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... [continues]
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(1999, 10). Viruses: Complex Molecules or Simple Life Forms?. StudyMode.com. Retrieved 10, 1999, from http://www.studymode.com/essays/Viruses-Complex-Molecules-Simple-Life-Forms-442.html
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