Viruses

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.

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

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

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

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.

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

Stages of viral replication

VIRUS REPLICATION -- Summary

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.

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.

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.

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.

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.

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

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

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

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

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.

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