Download Phylogenetic tree Basic properties of cells

Cells
Phylogenetic tree
Basic properties of cells
Last time: Defining line
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Most basic definition of life:
Life is something that can reproduce and evolve through natural
selection.
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Perhaps, for the purpose of searching for life elsewhere, we don’t need to
have a perfect definition:
◦  Just understand the main properties and the expected biosignatures
Phylogenetic Tree of Life - Evolution
Phylogeny = study of the evolution of life.
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Phylogenetic tree of life based on small subunit RNA sequence.
Illustrates the biochemical and genetic relationships between the
different domains of life.
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Based of cellular biochemistry, life can be classified into 3 domains of life:
◦  Bacteria,
◦  Archaea, and
◦  Eukarya
Prokaryotes
Eukaryotes
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Different classification compared to the old classification into
“kingdoms” based solely on structural and physiological differences.
◦  Branch lengths are a measure of the amount of genetic difference.
◦  When two lines converge – two types of organisms derive from a
common ancestor
At the root is the common ancestor of all life on Earth.
Microscopic life
Microscopic structure cells on Earth:
1.  Without a nucleus – prokaryotes.
2.  With a nucleus – eukaryotes.
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Cell nucleus: internal membrane that effectively walls off the
genetic material (DNA) from the rest of the cell.
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All prokaryotes are unicellular (bacteria).
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Eukaryotes can be:
◦  unicellular (amoeba)
◦  multicellular (humans, plants, animals).
◦  All multicellular organisms are eukaryotes.
Note:
!  Microbes (prokaryotes) are the form of life on earth that show the
greatest diversity
!  Plants and animals are two small branches of the domain Eukarya.
!  Thermophilic (heat-loving) organisms populate the deepest branches
of the tree, suggesting they are in the evolutionary sense closest to
the origin of life.
Microscopic life is the dominant life form on Earth
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Some are harmful
◦  E. Coli, salmonella, pneumonia, helicobacter pylori (gastric
ulster), nanobacteria (kidney stones), streptoccal bacteria
Helicobacter
Pylori
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Some are crucial for our survival:
◦  Intestinal bacteria provide vitamins
◦  Cycling carbon (decomposing) organic matter, soil and
atmospheres
◦  Fermentation cheese, genetic engineering, antibiotics
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Total mass of microbes in the oceans is about 5,000 times larger
that of all humans!
Bacteria and Archaea used to be grouped together as prokaryotes, based on
lack of cell nuclei
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Now they are different domains because of different biochemistry
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For example, Bacteria and Archaea have different types of lipid structures
in their cell membranes and synthesize proteins differently.
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Archaea seems to be closer related to Eukarya than to Bacteria.
Ideal would be to sequence the entire genome of all species.
But most molecular evolution studies compare a limited number of genes
from a large number of species.
Lateral gene transfer
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The transfer of genes horizontally in the
phylogenetic tree, acquisition of genes
from other species
◦  Makes tracing of species difficult!
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Seems to be more common in Bacteria
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Many enzymes in the metabolism of
eukaryotes are of bacterial and not
Archaeal origin, in spite of closer
relationship between Eukarya and
Archaea.
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Lots of current research concentrates
on the importance of lateral gene
transfer
Cells – basic units of life
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Basic biological structures of life are called cells
◦  Single cells organisms…
◦  up through very complex organism with trillions of cells working
cooperatively, having specialized tasks
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Life elsewhere may be composed of cells
◦  might not have the same biochemistry as on Earth
Biochemistry and origin
All cells used in life on Earth based on the same biochemistry suggesting
that all life on Earth shares a common ancestor
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Only left handed amino acids (space has both)
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Use the same molecule (ATP) to store and release energy
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Passes hereditary information in the same way with DNA
Living cells on the Earth
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Living organisms made of cells
◦  Earth life cells contain O, C, H, N
◦  Even though O dominates, the cell structure depends on carbon –
carbon based life.
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All life forms on Earth are built upon cell structure
◦  Cell walls/membranes
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All life follows the same basic biochemistry
◦  Using about 20 of 70+ known amino acids
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All life on Earth is left-handed on a chemical level
Earth life is carbon based
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Life on Earth made of more
than 20 elements
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More than 96% of the mass
of living cells are made of C,
H, O and N
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The molecule that accounts
for cell structure and
function is C
Carbon versus Silicon based life
We think life elsewhere is also carbon based:
C ideal as fundamental building block of biological molecules
Ethylene
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Easily forms chemical bonds with itself and other
elements, allowing a wide variety of complex molecules to
form
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Max number of bonds is 4, can form single, double and
even triple bonds
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Chemists refer to molecules containing carbon atoms as
organic
Ethane
Silicon based life – is it out there?
Is Si an obvious alternative to C as a building block for biological molecules?
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It has a similar electronic structure to C, forming max 4 bonds – but
does it have a similar chemistry?
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Reacts faster than C, suitable for colder environments
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Si forms it strongest bonds with O.
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Si more abundant in the Earth’s crust than C
Actually, seems unlikely:
1. 
Si-Si bonds are much weaker than C-C bonds
2. 
Si does not form multiple bonds with itself, limiting the complexity of
molecules it can form
3. 
Due to its size, it is much more difficult to form bonds with other
atoms for Si than for C
4. 
SiO2 is a high melting point solid and cannot be used to transfer Si
atoms like gaseous CO2 can be used to transport C atoms
5. 
Si not soluble in water (unstable, bonds break)
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C (organic) molecules found in space – meteoroids, interstellar dust,
Jovian satellites – Si is rare
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However, Si is more abundant in Earth’s crust than C, yet Earth’s life is
based on C
◦  Perhaps this shows how much easier C has to form bonds with other
atoms.
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Boron?
◦  Other possibilities exist, but all appear to have flaws
◦  Boron, e.g., is very rare in the universe
Molecular components of cells
Four molecular components: Carbohydrates, lipids, proteins and nucleic
acids.
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Carbohydrates
◦  Provides the main structure of cell – e.g., cellulose
◦  Can store energy - sugar, starch
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Lipids
◦  Can store energy – fats
◦  Needed for life since they form the cell membranes, protecting the
cell.
◦  Can form spontaneously in water – crucial for life to form!
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Proteins
◦  Perform many different functions in a cell
◦  For example, enzymes are responsible for biochemical reactions like
copying DNA
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Proteins are built of smaller molecules named amino acids
◦  Life on Earth builds or adds on 20 different versions of the ‘R’ group
Carboxyl group
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Signs of common origin of life on Earth
◦  Same sets of amino acids used in all life forms
◦  Only left-handed amino acids found on Earth
◦  In other circumstances like non-living material (on earth) and
molecules in space we find a mix between right and left-handed ones
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Nucleic acids
◦  DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)
◦  Determines how cell function (change the DNA and you change the
nature of the organism)
◦  Changes of DNA allows organisms to evolve