CHAPTER
13
EARLY LIFE FORMS AND VIRUSES
Though we never think of micro organisms (like bacteria, protists,
and viruses) unless we hear about them making someone sick,
most of these organisms are not harmful.
Many are beneficial in that they serve as food for larger organisms
and keep nutrients cycling through ecosystems.
EVOLUTION OF THE FIRST CELLS
•
Scientists believe that the atmosphere of early Earth contained a mixture of
gases such as carbon dioxide, nitrogen, hydrogen, and water vapor from
spewing volcanoes with little to no oxygen.
•
Lightning flashes shooting through this atmosphere enabled the first organic
molecules (such as amino acids and sugars to form)
•
The next step in the evolution of the first cells had to be the concentration of
these organic molecules in order to form more complex organic molecules.
•
The early RNA world hypothesis suggests that, in the beginning, RNA
stored both genetic information and acted as an enzyme to produce proteins.
In fact, some RNA molecules still act as enzymes today. The RNA enzymes
could have copied themselves to be passed on to other cells.
•
Next, protocells evolved. Protocells are membranous sacs composed of
lipids that would have enclosed interacting organic molecules and would
have been able to divide on their own.
PROKARYOTES
• The first cells were probably anaerobic prokaryotes that arose 3-4 billion
years ago.
• They probably used dissolved carbon as their carbon source and
mineral ions for energy. This would make them chemosynthesizers.
• The two prokaryotic domains, Archae and Bacteria, diverged very early
in the history of life.
• Soon after, some members of each group evolved the ability to do
photosynthesis without producing oxygen. Some modern
photosynthesizers still do this.
• Then, about 2.7 billion years ago, a linage of prokaryotes evolved that
released oxygen when doing photosynthesis. This caused oxygen to
begin to build up in the atmosphere.
• This oxygen 1) prevented the formation of complex organic molecules
from nonliving materials, 2) selected for organisms that could thrive in
high-oxygen conditions, and 3)resulted in the formation of the ozone
layer.
EVOLUTION OF ORGANELLES AND
EUKARYOTES
•
Eukaryotes, with their membrane-bound nucleus, evolved about 1.8 million
years ago.
•
Scientists believe that the membrane around the nucleus, which is
continuous with the ER, evolved from an infolding of the plasma membrane
and that this infolding membrane around the nucleus was favored because it
protected the cell’s genetic material.
•
Mitochondria and chloroplasts, wither their own prokaryotic DNA and
ribosomes are believed to have arisen according to the Endosymbiotic
Theory.
•
The first eukaryotes were also the first protists, red alga that lived about 1.2
billion years ago.
•
They were multicellular-some cells held the alga in place, while others
produced sexual spores; therefore, this red alga was one of the first sexually
reproducing organisms.
•
The first animal appeared about 570 million years ago.
•
By the end of the Cambrian period, 543 million years ago, all major animal
lineages, including vertebrate, were represented in the seas.
THE PROKARYOTES
• No nucleus or other membrane-bound organelles, very small, single
circular molecule of DNA called a nucleoid
• Porous cell wall around cell membrane, can be spherical (coccus),
spiral (spirillum), or rod-shaped (bacillus)
• Move using propeller-like flagella or pili.
• Existed for 2 billion years before eukaryotes evolved.
• Very successful: over 5,000,000,000,000,000,000,000,000,000,000
cells prokaryotic cells exist on Earth right now.
• Can be found in our bodies, in hot hydrothermal vents, glacial ice,
acidic springs, and even the driest desert.
• Four different ways of meeting their energy requirements: 1)
photoautotrophic, chemoheterotrophic, photoheterotrophic,
chemoautotrophic
PROKARYOTIC REPRODUCTION
• Many divide every 20 minutes by a process called prokaryotic or
binary fission, which yields 2 equal sized, genetically identical
daughter cells.
• Don’t sexually reproduce but can transfer genetic material in the form
of a plasmid using a sex pilus in a process called conjugation.
• In addition, bacteria can also take up a plasmid from their
environment in a process called bacterial transformation.
• Finally, viruses that infect bacteria, called bacteriophages, can move
genes from one cell to another, changing the bacterial genome.
