Chapter 2

• Viruses do not fit in the six-kingdom system because they do not display most of the characteristics of living cells.
• Viruses are not cellular and can only live as parasites, occupying a position between nonliving and living.
• Viruses range in size from 20 - 400 nm, with 1 nm being 10-9 m.
• Viruses are tiny sub-cellular particles made up of nucleic acid and a protein covering called a capsid.
• The capsid is made up of hundreds of protein molecules and accounts for 95% of the total virus, giving it its particular shape.
• Nucleic acid is a single-stranded RNA or double-stranded DNA.
• The largest and most complex viruses are those that attack and infect bacteria – bacteriophages.
• Other examples of viruses include HIV, polio, the tobacco mosaic virus, etc.
• Viruses must enter cells to carry out life processes.
• Not every virus is considered to be disease-causing.
• Viruses are generally selective, and, in most cases, specific viruses enter only specific host cells.
• Viruses can reproduce by either the lysogenic or lytic cycle.
• The lytic cycle can be summarized in four steps: Attachment and Entry, Synthesis, Assembly, and Lysis (Release).
• In the lysogenic cycle, the virus’ genetic material enters the cell, enzymes copy viral RNA into DNA, and then becomes part of the host cell’s chromosome.
• The viral DNA that has become part of the chromosome is referred to as a provirus, which can invade a cell, but does not kill it.
• In many cases, the provirus gene is not activated until later, and once it is, it results in a continuation of the lytic cycle.
• Prokaryotes are smaller in size compared to Eukaryotes, with a range from 1 to 10 um.
• Eukaryotes are larger in size, ranging from 100 to 1000um.
• Prokaryotes have single, circular DNA and no membrane.
• Eukaryotes have DNA organized into chromosomes and are enclosed in a nucleus.
• Prokaryotes perform binary fission and conjugation for cell division.
• Eukaryotes perform mitosis and meiosis for cell division.
• Prokaryotes reproduce asexually, which is the most common method.
• Eukaryotes reproduce sexually, which is also the most common method.
• Prokaryotes are unicellular, meaning they exist as a single cell.
• Eukaryotes are multicellular, meaning they exist as multiple cells.
• Prokaryotes have no organelles, while Eukaryotes have them present.
• Cells are prokaryotic, all are single-celled, the smallest organisms on Earth, contain no membrane-bound organelles, have a single chromosome, and reproduce asexually by binary fission.
• Cells can be classified according to appearance into spherical (cocci), rod-shaped (bacilli), and spiral (spirilla).
• There are also bacteria that do not fit into these three categories and can vary in shape.
• Bacteria can obtain energy through photosynthesis, consuming other organisms, or from inorganic compounds such as sulfur or iron.
• Archaea undergo methanogenesis, an anaerobic process which produces methane as a product.
• Most bacteria live in anaerobic conditions, but some can live in aerobic conditions.
• Some Archaea are extremophiles, often living in environments where most other organisms cannot survive.
• Three major groups of thermophiles include Extreme Acidophiles, Extreme Thermophiles, and Extreme Halophiles.
• Most bacteria are mesophiles, living in moderate conditions.
• All bacteria reproduce asexually by binary fission under normal conditions.
• The single strand of bacterial DNA replicates, resulting in identical genetic material being transferred to each new cell.
• Following replication of genetic material, a bacterium produces a septum and divides into two identical cells.
• When conditions are not ideal, some bacteria will reproduce sexually by conjugation.