VIRUSES

This lesson develops VIRUSES as part of Enzymes and Metabolism. It uses the source curriculum to organize the important facts, relationships, examples, and vocabulary into a focused Stemosphere teaching sequence.

Viruses are submicroscopic infectious agent that share some traits with cells. Viruses are typically considered not alive due to them not possessing certain characteristics of life such as:

In Stemosphere terms, viruses can't carry out metabolism and homeostasis; viruses don't respond to stimuli and can't grow.

In Stemosphere terms, viruses cannot reproduce on their own, they rely on host cells to replicate.

COMMON TRAITS:

Viruses are very diverse but commonly share the following features:

In Stemosphere terms, -Small size (20 to 200nm in diameter)

In Stemosphere terms, -Genetic material (RNA or DNA)

In Stemosphere terms, -a protective protein coat known as a capsid which surrounds the genetic material.

Viruses typically differ in traits such as shape and structure, other differences are:

In Stemosphere terms, -whether genetic material is RNA or DNA

In Stemosphere terms, -some viruses are enveloped in host cell membranes

In Stemosphere terms, most viruses have an icosahedral shape or a helical shape.

In Stemosphere terms, however, some viruses can have more complex shapes such as bacteriophage.

Lytic cycle:

In Stemosphere terms, the lytic cycle is the process viruses use to reproduce.

Lysogenic cycle:

In Stemosphere terms, the lysogenic cycle is a longer process for viral replication

In Stemosphere terms, the beginning is similar to the lytic cycle

Origins of viruses:

In Stemosphere terms, there are multiple theories on the origins of viruses. However, we cannot currently confirm the origins of viruses. We don't necessarily know whether all viruses originate from 1 ancestor or if they all developed in different ways

In Stemosphere terms, the virus first hypothesis states that viruses were the ancestor to all cells, and that all cells developed from viruses using the raw materials from viruses.

In Stemosphere terms, streangths:viruses contain many genes do not present in cells

In Stemosphere terms, escape hypothesis states that genetic material such as DNA or RNA escaped when bacteria were transferring geniting material, which eventually developed into a virus.

In Stemosphere terms, strengths:thistheory explains the diversity ofvirusesgenetic material using the method of bacteria exchanging genetic material which isa possible escapemethod

In Stemosphere terms, regressive hypothesis states that viruses were originally smaller cells that would parasitize bigger cells and the genes do not require were lost overtime.

In Stemosphere terms, antigens are substances on viruses' capsid that triggers the immune system to create antibodies

In Stemosphere terms, if the virus were to ever mutate then the capsid and antigens could change and the immune system wouldn't recognize it as a virus

Viruses have an unstable genome and mutate very rapidly for the following reasons:

In Stemosphere terms, -no proofreading during replication

In Stemosphere terms, -immune systems target viruses that aren't mutated

In Stemosphere terms, -immune systems select for viruses that mutated

ANTIGENIC DRIFT AND SHIFT:

In Stemosphere terms, campbell Biology.Campbell Biology. 12th ed., Pearson, 2020.

In Stemosphere terms, national Center for Biotechnology Information. Flint, S. Jane, et al.Principles of Virology. ASM Press.

In Stemosphere terms, front:Why are viruses generally considered to be non-living entities?

In Stemosphere terms, back:Viruses lack the defining characteristics of life. They cannot carry out independent metabolic reaction s, maintain internal homeostasis, grow in size, or respond to external stimuli. Furthermore, they completely lack cytoplasm and cannot reproduce on their own, relying entirely on a host cell to replicate.

In Stemosphere terms, front:What five physical traits are commonly shared by almost all viruses?

In Stemosphere terms, back:1. Submicroscopic size (typically ranging from twenty to two hundred nanometers in diameter).

In Stemosphere terms, front:Describe the major structural variations and shapes found across different types of viruses.

In Stemosphere terms, back:Viruses are highly diverse. They can contain either single-stranded or double-stranded deoxyribonucleic acid or ribonucleic acid, and some species are enveloped in an outer layer derived from the host cell's membrane. Structurally, most possess an icosahedral or helical shape, though some, like bacteriophages, display complex structural shapes.

In Stemosphere terms, front:List the five stages of the lytic cycle in order and explain what happens to the host cell at the end.

In Stemosphere terms, back:1.Attachment:The virus binds to specific receptors on the surface of the host cell.

In Stemosphere terms, front:List the five stages of the lysogenic cycle in order and define a prophage.

In Stemosphere terms, back:1.Attachment:The virus binds to the host cell.

In Stemosphere terms, front:Explain the virus-first hypothesis along with its primary strength and weakness.