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Home , Mitochondrion

Oceans: the cradle of ?

• Highest diversity of life, particularly archae, , and The origin of life • Will start discussion of life in the ocean with • are also believed to be the first cellular on earth • How might life on earth have started? • From molecules to the first

Cells: a sense of scale

Head of a needle Fig 25-UN8 Simple biological molecules can

1.2 bya: form under prebiotic conditions First multicellular 535–525 mya: Cambrian explosion 500 mya: (great increase • Conditions on earth during first billion 2.1 bya: Colonization in diversity of First eukaryotes (single-celled) of land by forms) fungi, years and animals 3.5 billion years ago (bya): • No free , no ozone layer to absorb First prokaryotes (single-celled) UV radiation

• Atmosphere = CO 2, CH 4, NH 3, H 2

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Simple organic molecules Laboratory are likely to “created” have been molecules produced under such conditions

• nucleobases Sugars such as Formation of polymers from Spontaneous polymerization of simple organic compounds random polymers • Amino acids (aa) can join by forming bonds • (nt) can join by forming phosphodiester bonds • In present day cells - polypeptides () are from aa, polynucleotides (RNA and DNA) are from nt.

Earliest polymers were of variable length and random Autocatalytic systems sequence • Mechanism of formation • Origin of life requires that some polymers – Heating of dry organic compounds must have a crucial property: the ability to – Catalytic activity of inorganic polyphosphates catalyze reactions that lead to the or other mineral catalysts production of more molecules of itself. • Polymers can influence subsequent • This can be accomplished by reactions by acting as a catalyst polynucleotides

Complementary templating

• Catalytic RNA molecule • Believed to be the basis for the first autocatalytic systems • Acts as both template and catalyst • Eventually specialization where DNA is the template and proteins are catalysts Self-replicating RNA systems • Start with a catalytic RNA molecule that polymerizes nucleotides to reproduce itself

Self-replicating RNA systems The beginning of

• 3.5 to 4 billion years ago - mixture of self- • Next step: replicating RNA systems and organic family of molecule precursors mutually • Systems with different sets of polymers supportive competed for the available precursors catalytic RNA molecules. • Success depended on the accuracy and One catalyzes speed with which the copies were made the and the stability of these copies reproduction of the others

From polynucleotides to Primative RNA/ systems • Polynucleotides are well suited for • Would have an advantage over RNA only information and storage but have limited systems due to better catalysis facilitated catalytic properties by proteins • Proteins have much greater catalytic capabilities • In modern cells, synthesis of proteins is catalyzed by (proteins and RNA) from RNA templates. Membranes defined the first cell

• An illustration of a , composed of a membrane encapsulating RNA ribozymes.

Membranes defined the first cell

• The development of an outer membrane was a crucial event • Proteins synthesized by a certain species of RNA would not increase reproduction of that RNA species unless they remained in the neighborhood of the RNA • Proteins could diffuse away and benefit competing RNA molecules

The need for containment is filled by amphipathic molecules • Amphipathic - property where one part of the molecule is hydrophobic and the other part is hydrophilic. • In present-day cells, these amphipathic molecules are primarily phospholipids. Membranes are readily formed • A simple fatty acid (far left) may have been a major componenent of early cell membranes. To the right of from amphipathic molecules the fatty acid is a phospholipid, which is the primary component of modern cell membranes. Vesicles and • Example: phospholipids micelles, shown on the right, are structures that can be – Hydrophobic tail group formed by fatty acids or phospholipids. Mouse-over the vesicle or micelle to see the whole structure – Hydrophilic head group • Mix in water and membranes spontaneously form

Summary of the hypothetical evolution of the first cells

DNA based systems have All present-day cells use DNA advantages over RNA based as their hereditary material systems • DNA acts as a permanent repository of genetic information – Is found principally in a double-stranded form which is more robust and stable – If there is breakage, there is a repair mechanism that uses the intact strand as a template. • DNA templates can become more complex From self-replicating RNA to From procaryotes to eucaryotes present day cells • RNA preceded DNA and proteins, having • 1.5 billion years ago there was a transition both catalytic and genetic properties. from relatively simple procaryotic cells to • Proteins eventually became the major larger more complex eucaryotic cells. catalysts • DNA became the primary genetic material • RNA continues to function in coding for proteins (mRNA) and catalysis (rRNA).

