Cells
Cell theory states that 1) all living organisms are made up of one or more cells, 2) nothing smaller than a cell is considered
alive, and 3) new cells are formed from existing cells. Cells must meet three criteria in order to be considered living. First,
they must be bound by a membrane and because of its selective permeability of the membrane, the cell can be different
than its surroundings. Secondly, cells must acquire energy through metabolic processes. Third, living organisms must have a
genetic system to regulate the metabolic processes and this genetic system must be capable of replication.
Nucleic acids are the one thing that viruses, bacteria and higher eukaryotic organisms have in common. The shape and
complexity of the nucleic acids differs between these three categories. Viruses are about 1/10 to 1/100th the size of a
typical bacteria or prokaryotic cell. They contain no cytoplasm. A virus has straight or circular DNA or RNA that is protected
by a protein capsule. A virus genome is about 1000 to 1,000,000 nucleotide bases.
Prokaryotic cells do not have nuclei or discrete organelles. They do contain DNA but it is not in the form of chromosomes.
Prokaryotic DNA is circular and contains only about 1,000,000 nucleotide bases (compared to 3 billion base pairs for
humans). Prokaryotic cells are 1/10 to 1/100 of typical eukaryotic cells. Prokaryotic cells do not contain organelles but they
do have ribosomes and internal membranes which conduct similar functions as eukaryotic organelles. Bacteria are
surrounded by a bi-lipid membrane. Often there is a thick cell wall and capsule coating outside the membrane. Some
species have a few to many external filaments (pilli, flagella) which might serve to facilitate attachment or locomotion in
the case of longer filaments (Fig. 1). Toxins and destructive enzymes (proteins) produced by bacteria are responsible for
tissue damage and symptoms of bacterial infection.
Fig. 1. A diagramatic bacteria.
Bacteria are broadly divided into categories defined by their shape and ability to be stained. Bacteria occur as cocci (round),
bacilli (rods) and spirochetes (spiral-shaped) (Fig. 2). They stain either positive or negative (not stained) with Grams stain.
So a bacteria might be described as a Gram Positive Bacillus. Shape and stain can be determined quickly from a sample and
assist in a course of treatment for a bacterial infection.
Fig. 2. Three shapes of bacteria.
Bacteria can exchange DNA and fragments of DNA (plasmids) by means of conjugation (Fig. 3). They can acquire external
DNA fragments from the environment. Bacteria divide by a simple process called binary fission or binary division. The
circular DNA duplicates, the cytoplasm divides and one of two circular chromosomes travels with each of the two dividing
portions of the bacterial cell.
Fig. 3. Bacterial conjugation.
Eukaryotic Cells
Eukaryotic cells contain organelles. The organelles include the nucleus which contains the DNA in the form of
chromosomes. The nucleolus inside nucleus manufactures the ribosomes which move to the rough endoplasmic reticulum.
Chromosomes are termed chromatin when the cell is not dividing. The rough endoplasmic reticulum (ER) contains circular
ribosomes which are where proteins are made. Other molecules are made in the smooth ER. The Golgi Apparatus packages
the molecules made by the ER into their final form and initiates their transport in the cell. Vessicles (vacuoles) transport
molecules to and from the cell membrane. Lysosome clean up unneeded particles and molecules. Microtubules and
filaments provide a weak skeleton of sorts withing the fluid cytosol (cytoplasm). Mitochondria are the energy plant. They
convert the breakdown products of mostly carbohydrates and lipds to ATP. Plants including algae have chloroplasts which
convert sunlight and CO2 to ATP and sugar. Mitochondria and chloroplasts have circular DNA which is similar to the circular
DNA of prokaryotes.
Fig. 4. A diagramatic eukaryotic cell.
DNA To Protein
Understanding the means by which proteins are made by cells is important if one is to understand how viruses infect cells
and replicate. Proteins are both structural and functional. They serve as enzymes to regulate 1000’s of metabolic processes.
Recall that proteins are sequences of dozens, hundreds or even more amino acids. The sequence gives the protein its
unique characteristics. The code for the sequence resides in the sequence of nucleotide bases. DNA in the chromatin state
must open up a bubble in the double helix (Fig. 5) in order to expose a sequence of nucleotide bases in order to
manufacture messenger RNA (mRNA). mRNA will have a complimentary sequence to the coding strand of DNA but Uracil
will replace Thymine in all RNA. The mRNA moves into the cytoplasm to the rough endoplasmic reticulum (rough ER). The
rough ER is so named because it has ribosomes which are the protein synthesis sites. Amino acids are brought into
sequence by transfer RNA (tRNA). tRNAs has only three nucleotide bases. These three bases will only align with
complimentary bases (e.g., AGC will align with UCG). Thus the sequence of the original DNA determines the sequence on
the mRNA which determines the sequence of amino acids (Fig. 6)
Fig. 5. A “bubble” opens up in the DNA double helix in order to make mRNA.
Fig. 6. tRNA’s bring amino acids to a growing polypeptide chain.
Viruses
Viruses do not meet one of the three functions all living things share. They do not have capacity to harvest energy through
metabolic processes. Viruses do contain a genetic system that can control metabolic processes of its host. Viruses are
protected by a membrane. It is not like the bi-lipid membrane of all living cells. Viruses are covered by a protein coating.
Viruses have a few different forms. Some are linear and some are multi-sided looking more like a machine.
Inside the virus is either DNA or RNA. Some viruses attach to a host’s cell at specific external sites and inject their nucleic
acid into the cell. Other viruses gain entry into the cell by being engulfed and taken into the cell in a vessel. The virus and
its nucleic acid is then able to escape the vessicle.
Viral Forms.
(1) (Fig. 7a) Viruses that contain DNA enter the nucleus and then proceed to make RNA which will travel to the rough ER
and begin to make more viral particles. (2) (Fig. 7a) Viruses that contain RNA utilize the rough ER to synthesize more viral
particles. (3) (Fig. 7b) Some viruses (retro viruses) have RNA which is reverse transcribed into DNA (the opposite of the step
where DNA is transcribed to mRNA) which is then incorporated into the host’s DNA. This new code is incorporated into the
host cell’s genome and then proceeds to manufacture RNA which will build more viruses.
Regardless of which of the three categories in terms of mode of action the result is that the genetic machinery of the host
cell is used to manufacture more viruses. Eventually the cell is ruptured releasing more viruses.
Fig. 7a. Pathways for non-retroviruses.
Fig. 7b. Pathway for retroviruses