Replication & Expression

Alight, now that the uncoating is done, whaat's next? Replication!

What is replication?



Replication involves assembly of viral proteins and genetic materials produced in the host cell.


Replication of nucleic acid. Replication of viral nucleic acid is a complex and variable process. The specific process depends
on the nucleic acid type.



NOTE: Symmetrical transcription of DNA gives rise to double-stranded RNA.

DNA virus replication -with the exception of the poxviruses, all DNA viruses replicate in the nucleus. In some cases one of the DNA strands is transcribed (in others both strands of a small part of the DNA may be transcribed) (step 4) into specific mRNA, which in turn is translated (step 5) to synthesize virus-specific proteins such as tumor antigen and enzymes necessary for biosynthesis of virus DNA. This period encompasses the early virus functions. Host cell DNA synthesis is temporarily elevated and is then suppressed as the cell shifts over to the manufacture of viral DNA (step 6). As the viral DNA continues to be transcribed, late virus functions become apparent. Messenger RNA transcribed during the later phase of infection (step 6) migrates to the cytoplasm and is translated (step 7). Proteins for virus capsids are synthesized and are transported to the nucleus to be incorporated into the complete virion (step 8).

Assembly of the protein subunits around the viral DNA results in the formation of complete virions (step 9), which are released after cell lysis.
The single-stranded DNA viruses first form a double stranded DNA, utilizing a host DNA-dependent DNA polymerase. They then undergo a typical replication cycle.


RNA virus replication -with the exception of the orthomyxoviruses and retroviruses, all RNA viruses replicate in the cytoplasm of the host cell. The exact process varies with the species of virus. The single-stranded RNA that is released after uncoating will act as either: (a) the mRNA to synthesize viral-coded proteins; or (b) a template to synthesize mRNA; or (c) a template to synthesize double stranded RNA, which is then used as a template to synthesize mRNA; or (d) a template to synthesize double-stranded DNA, which is then utilized as a template to synthesize mRNA. This latter process occurs only with the retroviruses (oncornaviruses).
The replication of poliovirus, which contains a single-stranded RNA as its genome, provides a useful example. All of the steps are independent of host DNA and occur in the cell cytoplasm. Polioviruses absorb to cells at specific cell receptor sites (step 1) , losing in the process one virus polypeptide. The sites are specific for virus coat-cell interactions. After attachment, the virus particles are taken into the cell by viropexis (similar to pinocytosis) (step 2) , and the viral RNA is uncoated (step 3) . The single-stranded RNA then serves as its own messenger RNA. This messenger RNA is translated (step 4), resulting in the formation of an RNA-dependent RNA polymerase that catalyzes the production of a replication intermediate (RI), a partially double-stranded molecule consisting of a complete RNA strand and numerous partially completed strands (step 5). At the same time, inhibitors of cellular RNA and protein synthesis are produced. Synthesis of (+) and (-) strands of RNA occurs by similar mechanisms. The RI consists of one complete (-) strand and many small pieces of newly synthesized (+) strand RNA (step 6). The replicative form (RF) consists of two complete RNA strands, one (+) and one (-).

The single (+) strand RNA is made in large amounts and may perform any one of three functions: (a) serve as messenger RNA for synthesis of structural proteins; b) serve as template for continued RNA replication; or (c) become encapsulated, resulting in mature progeny virions. The synthesis of viral capsid proteins (step 7) is initiated at about the same time as RNA synthesis.

The entire poliovirus genome acts as its own mRNA, forming a polysome of approximately 350S, and is translated to form a single large polypeptide that is subsequently cleaved to produce the various viral capsid polypeptides. Thus, the poliovirus genome serves as a polycistronic messenger molecule. Poliovirus contains four polypeptides.

Click this website and take a look at the animations to get a better picture of about virus replication.

Http://biology.about.com/od/virology/a/aa11108a.htm

Uncoating

Uncoating : Uncoating occurs simultaneously with or rapidly after penetration. Hence, this stage is difficult to be studied and remains a relatively poorly understood stage of the replication cycle.

Uncoating in general refers

o the events that expose the viral genome to the host cellular machinery and sets the stage for the viral genome to express its functions required for the replication.

This stage occurs simultaneously with or rapidly after penetration. In order to express the viral genome to the cell organelles, it is necessary that the virion coat

e removed partially or completely. Therefore, once virions are in the cytoplasm, they are generally uncoated to some extent by a variety of processes, including simple dissociation and/or enzyme-mediated partial degradation of the particles, to release the viral geno

me as a naked nucleic acid or as a nucleoprotein complex.

