Virus Notes

*Compare and contrast the structures and characteristics of viruses (lytic and lysogenic cycles) with non-living and living things*

Virus – a nonliving particle made up of a nucleic acid core and a protein coat

Bacteriophage ( phage) – virus that infects bacteria

Parts of a T₄  Phage

Capsid – protein coat

Capsomere – section of a capsid

Nucleic Core – either DNA or RNA but never both

Tail Fibers – protein fibers that attach the virus to the host cell

Head – contains the capsid and nucleic acid core

Tail – contains the tail fibers and sheath

Beijerinck – coined the term virus which literally means poisonous fluid

Wendall Stanley – isolated the first virus, the tobacco mosaic virus

Louis Pasteur – developed the rabies vaccine

Edward Jenner – developed the small pox vaccine

Jonas Salk – developed the polio vaccine

Steps of the Lytic Cycle

1. adsorption – phage attaches to the host cell with tail fibers
2. entry – phage injects viral nucleic acid (genetic material) into host cell

                   enzyme breaks down cell wall at receptor site

3. replication – viral nucleic directs host cell to make viruses
4. assembly – the parts of the virus are put together
5. release (lysis) – the host cell bursts releasing viruses

Virulent viruses enter the lytic cycle.  Virulent viruses cause disease immediately.

Steps of the Lysogenic Cycle 

1. adsorption – phage attaches to the host cell with tail fibers
2. entry – enzyme breaks down cell wall at receptor site

                   phage injects viral nucleic acid  (genetic material)  into host cell

3. prophage forms
4. prophage (viral nucleic acid) attaches to host DNA
5. prophage replicates with host DNA and host cell is unaffected
6. prophage becomes virulent and enters the lytic cycle
7. replication – viral nucleic acid directs host cell to make viruses
8. assembly – the parts of the virus are put together
9. release (lysis) – the host cell bursts releasing viruses

Temperate viruses enter the lysogenic cycle.  Temperate viruses do not cause disease immediately.

Compare viruses with living things:

• both have nucleic acids
• both have proteins
• both can reproduce in a living cell

Contrast viruses with living things:

• viruses do not carry out all the life processes such as metabolism, respiration, digestion, and homeostasis
• viruses must be in a living organism to reproduce
• viruses only have DNA or RNA but never both
• viruses are not composed of cells

Compare viruses with nonliving things:

• both do not carry out all the life processes
• both do not have both DNA and RNA
• both are not composed of cells

Contrast viruses with nonliving things:

·        viruses contain nucleic acids and proteins nonliving things do not

·        viruses carry out reproduction nonliving things do not

*Evaluate the medical and economic importance of viruses*

 

Viruses cause various human diseases such as measles, the common cold, the flu, polio, small pox, yellow fever, rabies and AIDS.  Retroviruses use the enzyme reverse transcriptase to make DNA from RNA.  HIV is a retrovirus. These diseases cause economic problems with missed work and insurance payments.  Viruses infect all organisms so viruses also cause economic problems with their effects on crops and animals. One positive side to viruses is that they are used to make the vaccines against them. Viruses also play a role in evolution by transferring genetic material between organisms and by killing many weaker organisms.