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Biology - Microorganisms


A bacterium is a single-celled organism and it usually has a cell wall.  A bacterium is a prokaryotic cell.  A typical bacterium has one ring-shaped chromosome made of DNA.  At some point in its life, the bacterium replicates the whole chromosome.  The bacterium then develops a cell wall across its transverse length and divides into two daughter bacteria, each of which has one chromosome.  The whole process is asexual and it is called binary fission.  Bacteria, however, have ways of mixing their chromosomes with chromosomes from other bacteria.  In that way, they achieve genetic recombination.

There are three words that signify the ways in which bacteria achieve genetic recombination; conjugation, transformation and transduction.  Conjugation is when two bacteria can get together and one can transfer some DNA to another through structures called pili.  One bacterium is called the donor and it has a plasmid called the F-factor that participates in the transfer of DNA.  The other bacterium is called the recipient.

Transformation is when one bacterium receives pieces of DNA from a second bacterium.  The two bacteria do not actually get together as they do in conjugation.  Instead, pieces of chromosome from the second bacterium have gotten loose and are floating around on their own, the first bacterium incorporates those pieces into its own chromosome.

Transduction is when a virus takes some DNA from bacterium and carries it to another.  Some bacteria are round; they are called cocci, singular is coccus.  Some bacteria are spiral, they are called spirilla, singular is spirillum.  Some bacteria look like straight rods, they are bacilli, singular is bacillus.

Remember, the following types of organisms and their method of obtaining nutrition.  Autotrophs are organisms that can produce their own nutrition via photosynthesis.  Heterotrophs are organisms that cannot perform photosynthesis and must derive their nutrition from an outside source.  Parasites are organisms that have no digestive system and absorb nutrients from the living body of a host organism at the expense of the host.  Saprophytes are organisms that have no digestive system and absorb nutrients from the dead bodies of other organisms.  Carnivores are organisms that have digestive systems and eat animals.  Herbivores are organisms that have digestive systems and eat plants.

Now remember, the following types of bacteria and their need for oxygen.  Obligate anaerobes derive energy via fermentation only.  They cannot survive in the presence of oxygen.  Facultative anaerobes derive energy via aerobic respiration or fermentation and can survive with or without oxygen.  Aerobes derive energy via aerobic respiration only and cannot survive without oxygen.  Some bacteria are called nitrogen-fixing bacteria, which mean that they have the ability to take nitrogen from the air and often provided to the plants.


A virus does not have a plasma membrane.  It does not have a nucleus and it does not have any of the organelles we have talked about.  A virus has a coat which is called a capsid.  This coat is made of protein.  Inside the virus’ protein coat, there is nucleic acid.  There are two kinds of nucleic acids, DNA and RNA.  Some viruses have DNA, and are called DNA viruses.  Some have RNA, and are called RNA viruses.  A virus cannot reproduce without the help of some other cell.

So when a virus wants to reproduce, here is what it does.  First, it latches on to some other cell.  It might latch on to a bacterial cell, fungal cell, plant cell or animal cell.  The other cell is called the host.  Second, once the virus has latched on to his host, it injects its nucleic acid into the host.  Third, the virus’ nucleic acid somehow uses the host cell’s equipment to reproduce itself many times.  While inside the host cell, the virus’ nucleic acid replicates itself repeated and each copy of the viral nucleic acid somehow causes a new protein coat to form around the nucleic acid.  Fourth, the new viral particles now burst out of the host cell and emerge.  Bursting is called host cell lysis.

The sequence of events in a lysogenic infection is a little different.  The virus latches on the cells, as usual, and injects its nucleic acid.  Then, the viral nucleic acid integrates into the host’s nucleic acid.  When the host cell replicates itself, the viral nucleic acid, which is now called prophage, also replicates.  Certain conditions can cause the prophage to become lytic and follow the lytic phase.

There is something about a virus’ protein coat that decides what kind of host cell it will attack.  A virus cannot settle on a host cell unless the host cell has some kind of receptor for the virus’ protein coat on its surface.  There is a type of virus called bateriophage.  A bateriophage is a virus that uses bacteria as its host.  The bateriophage, like all other viruses, has a protein coat and nucleic acid inside it.  The nucleic acid inside a bateriophage is DNA.  A bateriophage also has a tail.  A bateriophage reproduces just like other viruses, except that when it latches on to its host, it uses its tail.


It is hard to decide whether fungi are plants or animals.  It is also hard to decide whether a fungus is a single-celled organism or a multi-celled organism.  A fungus is like many cells all joined together in one and that is why it sometimes called a multi-nucleated cell.  A fungus is like a plant and that it has one big cell wall around it.  It is also a eukaryotic cell and that it has many, many copies of the usual organelles and many nuclei.  Fungi are usually haploid.  Some of them reproduce asexually and some reproduce sexually.  Some, however, can reproduce asexually or sexually.

Budding is when a piece of a fungus breaks off and becomes a brand new fungus.  Spores are tiny cells that a fungus expels usually from long structures called hyphae.  These spores germinate is some suitable environment, and a new fungus grows.  The new fungus is an identical twin to the original because its chromosomes are exactly the same as the original.  Fungi reproduce by each contributing a piece of itself by conjugation.  Each piece has a haploid number of chromosomes and when they get together to form a zygote, the zygote has the deployed number of chromosomes.

The zygote hangs around in a dormant state for a few months and does not do much.  This dormant state is the only part of the fungus’ life cycle that is diploid.  After the zygote is through dormancy, it undergoes mitosis and forms spores, each one having the haploid number of chromosomes.  The spores germinate somewhere and we get new fungi, each having the haploid number of chromosomes, so spores can be produce sexually and asexually.