VirusesChapter 19

Primitive - a word used by biologists to describe an organism group that has not changed significantly over time. The opposite of the term "primitive" is d__________.

Our inspection of life begins with the consideration of viruses - a genome fragment surrounded by a protein coat.

Resemblance to life forms by measure of the following criteria:

1. Their possession of nucleic acid (either DNA or RNA), 2. Their ability to reproduce, and 3. They act as agents of heredity and they are subject to

adaptation and evolution.

Viruses demonstrate adaptive radiation - the spread of a population into new and differing environments accompanied by adaptive evolutionary changes.

Nearly all living species are parasitized by viruses!!!!!!

Viruses are molecular, not cellular.

There is no place for viruses on the Phylogenetic Tree.

Viruses are often agents of disease (pathogens). Viruses must have evolved more recently than the first

life forms on our planet because they are obligate parasites.

The most current thinking in biology is that viruses are likely to be renegade strands of the original hosts' genome.

Viruses cannot grow, replicate on their own, metabolize, or move independently.

Viruses lack organelles (they don't even have ribosomes!) and all of the enzymes necessary for protein synthesis and metabolism.

Some forms do possess reverse transcriptidase and encode for other enzymes.

Virus Structure Tiny (smaller than bacteria) -

consist of at least TWO PARTS:

1. An outer coat of repeating protein subunits (a capsid).

2. An inner core of nucleic acid (either DNA or RNA)

3. (Sometimes) A few enzymes.

4. (Sometimes) An envelope around the capsid that resembles a plasma membrane.

Capsid - The capsid is the protein shell that encloses the nucleic acid; with its enclosed nucleic acid, it is called the nucleocapsid. This shell is composed of protein organized in subunits known as capsomers. They are closely associated with the nucleic acid and reflect its configuration, either a rod-shaped helix or a polygon-shaped sphere. The capsid has three functions: 1) it protects the nucleic acid from digestion by enzymes, 2) contains special sites on its surface that allow the virion to attach to a host cell, and 3) provides proteins that enable the virion to penetrate the host cell membrane and, in some cases, to inject the infectious nucleic acid into the cell's cytoplasm. Under the right conditions, viral RNA in a liquid suspension of protein molecules will self-assemble a capsid to become a functional and infectious virus.*

Envelope - Many types of virus have a glycoprotein envelope surrounding the nucleocapsid. The envelope is composed of two lipid layers interspersed with protein molecules (lipoprotein bilayer) and may contain material from the membrane of a host cell as well as that of viral origin. The virus obtains the lipid molecules from the cell membrane during the viral budding process. However, the virus replaces the proteins in the cell membrane with its own proteins, creating a hybrid structure of cell-derived lipids and virus-derived proteins. Many viruses also develop spikes made of glycoprotein on their envelopes that help them to attach to specific cell surfaces.*

Nucleic Acid - Just as in cells, the nucleic acid of each virus encodes the genetic information for the synthesis of all proteins. While the double-stranded DNA is responsible for this in prokaryotic and eukaryotic cells, only a few groups of viruses use DNA. Most viruses maintain all their genetic information with the single-stranded RNA. There are two types of RNA-based viruses. In most, the genomic RNA is termed a plus strand because it acts as messenger RNA for direct synthesis (translation) of viral protein. A few, however, have negative strands of RNA. In these cases, the virion has an enzyme, called RNAdependent RNA polymerase (transcriptase), which must first catalyze the production of complementary messenger RNA from the virion genomic RNA before viral protein synthesis can occur.*

Viral Replication Fundamental to your understanding of viruses is the

knowledge that viruses only reproduce inside of a host cell. That is to say, they are obligate intracellular parasites. Viruses have no metabolic machinery. They cannot

synthesize ATP.

Viruses have no ability to synthesize their own proteins. They lack ribosomes.

In their travels between hosts, viruses are simply aggregations of nucleic acids and proteins

Viruses are highly specific in their invasion of host cells. The cell type(s) invaded by a single type of virus is called the host range.

Host cell recognition is achieved by the union between a host cell receptor and a viral surface protein.

Viral infection of multicellular eukaryotes is usually tissue specific. HIV (Human Immunodeficiency Virus) invades only certain categories of Helper T cells. The cold virus only parasitizes cells in the upper respiratory tract.

Viruses function by entering host cell, using the host's protein-synthesizing machinery to make copies of itself, and then exiting the host, killing it upon exit.

Cell destruction by viruses is termed "lysis".

A viral infection begins when a virus binds to a host cell and the viral genome makes its way inside the host cell.

Once the viral nucleic acid gains entry, it “takes over” the cellʼs function by causing it to make viral proteins instead of host cell proteins.

The host cell provides free nucleotides (for new viral nucleic acid synthesis), enzymes, ribosomes, tRNA, amino acids (for new viral protein synthesis), and ATP.

Often, disassembly of the host cell DNA is part of the process.

Assembly of the newly-synthesized viral molecules into a virion is spontaneous – it occurs without host cell or viral regulation.

The number of virions assembled can range from hundreds to thousands.

Host cell destruction occurs upon the escape of the virions from the host cell.

