The Viral Genome

In our first installment, we said, "a virus is a container made of proteins and sometimes lipids that contains a payload of nucleotides (DNA or RNA)." But what does that actually mean? Let's start with the nucleotides.


Nucleotides are the molecules that form our genetic material. Each molecule has a sugar backbone, a phosphate, and one of four chemical compounds called bases. DNA, or deoxyribo-nucleic acid, is a chain of these molecules bound together, and it often forms the famous double helix because of bonds that form between pairs of the bases. In humans and other lifeforms, DNA encodes the instructions for creating all of the proteins that make up the lifeform; every cell carries inside of it DNA chains that have a specific ordering of the four bases (Adenine, Cytosine, Guanine, Thymine).

The DNA is "read" by proteins inside of the cell to make chains of RNA, or ribonucleic acid. RNA is very similar to DNA, but the sugar backbone has a slightly different chemical composition, and instead of the base Thymine, the fourth RNA base is Uracil. Special proteins called polymerases travel along a strand of DNA, and for each base they see, they add a corresponding RNA base to form a chain of nucleotides.

The strand of RNA that is formed is then read by ribosomes, which are a special set of proteins in cells that convert RNA to protein molecules. Every three bases of RNA yield one amino acid, which is a building block of a protein, and so the ribosomes translate the RNA strand into a chain of amino acids. Et voilĂ , we now have proteins that can do work inside of a living thing.

(It's worth stopping for a moment to marvel here-- there are strings of A/C/G/T encoded in DNA in every cell; DNA gets transcribed into a different chemical format called RNA; and then that RNA gets turned into amino acids that form proteins, but only if the order is exactly right to create functional proteins. It is in many senses shocking that we end up with fully formed cells and even multicellular organisms that work and think and love, even though we start with very low-level chemical and mechanical interactions.)

How do viruses fit in?

Now, all of this happens in normal cells, but it's also critical for viral replication. The virus contains a payload of either DNA or RNA. The full set of DNA or RNA that each virus carries is called collectively its genome. The virus's "goal" is to dump that genomic payload into a cell that knows how to read DNA and RNA. Once the virus's payload gets into a normally operating cell, the cell's own machinery starts to read and operate on the virus's nucleotides. For example, for a DNA virus, the cell's polymerase sees the viral DNA and converts it into RNA without realizing that DNA is foreign. The transcribed RNA then gets translated into amino acids by the cell's ribosomes, and the viral proteins that are created self-assemble to form a whole new viral capsid with a new copy of the viral payload inside. RNA viruses work similarly, but they skip the transcription into DNA, and go straight into the ribosomes.

In other words, the virus hijacks the cell's normal machinery to make copies of itself simply by encoding the right sequence of nucleotides. That's why we say that viruses do not grow themselves and that they are made from pre-formed components; without the host's cellular machinery, the virus itself cannot copy its nucleotide payload or create the proteins it needs for its capsid.

Next time, we’ll learn more about that capsid and its construction.



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