1. Viruses

2. Structure Tiny non-cellular structures
Have an outer protein coat called a capsid
In some the caspid is enclosed in a complex outer membrane
Inside is a nucleic acid: either DNA or RNA
Viral genes carry instructions to form new viruses
They don’t have ribosomes, mitochondrion, or other cytoplasmic organelles. They carry out no metabolic reactions & hence need a host to do this for them.

4. Viruses: Living or Dead?
They show features of living things but lack many others
It is thought that viruses represent pieces of nucleic acid that have escaped from other living cells.
They entered cells that were damaged and later evolved genes that allowed them to make protective covers to surround the nucleic acid

5. Viruses: Living or Dead
They are non-cellular and do not carry out metabolic reactions hence not in any of the 5 kingdoms
3 groups
Round shaped
Rod-shaped
complex in shape
Shapes of Viruses

7. Replication They are not cells so they cannot reproduce.
We say they replicate instead of reproduce.
They can only multiply by using the energy & structures of a live host cell.
They are said to be Obligate Parasites
A Bacteriophage is a virus that infects bacteria. They are the most complex and studied of all the viruses. Replication take about 30 minutes and is the way most viruses infect human cells

8. Stages of replication in Bacteriophage
Attachment
Virus attaches to host cell. Proteins from virus match receptors sites on the host cell.
Entry
Virus forms a hole into the host cell & viral nucleic acid is pushed through. The protein coat of a bacteriophage stays out side while other` protein of other viruses enter animal cells and are broken down.

9. Stages of Replication in Bacteriophage
Synthesis
The host nucleic acid is made inactive.
The viral nucleic acid uses the hosts organelles to produce new viral nucleci acid & proteins.
Assembly
New viruses are made inside host cell using the viral molecules that have been produced.
Release
The host cell bursts to release between 100 & 100,000 new viruses. Bursting of host cell is called Lysis

10. Stages of viral replication

11. VIRUS REPLICATION -- Summary

13. Retroviruses
Contain RNA (instead of DNA) and an enzyme that converts the virus RNA to DNA.
This DNA makes new copies of the virus RNA & new viruses inside the host cell.
The virus (HIV) that causes AIDS is a Retrovirus
Some viruses do not destroy the host DNA
Instead the viral DNA integrates with the host DNA & remains inactive.
When the host DNA is copied the viral DNA is copied and passed onto daughter cells.

14. Retroviruses
Diptheria, scarlet fever and botulism occur when a virus enters a bacterium in this manner. The virus causes the bacterium to release toxins which are responsible for the disease.

15. Medical & Economic Importance
Disadvantage
Human disease
Common cold, flu, polio, rabies, mumps, measles, chickenpox, warts, hepatitis, AIDS and some cancers
AIDS- (Acquired Immune Deficiency Syndrome) a person cannot make antibodies. This is due to infection with human immunodeficiency virus (HIV)
HIV enters body in fluids as blood & semen. Transmitted by sexual intercourse, infected blood products & shared needles.

16. AIDS
Effects
Once in the body, it enters a white blood cell & either remains dormant or disables the cell.
When the virus lays dormant, other WBC’s produce antibodies against it i.e. HIV. If they are present in a blood sample a person is said to be HIV positive. These antibodies do not disable the virus as it mutates too quickly.
It affects the Lymphocyte called helper T-cells this prevents a person from producing antibodies.
An AIDS suffer usually dies from a common infection they would normally fight of e.g. Pneumonia.
150 million people worldwide suffer with AIDS, growing by 20% each year.

17. AIDS Control and prevention No cure or vaccination
Prevention is vital in controlling the spread of the disease
Main method of prevention
Avoid sexual intercourse
Confine sex with 1 partner
Use a condom
Do not share needles
Avoid contact with blood
Do not donate blood if you are infected.

18. Disadvantage cont. Plant diseases Animal diseases
Tobacco mosaic disease
Potato and tomato mosaic disease
Animal diseases
Rabies Transferred by saliva.
Effects = choking, paralysis & death
Control = Quarantine
Foot & mouth Transferred on contact
Effects = Blisters in mouth & feet
Control = Destroy infected animals

19. Disadvantages of Viruses
Diseases of Humans, Plants and Animal
Human Diseases—common diseases, such as:
Measles
Mumps
Rubella
Cold
Warts

20. Plant Diseases
gain entry via a vector (carrier) such as insects
cause mosaic disease (striped patterns) in major crop plants.
Tobacco

21. Animal Diseases, such as :
Foot and mouth
SARS*
Rabies
*Sudden Acute Respiratory Syndrome

22. Benefits of Viruses
Genetic Engineering transfer genes from one organism to another in genetic engineering, such viruses called vectors
Control of infection Bacteriophage used to control bacterial infections. Help to reduce infections.

23. Advantages of Viruses
Genetic Engineering, where they are used inject a gene into target cells.
mosaic patterns give new variety to garden plants
Tulip