Essay by Anonymous UserA+, April 1996

download word file, 3 pages 4.6

Downloaded 85 times

A simple essay on computer viruses Could have added more examples

Viruses: Complex Molecules or Simple Life Forms?

Viruses have been defined as 'entities whose genomes are elements

of nucleic acid that replicate inside living cells using the cellular

synthetic machinery, and cause the synthesis of specialised elements that

can transfer the genome to other cells.' They are stationaryand are unable

to grow. Because of all these factors, it is debatable whether viruses

are the most complex of molecules or the simplest life forms. While the

definition of living organisms must be adapted, the majority of evidence

leads to the classification of viruses as living organisms.

Viruses are composed of a nucleic acid core, a protein capsid, and

occasionally a membraneous envelope. The nucleic acid core is composed of

either DNA or in the case of retroviruses, RNA, but never both. In

retroviruses, the RNA gets transcribed to DNA bye the enzyme reverse

transcriptase. The protein capsid is a protein layer that wraps around

the virus. There are four basic shapes of viruses. The tobacco mosiac,

adenovirus, influenza virus, and t-even bacteriophage are each examples of

a different virus structure. Each individual protein subunit composing the

capsid is a capsomere.

The tobacco mosiac virus has a helical capsoid and is rod shaped.

The adenovirus is polyhedral and has a protein spike at each vertex. The

influenza virus is made of a flexible, helecal capsid. It has an outer

membranous enevelope that is covered with glycoprotein spikes. The T-even

bacteriophage consists of a polyhedral head and a tail. The tail is used

to inject DNA into a bacterium while the head stores the DNA.

Basic life is defined as the simplest form capable of displaying

the most essential attributes of a living thing. This makes the only real

criterion for life the ability to replicate. Only systems containing nucleic

acids are capable of this phenomenon. With this reasoning, a better

definition is the unit element of a continuous lineage with an individual

evolutionary history. Because of viruses inability to survive when not in

a host, they must have evolved from other forms of life. The origin of

viruses is an easy thing to theorize about so many hypothesese have been made.

One such hypothesis is that viruses were once complete living

parasites. Over time they have lost all other cellular components. This is

backed up by the idea that all cells degenerate over time.

Some people think along very similar lines that viruses are

representatives of an early 'nearly living' stage of life. This goes along

with the first hypothesis in that it accounts for a loss of components. All

creatures that become parasitic can be seen losing their obsolete functions

and structures. An example of this is the flea. Fleas are eveolved from

flies but have discarded their unneeded wings.

This theory when applied suggests that atleast some branches of

viruses have evolved from bacteria because of their similar natures.

Scientists say that at one point viruses could have been independant

organisms. As they slowly became parasitic, the unsed structures for protein

and energy synthesis were lost, along with the inhibiting cell wall. While

viruses do need a host cell to complete many important functions of living

organisms, the should still be considered living themselves.

The ability to replicate is important to the classification of an

item as living. Within the host, viruses are able to replicate, evolve, and

even mutate. They are deeply intertwined in the life process by this

dependancy on a host.

Viruses are very specific to what they can use as a host. Despite

the specificness, many viruses can host withmembers of different species,

genus, and even phylums. A lock and key fit determines the host, or host

range. This works vert similar to that of an enzymes active site.

Once the virus has found a host cell, the virus uses the host's

nucleotides and enzymes to replicate it's DNA. Other materials and machinery

of the host cell produce the virus's capsid proteins. The viral DNA and

proteins then join to make a new copy of the virus.

While viruses are inactive when in transport between hosts, the

arguements are overwhelmingly in favor of considering viruses living organisms.

Through their parasitic nature, they are able to fulfill most qualities of

living organisms. Their behavior and complexness also lead to this

classification. While they are not the text book example of living organisms,

is has been in agreement that there always will be exceptions to the rules.

Viruses deserve to take their rightful place among the ranks of living