All of the viral offspring (virions) then have the potential to invade neighboring host cells and repeat this process.

Lysogenic Cycle The end-result of viral invasion by the Lytic Cycle is cell

death. Cell death occurs by lysis – the host cell breaks open (lyses) and releases the virions that were made within the host cell. If this invasion targets bacteria, the virus is called a phage or, alternately, a bacteriophage. A phage that utilizes cell lysis is called a virulent phage

Life Cycle – Lytic Cycle {Lysis: to destroy a cell} 1. Attachment of virion to a bacterial cell. 2. Viral DNA enters host. (Time = 0 minutes)

2. Host DNA disintegrates. (Time = 1 minute) 3. Viral DNA replicates. (Time = 5 minutes) 3. Multiple copies of coat protein appear in host. (Time =

8 minutes)

4. Assemblage of DNA and coat protein. (Time = 13 minutes).

5. Followed by production of lysozyme, an enzyme that will digest the host's membrane and wall. (Time =15 minutes)

7. Bacterial cell lyses and complete virions exit from the host. Host cell dies. (Time = 30 minutes)

The Lysogenic CycleSometimes, the viral nucleic acid enters the cell and joins

with the host DNA (instead of destroying the cell immediately). The invading phage is called a temperate phage. This is latent viral DNA.

When the cell divides (by mitosis), viral DNA is replicated and passed from one cell generation to the next. Organism cells (for now) remain unaffected.

Inactivity is maintained by a repressor protein . If the repressor is inactivated (typically by a change in the cell's internal environment, exposure to radiation or chemical mutagens), transcription and translation of viral DNA begin, resulting in host cell destruction.

In contrast to the lytic cycle, the host cell does not immediately die (lyse) upon viral nucleic acid entry. Instead, the viral DNA is incorporated into the host cell’s genome. When integrated in this way, the viral DNA is known as a PROPHAGE.

One of the prophage genes encodes for a protein that prevents transcription of the other prophage genes. In this state, the viral DNA is silent. The cell reproduces normally, producing many cells that house the prophage.

The bacterial population containing the prophage grows very quickly.

The viral genome becomes active when it exits the genome and initiates a lytic cycle.

This activation is signaled by the environment – UV radiation, the presence of a certain chemical, a change in local conditions.

Entry And Replication In Animal Viruses Many animal viruses have an envelope and an RNA

genome. Animal viruses use the envelope to aid in entry into the

host cell.

Protruding from the animal virus envelope are glycoproteins that bind to specific receptors on host cell membranes.

The viral envelope fuses with the host cell membrane, and the capsid and viral genome enter by endocytosis.

The capsid is digested away, releasing the viral RNA into the interior of the cell.

Viral RNA synthesizes a complement of itself, and then uses the complement as a template for the synthesis of new viral RNA.

Complementary RNA strands also function as mRNA, which is translated into viral proteins.

The viral proteins and new viral RNA self-assemble into new viral particles.

The viral particles fuse with the animal cell membrane and exocytosis occurs

Plant Viruses Plants, like other life forms, suffer viruses. Typically spread by insects during feeding or pollination

or May be spread when the plant is injured

Viruses That Could Affect Your Life Herpes Simplex I -

cold sores, fever blisters; cyclic shifts from lysogenic to lytic. The lytic cycle can be triggered by stress, disease, exposure to UV.

Herpes Simplex II - genital herpes, attacks mucous membranes in the mouth and genitals, highly contagious, can readily affect another party. Permanent and incurable. 10 - 20 million Americans affected.

Viral Diseases of Man smallpox, chickenpox, mumps, measles, German

measles (rubella), E bola, mono, genital warts, influenza, colds, AIDS, infectious hepatitis, polio, rabies, warts, Herpes

HIV HIV - (Human

immunodeficiency virus) - the virus responsible for AIDS (Acquired Immune Deficiency Syndrome).

•Host cell is the lymphocyte (more specifically, a type of lymphocyte known as a helper T { or T4} - cell.)

HIV is a retrovirus, meaning that its nucleic acid is a double strand of RNA.

Its envelope unites with the plasma membrane of a T4 cell, allowing the 2 strands of RNA to enter.

The cell uses reverse transcriptidase to transcribe to DNA. The new DNA strand then synthesizes a complimentary strand. The now-doubly stranded DNA is woven into the host T4 cell's DNA. (HIV is a temperate virus)

The HIV genome remains inactive for awhile, replicating right along with the host chromosome.

Dormancy may remain for years When activated, HIV's DNA goes into a wild,

uncontrolled transcription to RNA and translation to proteins, glycoproteins, and reverse transcriptidase.

New HIV virions are assembled and break away from the cell, taking a bit of the T4 cell's plasma membrane with them (as their envelope).

The host cell dies from the numerous pores left in the cell membrane.

Secondary infections end the victim's life. T4 cells are necessary to activate killer T cells and other lymphocytes.

HIV is spread only through the sharing of body fluids. If the envelope is exposed to atmospheric oxygen, the envelope dries or certain components are oxidized, and the virion loses its ability to fuse with a new T4 cell.