2.1 – A Microscopic Look at Life’s Organization
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There are two basic cell types – prokaryotic cells and eukaryotic cells.
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Prokaryotic cells are smaller and less complex than eukaryotic cells. Prokaryotic
cells do not have a membrane-bound nucleus or other membrane-bound
organelles.
Viruses
virus:
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a structure that contains strands of DNA or RNA surrounded by a protective protein
coat: it cannot live independently outside of cells
Viruses have no cellular structure therefore viruses are not considered to be
organisms. They are not classified in any kingdom of living things. Viruses have
no cytoplasm, organelles, or cell membranes. They do not carry out respiration or
many other common life processes.
Viruses consist of little more than strands of DNA or RNA surrounded by a
protective protein coat called a capsid.
Classifying Viruses
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Viruses can be classified by the size and shape of their capsid.
See Figure 2.2, pg 54
The polio virus resembles small crystals, HIV that causes AIDS has a spherical
shape, the tobacco mosaic virus has a cylindrical shape and the T4 virus that
infects bacteria has a head attached to a protein tail.
Reproduction in Viruses
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Viruses reproduce through replication within a host cell. The host cell can be either
a prokaryote or a eukaryote, depending on the type of virus.
They replicate through the lytic cycle or the lysogenic cycle.
See Figure 2.3, pg 55
In the lytic cycle, the entire replication process occurs in the cytoplasm of the
host cell.
Steps of the lytic cycle:
 Attachment: Proteins on the surface of the virus bind to protein
receptors on the surface of the host cell’s membrane.
 Entry: The virus injects its genetic material (DNA or RNA) into the
bacterial cell.
 Replication: The host cell makes more viral DNA or RNA and proteins.
 Assembly: New viral particles are assembled.
 Lysis and Release: The host cell breaks open and releases new viral
particles.
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In the lysogenic cycle, the virus’s genetic material enters the host cell’s
chromosome. Once this occurs, the infected cell has viral genes permanently.
The viral DNA that has become part of the host chromosome is then referred to as
a provirus. A provirus can invade a cell, but does not kill it.
Viruses and Disease
retrovirus:
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In this form, the viral genome can enter the chromosomes of the host cell and be
copied when the cell divides.
provirus:
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type of viral replication where a virus uses reverse transcriptase to copy
the viral RNA into DNA. The retroviral DNA is than integrated into the
host cell’s chromosome. An example is the human immunodeficiency
virus (HIV virus).
virus that undergoes a replication cycle in which its DNA becomes
integrated with the host cell chromosomes; the virus can invade the cell
but does not kill it.
When the host cell divides through the process of mitosis, it replicates the provirus
along with its own DNA. Every descendent of the host cell will carry a copy of the
provirus in its chromosomes. This process can continue for years, with no harm to
the host.
At any time, the provirus can separate from the host chromosomes and complete
the more damaging lytic cycle.
Patterns of Disease
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prion:
The replication strategies of viruses help explain certain patterns of disease.
For example, HIV forms a provirus in the host cell chromosomes, but it also
produces small numbers of new viruses while the cell continues to function
normally.
This explains why people may test positive for HIV but remain healthy for many
years.
Only when the infection spreads to more and more cells do the symptoms of AIDS
eventually appear.
The symptoms result from infections by other micro-organisms because the HIV
virus has destroyed the body’s T-lymphocytes, which help the immune system fight
off other diseases.
In the 1980’s, a new type of disease-causing agent called a prion was discovered.
an infectious particle that causes damage to nerve cells in the brain, and that
appears to consist mostly or entirely of a single protein
Viruses and Biotechnology
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Viruses can be useful tools for genetic engineers.
If researchers want to make a copy of a gene, they first insert the gene into the
genetic material of a virus. The virus then enters a host cell and directs the cell to
make multiple copies of the virus. Each new virus in each new cell contains the
added gene that the researchers want copied.
HOMEWORK:
pg 58 #1 - 12
2.2 – Comparing Bacteria and Archaea
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The two domains into which prokaryotes are classified are Bacteria and Archaea.
They are the smallest, simplest organisms on Earth and are abundant in the air,
water, soil, and on most objects.
All species in Archaea and Bacteria are unicellular, but some form colonies or link
up to form filaments.
Comparing Morphology
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Bacteria and archaea have three basic shapes:
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cocci (singular coccus) – spherical shape
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bacilli (singular bacillus) – rod shape
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spirilli (singular spirillum) – spiral shape
See Figure 2.5, pg 59
Bacteria often grow in characteristic patterns, or groupings.
