Viruses are inert in the extracellular environment

They replicate only in living cells, being parasites at the genetic level

The viral nucleic acid contains information necessary for programming the infected host cell to synthesize virus-specific macromolecules required for the production of viral progeny (new virus particle)

Attachment and adsorption

Uncoating (viral genome is injected to the host nucleus/cytoplasm)

Viruses must synthesize mRNA by using the host cell mechanisms

RNA viruses produce mRNA by several different routes

04.Viral mRNA is then translated in the host cytoplasm to produce viral proteins

05.Replication of viral DNA or RNA occurs in the host nucleus except for pox viruses where it takes place in the cytoplasm

06.Assembly of the viral proteins and genome

During the replicative cycle, numerous copies of viral nucleic acid and coat proteins are produced

07.Release of the viral particles

This takes place on the host cell plasma membrane

In order for viruses to reproduce, they must infect a cell. Viruses are not technically alive: they are like a brain with no body. In order to make new viruses, they must hi-jack a cell, and use it to make new viruses. Just as your body is constantly making new skin cells, or new blood cells, each cell often makes new proteins in order to stay alive and to reproduce. Viruses hide their own DNA in the DNA of the cell, and then, when the cell tries to make new proteins, it accidentally makes new viruses as well. HIV mostly infects cells in the immune system

Several different kinds of cells have proteins on their surface that are called CD4 receptors. HIV searches for cells that have CD4 surface receptors, because this particular protein enables the virus to bind to the cell. Although HIV infects a variety of cells, its main target is the T4-lymphocyte (also called the “T-helper cell”), a kind of white blood cell that has lots of CD4 receptors. The T4-cell is responsible for warning your immune system that there are invaders in the system.Once HIV binds to a cell, it hides HIV DNA inside the cell’s DNA: this turns the cell into a sort of HIV factory and replicates itself.

A virus consists of an outer envelope of protein, fat and sugar wrapped around a set of genes (in the case of HIV, genetic information is carried as RNA instead of DNA) and special enzymes.HIV has proteins on its envelope that are strongly attracted to the CD4+ surface receptor on the outside of the T4-cell. When HIV binds to a CD4+ surface receptor, it activates other proteins on the cell’s surface, allowing the HIV envelope to fuse to the outside of the cell.Entry can be blocked by entry inhibitors.

HIV’s genes are carried in two strands of RNA, while the genetic material of human cells is found in DNA. In order for the virus to infect the cell, a process called “reverse transcription” makes a DNA copy of the virus’s RNA.After the binding process, the viral capsid (the inside of the virus which contains the RNA and important enzymes) is released into the host cell. A viral enzyme called reverse transcriptase makes a DNA copy of the RNA. This new DNA is called “proviral DNA.”Reverse transcription can be blocked by: Nucleoside Reverse Transcriptase Inhibitors (NRTIs), and Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs).

The HIV DNA is then carried to the cell’s nucleus (center), where the cell’s DNA is kept. Then, another viral enzyme called integrase hides the proviral DNA into the cell’s DNA. Then, when the cell tries to make new proteins, it can accidentally make new HIVs.Integration can be blocked by integrase inhibitors

Once HIV’s genetic material is inside the cell’s nucleus, it directs the cell to produce new HIV.The strands of viral DNA in the nucleus separate, and special enzymes create a complementary strand of genetic material called messenger RNA or mRNA (instructions for making new HIV).Transcription can be blocked by antisense antivirals or transcription inhibitors (TIs), new classes of drugs that are in the earliest stage of research

The mRNA carries instructions for making new viral proteins from the nucleus to a kind of workshop in the cell. Each section of the mRNA corresponds to a protein building block for making a part of HIV.As each mRNA strand is processed, a corresponding string of proteins is made. This process continues until the mRNA strand has been transformed or “translated” into new viral proteins needed to make a new virus.

The final step begins with the assembly of new virus. Long strings of proteins are cut up by a viral enzyme called protease into smaller proteins. These proteins serve a variety of functions; some become structural elements of new HIV, while others become enzymes, such as reverse transcriptase.Once the new viral particles are assembled, they bud off the host cell, and create a new virus. The virus then enters the maturation stage, which involves the processing of viral proteins. Maturation is the final step in the process and is required for the virus to become infectious.With viral assembly and maturation completed, the virus is able to infect new cells. Each infected cell can produce a lot of new viruses.Viral assembly can be blocked by Protease Inhibitors (PIs). Maturation, a new target of companies developing anti-HIV drugs, may be blocked using Maturation Inhibitors

Viruses are classified according to

Morphology, size and capsid symmetry

Nucleic acid type

Most RNA viruses have helical capsid symmetry and DNA viruses have icosahedral capsid symmetry

For viruses to replicate viral proteins must be synthesized by the host cell protein synthesizing machinery

All human viruses with helical nucleocapsid poses an envelope

What are the important steps involved during viral replication?

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