Question

Differences between viruses that infect eukaryotes and prokaryotes. Describe the genome classification of viruses that infect eukaryotes according to their structure.

Answer 1

infections of viruses are said to be depend on two things

1 types of protien structure on surfaces of the viruses( the key)

2 types of sultures receptors on the surface of the host cell( the lock)

viruses that infect bacteria (PROKARYOTES CELLS) THROUGH BACTERIOPHAGES.

viruses that infect animals (EUKARYOTIC CELLS ) THROUGH ENDOCYTOSIS.

Viruses infect Eukaryotes
The process of endocytosis

EndocytosisVesicles (membrane-bound sacs) are formed around the viral particles after they penetrate the cell membrane.Viruses that are enveloped by a lipid membrane can also fuse directly to the cell membrane and release the capsid into the cell’s cytoplasm.Once inside the cell, eukaryotic viruses target the nucleus of the cell.

Bacteriophage Infection
Recall that bacteriophages only infect bacterial cells.One example is the T-bacteriophage which infects E. Coli. It has a 20-sided capsid connected to a long protein tail with spiky foot-like fibres.

Bacteriophage Infection
Notice that the capsid contains the genetic material.The tail and spikes help to attach the virus to the host cell.After attaching, the tail of the virus releases an enzyme that breaks down part of the bacterial cell wall.

Bacteriophage infection
After this step, the tail sheath retracts, and the tail core punches through the cell wall, injecting the DNA of the bacteriophage into the bacterial cell.The bacteriophage (or phage for short), works like a syringe, injecting its DNA into the bacterial cell’s cytoplasm.Remember: bacteria don’t have a nucleus, so the cytoplasm is where their DNA is found.

Viruses Cause Two Types of Infections

Lytic Infection A lytic infection is an infection where the host cells bursts, or lyses.When the cell lyses, new viruses are released from it and each one of them infects another cell.The following are the steps of a lytic infection (taken from page 551 of the textbook).

Lytic Infection Steps  When the viral DNA enters the host cell, it takes over control of the host cell’s DNA, turning on the genes necessary to copy the viral genes.Under direction of the viral genes, the host’s DNA undergoes transcription and translation, producing capsids and enzymes. The enzymes then help in the copying of the virus’s DNA.Using energy from the host cell, the capsids and viral DNA assemble into new virions. Viral enzymes dissolve the host cell membrane, releasing the new virus particles into the host cell’s bloodstream or tissues – and destroying the host cell in the process.

Lysogenic Infection In a lysogenic infection, a phage (virus) combines its DNA into the host cell’s DNA.After entering the host cell, the viral DNA combines with the host’s DNA, forming a new set of genes called a prophage. A prophage is the phage DNA inserted into the host cell’s DNA. In organisms other than bacteria, this stage is called a provirus.

Lysogenic Infection The prophage is copied and passed to daughter cells, with the host’s own DNA, when the host cell undergoes mitosis (cell division). Although this process does not destroy the cell, it changes some of the cell’s traits.After the cell has been copied, there are two possible paths. A trigger, such as stress, can activate the prophage, which then uses the cell to produce new viruses. Or the prophage can remain as a permanent gene.

GENOMIC CLASSIFICATTION OF VIRUSES BASED ON BALTIMORE CLASSIFICATIONS

Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication. Named after David Baltimore, a Nobel Prize-winning biologist, these groups are designated by Roman numerals. Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar. In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research. Viruses can be placed in one of the seven following groups:[11]

• I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
• II: ssDNA viruses (+ strand or "sense") DNA (e.g. Parvoviruses)
• III: dsRNA viruses (e.g. Reoviruses)
• IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g. Picornaviruses, Togaviruses)
• V: (−)ssRNA viruses (− strand or antisense) RNA (e.g. Orthomyxoviruses, Rhabdoviruses)
• VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate in life-cycle (e.g. Retroviruses)
• VII: dsDNA-RT viruses DNA with RNA intermediate in life-cycle (e.g. Hepadnaviruses)

DNA VIRUSES

• Group I: viruses possess double-stranded DNA. Viruses that cause chickenpox and herpes are found here.
• Group II: viruses possess single-stranded DNA.

RNA VIRUSES

• Group III: viruses possess double-stranded RNA genomes, e.g. rotavirus.
• Group IV: viruses possess positive-sense single-stranded RNA genomes. Many well known viruses are found in this group, including the picornaviruses (which is a family of viruses that includes well-known viruses like Hepatitis A virus, enteroviruses, rhinoviruses, poliovirus, and foot-and-mouth virus), SARS virus, hepatitis C virus, yellow fever virus, and rubella virus.
• Group V: viruses possess negative-sense single-stranded RNA genomes. The deadly Ebola and Marburg viruses are well known members of this group, along with influenza virus, measles, mumps and rabies

Reverse transcribing viruses

• Group VI: viruses possess single-stranded RNA viruses that replicate through a DNA intermediate. The retroviruses are included in this group, of which HIV is a member.
• Group VII: viruses possess double-stranded DNA genomes and replicate using reverse transcriptase.