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The nature of viruses
Viruses are very small obligate parasites. This means that they cannotsurvive without taking over a host cell.
Viruses contain DNA or RNA ( nucleic acid) surrounded by a coat whichis usually protein.
After a virus invades a host cell it forces the cell to replicate viralDNA/RNA, make copies of its protein coat and self-assembles resultingin the release of large numbers of viruses.
Tobacco mosaic virus HIV Bacteriophage
Infects plants Infects Humans Infects Bacteria
DNA or RNA
Protein Coat
DNA or RNA
Protein Coat
Structure of Viruses
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Invasion of a cell by a virusDNA or RNA
Protein Coat
1. Viral DNA is
injected
2. Viral DNA
replicated
3. Viral mRNA for
protein coat
transcribed by host
4. Virus protein coats
translated by host ribosomes
5. Viruses self-assemble
6. Lysosome enzyme
used to help virus burstout of cell
Host
Cell
Nucleus with
host DNA
Host cell supplies virus with:
Nucleotides
Amino Acids
ATP Enzymes
Invasion of a cell by a virusDNA or RNA
Protein Coat
DNA or RNA
Protein Coat
1. Viral DNA is
injected
2. Viral DNA
replicated
3. Viral mRNA for
protein coat
transcribed by host
4. Virus protein coats
translated by host ribosomes
5. Viruses self-assemble
6. Lysosome enzyme
used to help virus burstout of cell
Host
Cell
Nucleus with
host DNA
Host cell supplies virus with:
Nucleotides
Amino Acids
ATP Enzymes
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Cellular response in defence in animals
Any invading foreign particles are known as antigen. These include bacteria,viruses, fungi and cells foreign to the body.
White bolld cells known as phagocytes engulf and digest antigen by the processofphagocytosis.
The antigen is enclosed in a vacuole within the cell
Lysosomes containing powerful protein digesting enzymes fuse with thevacuole
The antigen is digested and the breakdown compounds diffuse into cytoplasm
Lymphocytes are a second type of white blood cell that recognise foreignantigen and produce special proteins calledantibodies which bind with anddestroy the antigen.
The combined effects of phagocytes and lymphocytes is a particular problemduring tissue transplantation. The foreign tissue is treated as an antigen andrejected by the body. It is necessary to use
Donors which closely match tissue type of recipient
Use immuno-suppressors which reduce inhibit immune system.
TOP TIP - Try the data interpretation comparing primary and secondaryresponses to immunisation injections
Phagocytotic WBC
AntigenWBC engulfsantigen trappingit in a vacuole
Lysosomes containing
powerful protein digestingenzymes fuse with the vacuole
Enzymesreleased into the
vacuole
Antigen digestedDigested parts diffusesinto cytoplasm
Phagocytotic WBC
AntigenWBC engulfsantigen trappingit in a vacuole
Lysosomes containing
powerful protein digestingenzymes fuse with the vacuole
Enzymesreleased into the
vacuole
Antigen digestedDigested parts diffusesinto cytoplasm
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Cellular defence mechanisms in plants.
Plants dont have an immune system but must protect themselves from herbivores
Plants can protect themselves by producing a variety of toxic compounds
Tannins- These are acidic chemicals produced when a parasite such as insect orfungus penetrates the plant. They are produced inside swellings called galls whichhelp isolate the parasite
Cyanide Hydrogen cyanide is a poison which blocks a herbivores cytochromesystem and is produced by some plants (e.g. white clover) in response to tissuedamage. The production of hydrogen cyanide is known as cyanogenesis
Nicotine This is a poison produced by tobacco plants which over stimulates thecentral nervous system of herbivores
a) Plants can protect themselves by isolating injured areas using barrier substancessuch as
Resins These are glassy substances such as amber which exude from plants and
trees and help isolate possible routes by which the plant may be invaded e.g. xylemand phloem vessels
Lignin Lignin is normally part of the xylem vessels but if a vessel is invaded byfungi extra lignin is produced to clog the vessel completely
This photographs shows
swellings called galls which are
produced by the leaf after aninsect lays eggs on it. This
isolates the leaf from the egg.
When the insect hatches it eats
its way down towards the leaf
but gets a mouthful of acidic
tannins which help to stop it
getting any further.
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