(Fig 2.6, pg 59)
diplo- describes cells arranged in pairs
staphylo- describes cells arranged in clusters resembling grapes
strepto- refers to bacteria arranged in a chain
Comparing Nutrition
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Species in Archaea and Bacteria obtain energy: by carrying out photosynthesis,
consuming other organisms, or getting energy from various inorganic compounds
such as hydrogen sulfide or iron.
One method of obtaining energy that is unique to Archaea is methanogenesis,
which produces methane gas as a by-product.
Methanogenesis is an anaerobic process that occurs in environments that lack
oxygen, and it is often one of the final stages of decomposition.
Another key difference between Archaea and Bacteria is photosynthesis.
Photosynthetic bacteria are called cyanobacteria that use solar energy to convert
carbon dioxide and water into sugar.
Researcher are still looking for evidence of photosynthesis in archaea.
Comparing Habitats
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Both archaea and bacteria live in aerobic (with oxygen) and anaerobic (without
oxygen) environments.
Methanogens are abundant in anaerobic environments such as landfill sites, guts of
cattle, sediments of swamps, and sewage-disposal plants.
Anaerobic bacteria are found in the human gut and many other environments.
Archaea are referred to as extremophiles because of their ability to live in
extreme environments.
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Types of Extremophiles are:
 Thermophile (heat-lovers) – live in extremely hot environments such as
deep sea vents and hot springs
 Acidophile (acid-lovers) – live in extremely acidic environments such as
volcanic crater lakes and mine drainage lakes
 Halophile (salt-lovers) – live in extremely salty environments such as salt
lakes and inland seas that have salt concentrations higher than 20 per cent
See Table 2.2, pg 61
Most bacteria are mesophiles – organisms that occupy environments with
moderate (less extreme) conditions.
Comparing Reproduction
Asexual Reproduction
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Archaea and Bacteria divide by the process of binary fission, or cell division. (See
Fig 2.9, pg 62 for binary fission in prokaryotes)
binary fission:
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the asexual form of reproduction used by most prokaryotes (and some
eukaryotic organelles), in which a cell divides into two genetically identical
cells (or organelles)
As a cell grows, it makes a copy of its original, single chromosome. When the cell
reaches a certain size, it elongates, separating the two chromosomes. The cell
then builds a partition between them, and eventually the original cell splits into two
smaller, genetically identical cells.
Sexual Reproduction
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In less favourable conditions, Some bacteria and archaea are able to exchange
DNA by a process called conjugation.
conjugation:
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a process in which there is a transfer of genetic material involving two cells
During conjugation, one cell becomes linked to one another through bridging
structures called pili.
The cell transfers all or part of its chromosome to the other across this structure.
(See Fig 2.10, pg 63)
With its new gene content, the receiving cell then undergoes binary fission to
produce more cells with the same genetic make-up.
Plasmids: Small Loops of DNA
In most bacteria and archaea, the chromosome is not the only part of the cell
containing genes.
Plasmids are small loops of DNA that are separate from the main chromosome
and contain one to a few genes that are different from those found in the
chromosome.
Plasmids can split from the chromosome and rejoin it, and they are an important
method of producing genetic recombination in prokaryotes.
Plasmids may be transferred from one cell to another during the process of
conjugation.
Scientists can use this mechanism as a tool for genetic engineering to introduce
new genes into cells. This technique can be used to clone a human gene for
producing insulin.
Endospores: Protecting Genetic Material
Some bacteria are able to survive an unfavourable environment by developing a
tough outer covering that surrounds their DNA and a small amount of cytoplasm.
This forms small seedlike structures called endospores.
While in the endospore stage, bacteria do not grow and reproduce.
Endospores can resist extreme heat or cold, drying out, and damaging chemicals.
When favourable conditions return, the endospore loses its coat and begins to grow
and divide like a normal cell.
Endospores have not been found in archaea.
Classifying and Identifying Bacteria and Archaea
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One way of classifying bacteria is the use of Gram stain.
Gram-positive bacteria have a thick protein layer on their cell wall and stain
purple.
Gram-negative bacteria have a thin protein layer on their cell walls and stain
pink.
See Figure 2.12, pg 64
Other traditional methods used to classify and identify prokaryotes include size and
shape, nutrition, movement, and genetic components.
Modern biologists use DNA sequences.
