Viral Genetics

Phage T4 - invades e.coli

Viruses are rather simple. They usually consist of DNA or RNA packed into a protein head. They are incapable of many metabolic tasks by themselves and hence must invade a host cell in order to reproduce.

Virus Structure

I am sure all of you guys have seen those pictures of viruses which makes them look like martian spaceships, some of them indeed do look like that but their structure is vary varied.

They are rather small and range between 20 to 14000 nm in length

A virus structure can be split into two parts

1. Viral Nucleic Acid
2. The Protein Coat

Viral Nucleic Acid - It is very varied. It can be single-stranded, double-stranded, DNA or RNA. The genetic material can be found to be either linear or circular

Protein Coat - The main function of the protein coat or capsid is to protect the nucleic acid from the environment. The capsid is made up of repeating units called capsomeres which are proteins synthesized by the viral nucleic acid. Some viruses are proteceted by addition layers containing lipids and carbohydrates. The structure of the protein coat is coded in the viral DNA sequence and varies from virus to virus.

As we already said viruses can only reproduce within the host cell?

How do they recognize the host cell? The host cell has particular receptors which are recognized by the virus, which then allows it to enter the cell. The types of cell a virus can invade varies; some viruses can invade only one type of host cell while other types of viruses can invade a range of host cells.

A typical Viral Life cycle:

1. Recognize host cell

2. Enter host cell

3. Replicate using host cell machinery (nucleotides, ribosomes, tRNA, etc)

4. Self-assemble capside and pack genome into capsid head

5. Release itself from host cell (possibly killing the host cell)

Bacteriophages - are viruses that invade bacteria. Examples include T4 and lambda which infect E.coli.

T4 - multiplies by the lytic cycle which kills the host

Lambda - multiplies by the lysogenic cycle. It stays put as a prophage until due to some trigger enters the lytic cycle.

Lytic Cycle - We briefly mentioned the lytic cyle in the last section when talking about bacterial transduction. Now we are going to describe the steps in further detail. Nasty Stuff.

1. Complex fibers on the tail of T4 allows it to contact and bind the correct receptor on the surface of e.coli virus. The virus is only weakly adhered to bacteria.
2. The T4 releases an enzyme called lyzoszyme which breaks the cell wall at the place of injection and allows the T4 virus to inject its DNA into the bacteria. The empty capsid remains outside
3. Host protein synthesis is stopped and T4 uses host nucleotids to replicate its DNA
4. It uses the host protein machinery to synthesize capsid proteins and other important proteins
5. A trigger is sent out that the T4 is matured, which causes the spontaneous self-assembly of the DNA as well as capsid and the tail fibers
6. T4 virus releases lysoszyme which breaks the cell wall and allows the virus to leave the cell, in effect killing the cell.

Lysogenic Cycle

The lambda virus can enter either the lytic or lysogenic cycle, these types of phages are referred to as 'temperate phages'

1. When lambda enters the lysogenic cycle, its DNA combines with the host DNA via recombination

2. This is called a propahge

3. Genes called repressor proteins are synthesized by the bacteria which prevents the synthesis of most viral genes

4. A random event triggers the bacteria to enter the lytic cycle and break out of the cell. The prophage can remain as one for many generations, and is passed on each time the bacterial chromosome is replicated.

HIV - HIV is a unique virus is that its genetic material is a single-stranded RNA. These types of viruses are called retroviruses.

Once the HIV virus enters the cell, it activates an enzyme called reverse transcriptase.

This enzyme uses the RNA of the virus to synthesize complimentary double stranded DNA. The process of reverse transcription is very error-prone, hence there is a large degree of mutation which is why finding a cure for AIDS is so difficult.