Microbiology

The promoter is a regions of DNA that facilitates the transcription of a particular gene. Promoters are typically located near the genes they regulate, on the same strand and upstream.

Early genes are genes that code for enzymes and regulatory proteins that are needed to start the viral replication process

Late genes are genes that encode for structural proteins, proteins needed for assembly of the mature virus. These genes are read after the expression of the early genes.

Type I viral replication refers to viruses whose genomes consists of a single strand of positive polarity RNA. This virus does not require any replicative enzymes because the positive polarity RNA acts as mRNA and can immediately be translated into proteins by the host cell.

Type II viral replication refers to viruses whose genome consists of a single strand of negative polarity RNA. This type of virus must posses an RNA polymerase within its capsid in order to be able to transcribe its complementary positive polarity RNA strand within the cytoplasm of the host. Once the positive polarity strand has been synthesized it can act as the mRNA and translation may begin.

Type III viral replication refers to a virus whose genome consists of a double strand of RNA. The two strands simply separate once they have been injected into the host cell. The positive strand goes on to make proteins and act as mRNA while the negative strand serves as a template for the positive strand.

Type IV viral replication always refers to a retrovirus. These viruses have a genome with a single stranded positive polarity RNA. Usually positive polarity RNA can immediately serve as mRNA, but in retroviruses it is injected into the cytoplasm along with reverse transcriptase which immediately acts to synthesize a negative polarity strand of DNA. Next a positive polarity DNA strand is synthesized and incorporated into the hosts DNA enzyme integrase. From there the DNA can be transcribed into mRNA and proteins can be made to construct new virions.

A retrovirus is a virus that undergoes Type IV replication. It contains a positive polarity single strand of RNA that under other circumstances would immediately be used as mRNA within the host cell but in a retrovirus is used to synthesize a negative polarity single strand of DNA. This is accomplished by the enzyme reverse transcriptase. Reverse transcriptase is an enzyme that is included in the capsid with the virus. Once the DNA is created it is incorporated into the hosts genomes where it will continually reproduce more viruses. an example is the HIV virus.

RNA-dependent RNA polymerase is an enzyme that produces RNA from RNA. This enzyme is found in virions. For viruses RNA-dependent RNA polymerase is packaged in the virion with the genome to allow for mRNA synthesis upon arrival in the hosts cell. The only type of virus that does not contain it in its capsid is a Type I replicating virus. It already has a positive polarity single strand of RNA that acts as mRNA.

Reverse transcriptase is an enzyme present in viral capsids of retroviruses. The function of the enzyme is to transcribe a negative polarity single strand DNA from the viruses positive polarity single strand of RNA.

Integrase is an enzyme that enables viral DNA to be integrated into its hosts DNA for further replication.

A provirus is a retrovirus that integrates its genome randomly into its hosts cell and is passed onto the hosts cells offspring. The virus may lay dormant for long periods until its stimulated by a particular environmental stimulus. It's also associates with oncogenes and triggering cancer.

Uncoating is the third step of viral replication. Once the virus has attached and penetrated the hosts cells, its capsid is degraded by viral or host enzymes. This degradation allows for the insertion of the viral RNA into the host.

Penetration is the second step of viral replication. It is the successful insertion of the whole virus (capsid) into the hosts cell. Penetration happens one of two ways: 1-Receptor mediated endocytosis which is when one of the glycoproteins of the virus is able to dock with a receptor of the host cell and trigger endocytosis (where the cell brings the virus into itself through and invagination of its plasma membrane). 2-Translocation is when the lipid bilayers of both the virus and the host cell fuse and allow the capsid to enter the host cell.

Receptor mediated endocytosis in viral replication is one way in which a virus can penetrate (penetration being step 2 of viral replication) its host cell. The process by which a virus is able to dock to a hosts cells receptors with one of its glycoproteins and trigger endocytosis. Endocytosis is the process by which cells "engulf" nutrients into themselves through the invagination process of their lipid bilayer. In this case the virus in engulfed into the host cell where it then can release its RNA.

Translocation in viral replication is a method of penetration (penetration is step two of viral replication). In this method the membranes of the virus and host cell fuse allowing the capsid to enter the host cell.

Adsorption

Penetration

Uncoating

Replication

Adsorption is the attachment of the virus to the host cell through receptors. It is the first step of viral replication. It occurs for both enveloped and naked viruses. For enveloped viruses it occurs via the glycoprotien spikes. For naked viruses it occurs through surface proteins.

• 1) Adsorption
• 2) Penetration
• 3) Uncoating
• 4) Replication
• 5) Assembly
• 6) Release

Assembly is the fifth stage of viral replication. It is when the proteins synthesized are put together to create the capsid and fill the capsid with the newly replicated RNA (be it + or - ss or ds) along with the appropriate enzymes to make the virion infectious.

Release is the 6th and final stage of viral replication. It is the final release of the fully assembled and infectious new virus into the hosts body. With naked viruses release usually occurs via cell lysis. With enveloped viruses release usually occurs via budding or exocytosis.