PROKARYOTIC CLASSIFICATION
• Classification can be difficult since bacteria can exchange gene
between species, even with those in the other domain, and because
many cannot be grown in a lab.
• Scientists define prokaryotic species by focusing on shared
ancestry as revealed by gene sequence studies and shared
phenotypic traits.
• Are also categorized into strains within species based upon specific
traits.
• Eukaryotes are more closely related to Archaebacteria than
Bacteria.
DOMAIN: ARCHAE (ARCHAEBACTERIA)
• More recently discovered but less well known since they can live in
very extreme environments.
• Extreme thermophiles: can live in very hot places
• Extreme halophiles: can live in very salty water
• Methanogens: make methane; cannot tolerate oxygen; live in our
guts and cause us to “pass gas”
• Abundant in deep ocean
• Not particularly harmful to humans
DOMAIN: BACTERIA (EUBACTERIA)
• More diverse and better studied
• Plasma membrane, cell wall, and flagelle are built differently than
archaens.
• Genetically distinct from archae
• Includes cyanobacteria that are the ancestors of chloroplasts
• Cyanobacteria not only produce oxygen but also participate in the
nitrogen available to plants so that it can enter living things-called
nitrogen fixation.
• Bacteria also serve as decomposers, breaking down organic
matter and turning it back into its inorganic subunits.
• Beneficial prokaryotes that normally live in or on our body are
referred to as our normal flora.
DISEASE-CAUSING BACTERIA
• Called pathogens, organisms that infect other species and cause
diseases
• An animal that transmits a pathogen is called a vector (for
example, a tick is the vector that transmits Lyme disease).
• Sexually transmitted bacterial diseases include: gonorrhea,
syphilis, and chlamydia.
• May also be taken in when eating or drinking contaminated food.
• Many very harmful pathogens (such as anthrax and the bacterium
that causes lockjaw) can form spores, which is a dormant state that
the cells enter until conditions are more favorable.
• These spores (also called endospores) can survive boiling,
irradiation, and dessication.
PROTISTS
• Most are single-celled; a few are colonial or multi-cellular
• Many are photoautotrophs, but some are predators, parasites, or
decomposers
• Are very diverse, consist of may different lineages
• Many lineages are more closely related to plants, animals, or fungi
than they are to each other.
PROTISTS
FLAGELLATED PROTOZOA
FORAMINIFERANS
•
Heterotrophic protists that live as
single cells
•
No cell wall
• Single-celled predators that
secrete a shell of calcium
carbonate
•
Have one or more flagella
•
Have a pellicle (layer of elastic
proteins) that help cell keep its shape
•
Include Trichomonas (STD), Giardia,
Trypanosomes (parasitic), Euglena
(freshwater)
•
Euglena have contractile vacuoles
that allow water that diffuses into cell
to be pumped back out.
• Live on the sea floor
• Some are plankton,
microscopic organisms that
drift or swim in the open sea
• These plankton often have
photosynthetic protists that
live inside their cells
PROTISTS
CILIATES
DINOFLAGELLATES
• Ciliated protozoans
•
Means “whirling flagellate”
•
Single-celled with two flagellacell
rotates as it moves forward
•
Deposits cellulose beneath cell
membrane, forming think protective
plates
•
Live in freshwater and ocean
•
May be predators, parasites, or
photosynthetic
•
Some are bioluminescent, converting
ATP into light
•
One of the protists that undergoes
algal bloom in nutrient-enriched
waters
• No cell wall
• Have many cilia
• Most are predators that
feed on bacteria, algae, or
one another
• Live in the gut of
mammalian grazers,
helping them to digest
plant material
PROTISTS
APICOMPLEXANS
WATER MOLD, DIATOMS,
BROWN ALGAE
• Parasitic protists
•
Water molds are heterotrophs also
called oomycetes
• Also called sporozoans
•
Most water molds decompose
organic debris and dead organisms
in water but some are parasites that
have a significant economic impact
•
Diatoms have a two-part silica shell
that fit together like a shoe box
•
Deposits left by these millions of
years ago are quarried to make
filters, abrasive cleaners and
