم ُمحلل‬
‫الهيكل الخلوى‬
‫‪19‬‬
‫‪Fig. 7.7, Page 114‬‬
‫فجوة مركزية‬
‫بالستيدة خضراء‬
‫الجدار الخلوى‬
‫ثقوب بينية‬
‫‪20‬‬
‫‪Fig. 7.8, Page 115‬‬
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