The steps involved in the process of disintegration of the protein coat or capsid to release its genome into the cell is called as uncoating. Uncoating may be achieved by the complete or partial removal of the capsid.

Penetration

The next step in the virus replication cycle after the attachment is penetration. Penetration is the process by which the virus goes into the plasma membrane and prepare for is uncoating. Penetration of eukaryotic cells occurs in three ways.

The first way is the Receptor-mediated (enveloped-virus). It is a process by which cells internalize molecules by the inward budding cell membrane vesicles containing proteins with receptor sites specific to the molecules being internalize.

The virus binds to its specific receptor and goes into the plasma membrane.

The function of the receptor-mediated endocytosis is used for the specific uptake of certain substance required by cell.

Clathrin is another way for the penetration. Though both receptor mediated and clathrin are almost the same, clathrin usually occur for naked viruses whereas receptor mediated are for enveloped viruses.

The last one would be the fusion of enveloped virus. It is said to be a direct penetration method.

I'll end here for penetration. Enjoy!

Attachment

When we talk about virus-host interaction, one thing comes into our mind is the General Virus Replication Cycle(GVRC). GVRC is a cycle that gives us a brief information about how the virus would infect a host and whats during this infection. Through this cycle, we can understand how viruses infect humans and how it could spread to other host.

A diagram of a General Virus Replication Cycle

Today, we would be talking about the first step of the General Virus Replication Cycle(GVRC): Attachment.

"A Virus requires a host in order for it to infect or duplicate".

Attachment only happens when the attachment site, in this case the virus, binds itself to the receptor site, the host. A receptor is needed in order for the attachment to be successful. Receptors may be, proteins, glycoprotein, glycolipid, CD4 (for HIV), CR2 also known as, complement receptor 2 (for Epstein-Barr Virus), Glycophorin A (for Influenza A), ICAM-1 (for Rhino Virus) and many other more.

If the acceptor or receptor lacks the necessary components needed, it won't bind till it finds its necessary component.. Hence, this allows the host to be inherently be resistant to that particular virus. For example, mice lack receptors for polio viruses and thus are resistant to polio virus. Similarly, humans are inherently resistant to plant and many animal viruses.

I'll end my attachment of virus part here. Stay in tune for the next part, which is the penetration part.

Regards,
Faradila
Karen
Khairul
Shobana
Thiviyan

Introduction to Viruses

One fine day, the teacher wrote "Virus" on the whiteboard. Suddenly...

Student: Teacher, what virus are you talking about? Computer Virus?

*Everyone in the class laughed*

Teacher: No my child. This virus that i'm talking about is not a computer virus. Its just your normal virus that causes tons and tons of infection across the world. From your recent H1N1 to the SARS outbreak.

All the students: Ohhhhhhh~

Student: So what's a virus?

A very good question indeed by one of the students. So what in the world is a virus? A virus, as we all know infects a human which causes its body system to weaken or malfunction. It is, in other words, a small infectious agent that can only replicates upon attaching itself to a host; the human.

How a virus would normally look like.

The discovery of virus was by a Dutch Microbiologist and Botanist, Martinus Willem Beijerinck. It was just a normal day for Martinus, doing his filtration experiment in 1898 on tobacco mosaic disease. He was trying to prove that there is an agent smaller than a bacteria. Hence, he named that new pathogen, virus. The proper term for studying viruses is called Virology.

Through his findings, we can clearly understand that viruses are smaller than bacteria.

So what's in/on the virus? Well basically, what you can find in a virus are 3 important things.

• DNA or RNA
• Long molecules that contains genetic information
• Protein coat that protects the genetics or an envelop of fats that surrounds the virus when outside a cell.

Without this 3 things, infecting a human is like injecting more blood into a human; nothing will happen.

As we all know, a virus requires a host in order for it to replicate itself. But we don't know what happens before, during or after this replication. As we go along, we will understand further what happens to the virus itself. We would also talked about examples of recent virus discoveries.

Till then, stay in tune.

Regards,

Faradilla

Karen

Khairul

Shobana

Thiviyan

Introduction

Hi and welcome to Microvirusey@blogspot.com.

In this blog, we will be talking about viruses and how it infects and affects homo sapiens like us. We hope that you would enjoy reading this blog as we will try our best not to bore you guys from reading it.

There will be words, pictures and also a definition of each difficult so as to allow you all to further understand the meaning!

Having an enjoyable time reading!