Powerpoint: February 22 Viruses
Keywords: Virus, head, tail, DNA, RNA, protein capsid/coat, lipid membrane (or membrane envelope), antigens, lytic cycle, lysogenic cycle,
Textbook Pages: 17-1
Learning Objectives
- Identify the structures of bacteriophages and retroviruses
- Describe the life cycle of (1) Lytic and (2) Lysogenic viruses
- Describe the body’s defence mechanisms against a viral/bacterial infection
What are Viruses?
Viruses are defined as “noncellular particles made up of genetic material and protein that can invade living cells”. They do not belong to any of the five kingdoms of organisms. It isn’t even clear whether viruses are living or non-living. While they do evolve in so much as only successful viruses can propagate their genetic information, they do not have cells, do not grow or reproduce, and do not use energy the way living things do. They also cannot live independently of cells, as they depend on cells to replicate.
Nevertheless, viruses are fascinating tiny machines of nature that have been used and studied for a variety of purposes, such as in genetic engineering, combatting cancer, and to destroy harmful bacteria in foods. Some viruses are also responsible for some of the most debilitating diseases known to man, including HIV (Human immunodeficiency virus) leading to AIDS, mono, shingles, herpes and chickenpox.
Identifying Viruses
Viruses come in many shapes, sizes and life cycles. There are two specific groups of viruses we will focus on here.
Bacteriophages |
Retroviruses |
Bacteriophages have a distinct shape and structure. There are two distinct parts: a head and a tail. The head is composed of a protein capsid or coat. It contains the DNA information. The tail and fibers at the base are used to attach the virus to bacteria, where it can then inject its genetic information into the cell. |
Retroviruses are named for their ability to insert their genetic material into the existing genetic information of the host cell. Normally DNA is “transcripted” to RNA (nucleic acid made of a single chain of nucleotides, in contrast to two strands in DNA). However, the genetic information in retroviruses are made up of RNA. When the RNA enters the host cell, enzymes attach the piece of RNA to the DNA of the cell. Complementary nucleotides then attach to the RNA, making it double stranded and indistinguishable from host cell genome.
Just like the head of bacteriophages, made of a protein capsid coat with genetic information inside, retroviruses a protein capsid with RNA. However, they are also surrounded by a lipid membrane (membrane envelope) and often antigens. Antigens are structures on the outside of the virus, toxins or proteins. The antigens act like the face of a virus, which can be recognized by the body and targeted by the body. |
DNA Replication
DNA do not techninically reproduce the way living organisms do. However, they do invade host cells, causing them to create more copies of viruses. There are two ways that this happens.
Lytic Cycle |
Lysogenic Cycle |
|
NOTE: Bacteriophages can also replicate through the lysogenic cycle. Retroviruses almost always replicate through the lysozenic cyccle.
1, Virus attaches itself to host cell |
Thankfully, the human body also has some ways of protecting ourselves against viruses, at different levels of specificity.
| Primary Line of Defense:
These forms of defense are non-specific, as in, it is to protect against any form of pathogen, virus, bacterial, protist, etc. |
– Skin
– Oil and Sweat – Hairs and cilia in mouth and nose – Stomach (acids) – Saliva, sweat and tears (lysozyme) |
| Secondary Line of Defense:
These forms of defense are also non-specific, but are generally activated only when pathogens have invaded. |
– Inflammatory response: white blood cells
– Fever |
| Tertiary Line of Defense
The most specific form of defense. These forms of defense are activated specifically to target the pathogen. This is usually done through antibodies that are produced based on the antigens on the surface of the pathogen. The antibodies can then target the pathogens. |
–Interferons : produced by cells to interfere with virus replication
– Antibody production: white blood cells produce antibodies |