DNA information is found in units called genes. One gene codes roughly for one protein. The proteins perform most of the functions of the body at the cellular level. Examples are digestion, the body's defense against disease, and transporting substances throughout the body. Thousands of proteins are needed to do the work of a single cell and of proteins within the multi-cellular human body. The proteins are coded in the DNA by these genes (Ridley). Ribonucleic acid or RNA molecule is a single-strand structure, consisting of ribonucleotides (Tutor Vista 2008). It contains the pyrimidine uracil while DNA contains thymine. It has three types, according to cellular composition (Tutor Vista).

The processes of transcription and translation decode DNA information in a gene

(Ridley 2009). A protein molecule evolves from these processes through which DNA produces its effects on every cell of every living organism. Information moves from DNA to RNA to protein on a one-way direction. This is referred to as the central dogma, which explains why acquired characters are not inherited (Ridley).

The process of transcription produces the three forms of RNA, namely messenger RNA or mRNA; ribosomal RNA or rRNA; and transfer RNA or tRNA (Ridley 2009).

Virus -- This is an infectious agent, which affects all life forms, such as humans, animals, plants, fungi and bacteria (Hardwick 2009). It consists of either DNA or RNA and surrounded by a protein, called capsid. It is 20 to 100 times smaller than bacteria so that it is visible only under light microscopy. A virus is not considered a living organism, as it cannot reproduce outside a living host. It can only replicate, that is, transmit its genetic information from one cell to another (Hardwick).

At first contact, an outer viral structure connects with and binds to a specific molecule on the host's cell surface (Hardwick 2009). The virus crosses the cell membrane often by fusing its lipid envelope that cell membrane. In the process, it releases nucleocapsid into the cytoplasm of the host's cell. When it gains entry, viral genes replicate. It produces proteins by the host's cellular machinery. The first proteins are often the enzymes used to copy the viral genome up to thousands of times. Later, the proteins in the capsid...

The virus completes the replication process by leaving the cell. It does this by budding out of the cell's plasma membrane, releasing new viral particles to infect other cells or move from one cell to an adjacent cell. A small virus can replicate in a few hours while a large virus can replicate for several days (Hardwick).
Examples of human illnesses caused by infective viruses are influenza, hepatitis B

and herpes simplex. #

BIBLIOGRAPHY

Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; and Walker, Peter. Catalysis and the Use of Energy by Cells. Molecular Biology of the Cell, 4th edition, 2002. New York: Garland Science. Retrieved on September 11,

2009 from http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section

Buckley, James M., Jr. Living vs. Non-Living. Dewego City Schools

District: Regents Exam Prep Center, 2003. Retrieved on September 11, 2009

from http://regentsprep.org/Regents/biology/Units/organization

Hardwick, J. Marie. Virus. Microsoft Encarta Online Encyclopedia: Microsoft

Corporation, 2009. Retrieved on September 11, 2009 from http://encarta.msn.com/encyclopedia_761575740/Virus_(life_science).html

Microsoft Encarta. Mendel's Laws. Microsoft Encarta Online Encyclopedia:

Microsoft Corporation, 2009. Retrieved on September 11, 2009 from http://encarta.msn.com/encyclopedia_761578133/Mendel's_Laws.htm

Smith, Brian Richard. Blood. Microsoft Encarta Online Encyclopedia: Microsoft

Corporation, 2009. Retrieved on September 11, 2009 from http://encarta.msn.com/encyclopedia_761578429_2/Blood.htm

Taylor, Maxine. Understanding Cellular Respiration. Essortment.com: Pagewise, 2002.

Retrieved on September 11, 2009 from http://www.essortment.com/all/cellularrespira_rmpr.htm

Tutor Vista. Biomolecules, 2008. Retrieved on September 11, 2009 from http://www.tutorvista.com/content/chemistry/chemistry_IV/biomolecules/biomoleculesindex.php

ThinkQuest Team. Characteristics of Life. Astrobiology: ThinkQuest, 2000. Retrieved

on September 11, 2009 from http://library.thinkquest.org/C003763/index.php?page=origin06