Bacteria and Human Health
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Food spoilage (botulism) and the spread of disease (strep throat, ear infections,
and cavities) are the result of bacteria carrying out their normal life functions.
Bacteria and the Environment
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Bacteria are decomposers that break down organic materials and release carbon,
hydrogen, and other elements into the environment for use by other organisms.
Cyanobacteria are major producers of oxygen through the process of
photosynthesis.
Archaea and Biotechnology
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Most enzymes break down and stop working when they are exposed to extreme
conditions however, archaeon enzymes do not.
Polymerase chain reaction (PCR) is a technology that produces millions of copies of
DNA.
DNA polymerase from archaea is essential in the PCR process since extreme heat is
used in the process.
HOMEWORK:
pg 66 #1 - 12
2.3 – Eukaryotic Evolution and Diversity
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Scientists hypothesize that eukaryotic cells developed in the past through
endosymbiosis.
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In endosymbiosis, one cell engulfs a different type of cell and that cell survives and
becomes an integral part of the engulfing cell.
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The two organelles that present the strongest evidence of endosymbiosis are
chloroplasts (converts solar energy into sugar) and mitochondria (extract
energy stored in sugar so that cell can do work).
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See Figure 2.14, pg 67.
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There is a lot of evidence supporting the theory of endosymbiosis.
 The membranes of chloroplasts and mitochondria are similar to those of
living prokaryotes.
 The ribosomes are similar to prokaryotic ribosomes.
 These organelles reproduce by binary fission.
 Many of the genes sequences found on the circular chromosome of these
organelles match those of living prokaryotes.
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After eukaryotes appeared on Earth, a great variety of unicellular forms evolved,
but multicellular forms came later.
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In eukaryotes, sexual life cycles vary, but they alternate between meiosis, which
produces haploid cells (sperm and eggs), and fertilization, which produces diploid
cells (joining of sperm and eggs).
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See Figure 2.17, pg 70
HOMEWORK: pg 71 #1 - 13
2.4 – Protists: The Unicellular Eukaryotes
protist:
a eukaryotic organism, usually unicellular, that is not a fungus, plant, or animal
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Protists are generally divided into three groups based on how they obtain nutrition:
animal-like protists, fungus-like protists, and plant-like protists.
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See Table 2.3, pg 72
Animal-like Protists (Protozoa)
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Protozoa obtain their food by feeding on other organisms or dead matter
(heterotrophs).
Most protozoa can move, and species are traditionally classified in four phyla based
on their methods of locomotion.
The four phyla are: Sarcodines, Flagellates, Sporozoa, Ciliates
Sarcodines (Sarcodina)
 use pseudopodia for amoeboid movement
 (e.g. Amoeba, Foraminifera)
 pseudopodia: limb-like extensions of the cytoplasm
Flagellates (Zoomastigina)
 move by means of whip-like flagellae
 (e.g. Trypanosoma, Giardia, Euglena)
Sporozoans (Sporozoa)
 are mainly non-motile parasites
 (e.g. Plasmodium)
 See Fig 2.74, pg 74 for the life cycle of Plasmodium.
Ciliates (Ciliophora)
 move by beating numerous hair-like cilia
 (e.g. Paramecium)
Some protozoans, such as Entamoeba hystolitica and Balantidium coli, can cause
intestinal infections in humans. Several types of sporozoans cause malaria in
humans.
Fungus-like Protists
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Fungus-like protists get their energy by absorbing nutrients from living organisms,
dead organisms, and wastes (heterotrophs).
Members of this group include slime moulds and water moulds.
See Table 2. 4, pg 75
Plant-like Protists
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Plant-like protists include diatoms, dinoflagellates, and euglenoids.
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Diatoms (Chrysophytes)
 unicellular
 Diatoms are the biggest components of plankton. As photosynthetic organisms,
they are also a major source of atmospheric oxygen.
 Diatoms reproduce asexually and sexually. (See Fig 2.23, pg 76)
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Dinoflagellates (Pyrrophytes)
 unicellular
 Reproduction is mainly by cell division, although they are also capable of sexual
reproduction
 Dinoflagellates can cause red tides when nutrients are plentiful.
 See Figure 2.24, pg 77
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Euglenoids (Euglenophytes)
 unicellular
 (e.g.) Euglena are small freshwater organisms with two flagellae.
 Euglenoids can be autotrophic (in the sunlight) or heterotrophic (in the dark).
HOMEWORK:
pg 78 #1-13