Chapter 18: Viruses

The Usual Transcription Process of Genetic Material:

A DNA strand is used to make a complementary strand of RNA.

Capsomer

A Sub-unit of protein that makes up the capsid.

Protein

A functional unit composed of one or more polypeptides. Each polypeptide is composed of a linear sequence of amino acids.

Episome:

A genetic element that can replicate independently of the chromosomal DNA but also can occasionally integrate into chromosomal DNA.

Polypeptide

A linear sequence of amino acids; the term denotes structure.

An Enzyme is:

A protein that acts as a catalyst to speed up a chemical reaction in a cell.

Reverse Transcriptase

A viral enzyme that is carried in the capsid and released into the host cell along with the viral RNA. Viruses are then able to use the viral RNA strand to make a complementary copy of a host DNA strand, and then the viral RNA uses the complementary copy of the host DNA strand, as a template to make double-stranded viral DNA The viral double-stranded DNA enters the host cell nucleus and is inserted into a host chromosome via ingrase. Viruses that follow this mechanism are called retro-viruses.

Viral Reproduction Step 3: Integration

A viral gene is expressed soon after entry so that ingrase enzyme is made. Ingrase cuts the host's chromosomal DNA and inserts the viral genome into the chromosome. This step may be skipped and the cycle would proceed from step 2 to step 4.

Viral Reproduction Step 6: Release

After the phages have been assembled, a phage-encoded enzyme called lysozyme digests the bacterial cell wall, causing the cell to burst. This event is known as cell lysis, or just lysis. Lysis releases new phages.

Lysogenic

After viruses integrate their genomes into a host chromosome the prophage, or provirus may become inactive, or latent. When this occurs both the prophage or the provirus, and the host cell are said to be latent.

Spike Glycoproteins:

Also called spikes or peplomers.

Retrovirus:

An RNA virus that utilizes reverse transcription to produce viral DNA that can be integrated into the host cell's genome.

Excisionase:

An enzyme needed to excise the prophage from the host chromosome.

Lysozome

An enzyme that digests the bacterial cell wall , causing the cell to burst (cell lysis).

Ingrase

An enzyme that splices the host chromosomal DNA and integrates the viral genome. This occurs during Step 3 of the Viral Reproduction Process:Integration.

Step 5 HIV Virus Assembly: Stage 1

Capsid proteins assemble around 2 molecules of HIV RNA and 2 molecules of reverse transcriptase.

Temperate Phage:

Certain types of viruses that may spend some of their time in the lysogenic cycle are known as bacteriophages.

Viral envelope:

Consists of a lipid bilayer that is derived from the plasma membrane of the host cell and is embedded with virally encoded spike glycoproteins that encloses the capsid.

What are the functions of non-caspid proteins?

Function as enzymes that modify capsid proteins, while others serve as scaffolding for the assembly of the capsid.

Step 4 HIV Component Synthesis:

In the case of HIV, the DNA provirus (viral DNA integrated into a eukaryotic cell's chromosomal DNA), is not excised from the host chromosome. It is transcribed in the nucleus to produce many copies of viral RNA. These copies enter the cytosol where they are used to make viral proteins and serve as the genome for new virus particles.

Provirus:

Integrated viral DNA in a eukaryotic cell.

Lysogeny:

Latency

Capsids are composed of:

One or several different protein subunits called capsomers.

Why is Attachment cell specific?

Proteins in the virus recognize and bind to specific molecules on the cell surface.

Viral Reproduction Step 5: Viral Assembly

Some simple viruses self-assemble; the viral components spontaneously bind to form a complete virus particle. Other viruses require the help of non-capsid proteins not found in the completed phage particle. Some of these non-capsid proteins function as enzymes that modify capsid proteins, while others serve as scaffolding for the assembly of the capsid.

Lytic Cycle:

Steps 1 Attachment, 2 Entry, 4 Synthesis of Viral Components, 5 Viral Assembly, 6 Release

Amino Acids

The building blocks of proteins. They have a common structure in which a carbon atom, called the alpha-carbon, is l;inked to an amino group (NH2), and a carboxyl group (COOH). The alpha-carbon is also linked to a hydrogen atom and a particular side chain.

Virulent Phage:

The genome of a virulent phage is not capable of integration with the host chromosome; they only have Lytic cylcles.

Step 5 HIV Virus Assembly: Stage 2

The newly formed capsid, of the HIV virus, acquires its outer envelope in a budding process. This stage occurs in Step 6-Release, as the virus is released from the cell.

Prophage

The phage DNA integrated in the bacterial chromosome.

Viral Reproduction Step 2: Entry

The phage injects its genetic material into the cytoplasm or the virus fuses with the plasma membrane, so that both the capsid and its contents are released into the cytosol.

Viral Reproduction Step 4: Synthesis of Viral Components

The prophage must be excised from the host's chromosomal DNA, before synthesis of new viral components can occur. Excisionase is required for this process. After excision, host cell enzymes copy the phage DNA and transcribe the genes into mRNA. Expression of phage genes leads to the degradation of the host DNA.

Budding:

The release of enveloped viruses from an animal cell does not lyse the cell. New virus particles (HIV) associate with the plasma membrane containing glycoproteins. The membrane of the host cell, enfolds the viral capsid, and the newly formed virus particle buds from the surface of the cell. This is how the new virus acquires its envelope, which is a piece of cell membrane studded with viral glycoproteins.

Viral Reproduction Step 1: Attachment

The virus attaches to the surface of the host cell; this is usually specific for one or a few types of cells.

Uncoating:

Virus capsid proteins are removed by the host cell enzymes, and in HIV a retro-virus; this releases 2 copies of viral RNA and 2 molecules of reverse transcriptase into the cytosol. This occurs in Step 2: Entry, of the viral reproduction cycle.

Bacteriophage or Phage

Virus that infects bacteria.