The earliest procaryotes were Procaryotes generally have no like present day bacteria obvious internal structures • Bacteria – Simplest organisms – Spherical or rod shaped several microns long – Tough protective coat () – Plasma membrane enclosing a single cytoplasmic compartment • DNA, RNA, proteins, and small molecules

Originally, there was little need Metabolic reactions evolve for so many metabolic reactions • A bacterium growing in a simple solution • Cells with simple chemistry could survive containing must carry out and grow on the molecules in their hundreds of different reactions. surroundings. • Glucose used for chemical and as • Eventually these molecules would become a precursor for all organic molecules the limited and cells devised ways to cell requires. manufacture what they couldn’t acquire. • This led to metabolic complexity. , the oldest metabolic Cyanobacteria can fix CO 2 and pathway N2 • A strong selective advantage would • Degradation of sugar phosphates in the eventually be gained by organisms able to absence of oxygen occurs by glycolysis use C and N directly from the atmosphere. • Glycolysis is similar in all kinds of • CO 2 and N 2 are very stable and require a organisms -- suggesting an extremely large amount of energy as well as ancient origin complicated chemical reactions to convert • evolved by sequential them to organic molecules addition of new enzymatic reactions to existing ones. • In present-day organisms hundreds of chemical processes are linked to

Photosynthesis and nitrogen Atmospheric oxygen and the fixation course of evolution

• Process that used energy from sunlight

to convert CO 2 and N 2 into organic compounds. • Cyanobacteria (blue-green) algae are the most self-sufficient organisms that now exist. • The metabolic activity of these organisms set the stage for the evolution of more complex organisms

Origin of eucaryotic cells with Aerobic respiration distinct • Accumulation of oxygen in the atmosphere • What happened to the anaerobic led to the ability to oxidize more organisms? completely ingested molecules – Many found low oxygen niches • Also oxygen was toxic to many early • Some acquired aerobic cells as anaerobic organisms intracellular symbionts giving rise to • By around 1.5 billion years ago, organisms eucaryotes with aerobic respiration became widespread Features of eucaryotic cells Features of eucaryotic cells

• Outside the nucleus is the where most of the cell’s metabolic • Have a reactions occur. nucleus containing • The cytoplasm contains distinctive most of the organelles cell’s DNA – Prominent organelles are and mitochondria – Each is enclosed in its own membrane

Eucaryotic cells depend on mitochondria for aerobic Mitochondria respiration • Similar to free-living prokaryotic cells • Mitochondria • Resemble bacteria in size and shape are found in • Contain DNA, make protein, and virtually all reproduce by dividing in two eukaryotic • Without mitochondria eukaryotic cells cells would be anaerobic organisms including • By engulfing mitochondria, internal oxygen plants and concentrations are kept low animals

Acquisition of mitochondria may Some present day organisms have allowed evolution of new may resemble the hypothetical features ancestral eucaryote precursor • Plasma membrane of mitochondria and • They have nuclei but lack mitochondria procaryotes is heavily committed to energy • Example the metabolism • In contrast eucaryote membrane is not. • Eucaryote membrane developed new features – Example: channels allowing electrical signaling Giardia

• Live as parasites in the guts of animals (including humans) – Low oxygen environment – Rich in nutrients allowing them to survive on inefficient anaerobic metabolism

Chloroplasts are descendants of an engulfed procaryotic cell • Similarities to cyanobacteria – Carry out – Structural resemblance (size, stacked membranes) – Reproduce by dividing – Contain DNA nearly indistinguishable from portions of a bacterial

Some present-day cells contain authentic cyanobacteria

• Cyanophora paradoxa Fig. 25-9-1 Fig. 25-9-2

Cytoplasm Plasma membrane Aerobic Ancestral DNA heterotrophic prokaryote prokaryote

Mitochondrion Nucleus Ancestral heterotrophic

Fig. 25-9-3 Fig. 25-9-4 Cytoplasm Plasma membrane Ancestral DNA Photosynthetic prokaryote prokaryote

Endoplasmic reticulum Nucleus Nuclear envelope

Mitochondrion Aerobic heterotrophic Photosynthetic prokaryote prokaryote Mitochondrion Ancestral Mitochondrion heterotrophic Ancestral photosynthetic eukaryote Plastid eukaryote

Ancestral photosynthetic eukaryote

The postulated origin of Fig. 25-15 present-day eucaryotes

NORTH AMERICA Chicxulub Yucatán crater Peninsula • The Cretaceous mass extinction 65.5 • The presence of iridium in sedimentary million years ago separates the Mesozoic rocks suggests a meteorite impact about from the Cenozoic 65 million years ago • Organisms that went extinct include • The Chicxulub crater off the coast of about half of all marine species and many Mexico is evidence of a meteorite that terrestrial plants and animals, including dates to the same time most dinosaurs

Fig 25-UN8 Fig 25-UN9 - so - Ceno Me zoic oic ic z zo leo 1.2 bya: Pa First multicellular eukaryotes 535–525 mya: Cambrian explosion 500 mya: (great increase 2.1 bya: Colonization in diversity of First eukaryotes (single-celled) of land by animal forms) fungi, plants and animals Origin of solar system and Earth 3.5 billion years ago (bya): First prokaryotes (single-celled)

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Fig 25-UN11 - o - Ceno Mes zoic zoic c oi oz le Pa

Origin of solar system and Earth

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