Viruses are really small infection causing particles. They used to be thought of as bacteria before scientists realized that they were much much smaller. The structure of a virus is just a small piece of deoxyribose nucleic acid or ribonucleic acid. The sizes of the genomes vary from only seven nucleotides long, like Ebola, to viruses like the Megavirus, who genome is over 1,000,000 nucleotides long. Viruses are protected by a protein coat or capsid. The protein coat protects the genome and also works as a receptor which binds to the receptors on the surface of a cell. The cell will then let the virus in where it infects the cell. That is all a virus is. There is no metabolic process, no replication, just a free floating particle of nucleic acid surrounded by a protein coat. But there is a debate as to whether viruses are alive or not. The biological definition for life is having the capacity to grow, metabolize, respond to stimuli, adapt and reproduce. The reason that viruses are so much on the border of living and nonliving is that a virus depends on the host cell for all of this.
To some scientist a virus can not be living because it can not reproduce, metabolize or grow on its own, it relies on the host cell to do that. Also the fact that it does not become a living system until after its nucleic acid integrates with the DNA of the host cell. Also they have no energy or metabolic system of any sort. They are not made up of cells, which is the basic unit of everything living. It can survive for incredibly long periods of time without a host cell. The most common theory is that viruses are just small bits of DNA or RNA shed by cell. But what about reproduction, a virus creates more viruses, as long as it has a host cell. I believe that viruses are not living due to information above. Without a host cell, viruses would not be able to reproduce or grow, they are completely depended which goes against the biological definition of what living is.
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