insecticides
•
Brown algae are multicellular, live
in temperate or cool seas
•
Range from microscopic filaments to
giant kelps that stand 100 feet tall in
coastal waters
•
Both diatoms and brown algae are
photosynthetic
• Possess a complex of
microtubules at their
apical (top) end that
allows them to enter a
host
• Example: malaria
PROTISTS
RED ALGAE AND GREEN
ALGAE
AMOEBOZOANS
•
Most red algae are multicellular
and live in tropical seas
•
No cell wall, shell, or pellicle so they
can change their shape
•
Have a branching structure
•
•
Some have calcium carbonate
cell walls and are a component of
coral reefs
Engulf prey and move by extending
lobes of cytoplasm called
pseudopods
•
Single-celled
•
Plasmodial slime molds are flat, slimy
masses that hang over logs on forest
floor that feeds on bacteria and is
composed of a single cell with 100’s of
nuclei
•
Cellular slime molds are individual
amoeba-like cells
•
When food runs out, cells come
together, forming a “slug” that migrates
in response to light and heat
•
Cellular slime molds are used in
research to research how signaling
pathways of multicellular animals that
are responsible for coordinated
behavior evolved.
•
Tinted red to black due to
pigments called phycobilins,
allowing them to carry out
photosynthesis in deeper waters
•
Green algae has single-celled,
colonial, and multicellular forms
•
Can be found in fresh water
mostly
•
Both red and green algae contain
chloroplasts and so they share a
common ancestor.
•
One lineage of green algae gave
rise to land plants
VIRUSES
•
A non-cellular infectious agent
•
Made of a protein coat wrapped around genetic material (RNA or DNA)
•
Can only replicate inside of living cells
•
Inserts its genetic material into the DAN of the host cell, essentially “hijacking” the
host cell’s cellular machinery to reproduce itself.
•
Each virus can only infect certain hosts
•
Bacteriophages infect prokaryotes
•
Since plants have thick cell wall, plant viruses can generally only infect a plant after
insects or pruning has created a wound where the virus can enter.
•
Adenoviruses are naked (no protein coat) viruses that infect animals
•
Human upper respiratory problems, hepatitis, polio, the common cold, and warts are
usually caused by adenoviruses
•
Usually, however, animal viruses have a coat that is composed of membrane derived
from the host cell.
•
Herpes, AIDS, rabies, rubella, bronchitis, mums, measles, yellow fever, and West
Nile are caused by enveloped viruses.
VIRUSES
Bacteriophage
Herpes virus
VIRAL
MULTIPLICATION
1.
Virus attaches to host cell by binding to a protein on host cell
membrane
2.
Virus/genetic material enters host cell
3.
Viral genes direct cell to replicate the viral DNA/RNA and to build viral
proteins
4.
These components self-assemble to form new viruses
5.
The new viruses are released from host cell and go to infect new cells
and repeat the cycle
•
Bacteriophages use two different pathways to get out of host cells
1.
2.
Lytic pathway: virus causes host cell to produce enzymes that
cause cell lysis; cell membrane breaks up, viruses escape and host
cell is killed
Lysogenic pathway: viral DNA becomes incorporated into host cell
DNA and is copied and passed on to descendant cells, awaiting a
signal to enter the lytic cycle
VIRAL ORIGINS
•
Three hypotheses:
1.
2.
3.
Viruses were parasites inside of other cells and became unable to
survive on their own.
Viruses are genetic elements that escaped from cells.
Viruses represent a separate evolutionary branch; they arose
independently from replicating molecules that preceded the origin of
life.
•
Viruses infect organisms in all three domains of life.
•
Viruses usually decrease a host’s ability to survive and reproduce.
•
Though this is bad new for the host, it can actually be beneficial.
•
It keeps population numbers of cyanobacteria in check in the ocean.
•
Viruses benefit humans when they target bacteria that cause human
disease or insects that eat our crops.
•
“The enemy of my enemy is my friend.”
•
Virus-based pesticides are safer than chemical ones because viruses are
specific for their hosts and they do not persist in the environment.