The origin of (Eukaryotic) cells

Phylogenetic relations between organisms Five kingdoms of life

The Cell: The Basic Unit of Life The cell theory states that: Cells are the fundamental units of life. All organisms are composed of cells. All cells come from preexisting cells. If all organisms evolved from an ancient original cell, can we propose the order of events?

Phylogenetic relations between organisms Phylogenetic relationships can be inferred from common characters

Mutations drive evolution Mutation is the molecular basis of change and speciation. The comparative study of genomic sequences is now used to infer phylogenetic relations. Genome based analysis confirms many of the traditional classifications of organisms.

Phylogenetic relations between organisms Sequence comparison between plants

Phylogenetic relations between organisms Based on sequence analysis biologists now categorize all life into three domains: Bacteria, Archaea, and Eukarya. Prokaryotes ~ 3.5 billion years ago

Three Domains All three domains had a single common ancestor. Present-day Archaea share a more recent common ancestor with eukaryotes than they do with bacteria. The common ancestor of all three domains was prokaryotic. Prokaryotes were the only life-forms for billions of years. The three domains are products of billions of years of natural selection.

Comparison between the three domains of life Prokaryotes Eukaryotes

EARLY EARTH AND THE ORIGIN OF LIFE Life began on a young Earth Planet Earth formed some 4.6 billion years ago The early atmosphere probably contained H2O, CO, CO2, N2, and some CH4 Volcanic activity, lightning, and UV radiation were intense

Stanely Miller re-creating his 1953 experiment Stanley Miller’s experiments showed that organic molecules could have arisen on a lifeless earth

Stanely Miller re-creating his 1953 experiment Simulations of such conditions produced amino acids, sugars, lipids, and the nitrogenous bases found in DNA and RNA CH4 “Atmosphere” Water vapor Electrode NH3 H2 Condenser Cold water Cooled water containing organic molecules H2O “Sea” Sample for chemical analysis

The first polymers may have formed on hot rocks or clay Organic polymers such as proteins and nucleic acids May have polymerized on hot rocks High temperatures, electrically charged sites on the clay particle and metal atoms, such as iron and zinc, functioned and catalysts of the polymerization reaction.

A hypothesis for the origin of the first genes Ribozymes G A C U 1 2 Formation of short RNA polymers: simple “genes” Assembly of a complementary RNA chain, the first step in replication of the original “gene” Monomers The RNA world

The beginnings of molecular cooperation Self-replication of RNA Self-replicating RNA acts as template on which poly- peptide forms. Polypeptide acts as primitive enzyme that aids RNA replication. RNA Polypeptide

Phospholipids Form a Bilayer

Protobionts Figures\Chapter04\High-Res\life7e-fig-04-01-0.jpg

Microscopic spheres made of phospholipids LM 650

Cooperation among membrane-enclosed macromolecules Semi-permeable Protobiont Membrane Polypeptide RNA Primitive metabolism in a protected environment

The Origin of the Eukaryotic Cell The evolution of eukaryotic cells included the following components: The origin of a flexible cell surface The origin of a cytoskeleton The origin of a nuclear envelope The appearance of digestive vesicles The endosymbiotic acquisition of certain organelles

The Origin of the Eukaryotic Cell The first step toward the eukaryotic condition may have been the loss of the cell wall by an ancestral prokaryotic cell. A surface that is flexible enough to allow for infolding lets the cell exchange materials with its environment rapidly enough to sustain a larger volume and more rapid metabolism. A flexible surface also allows endocytosis. An infolded plasma membrane attached to a chromosome within an ancestral prokaryote may have led to the formation of the nuclear envelope.

The Origin of the Eukaryotic Cell The early steps in the evolution of the eukaryotic cell likely included three advances: The formation of ribosome-studded internal membranes, some of which surrounded the DNA The appearance of a cytoskeleton The evolution of digestive vesicles

From Prokaryotic Cell to Eukaryotic Cell (Part 1)

Membrane Infolding

The Origin of the Eukaryotic Cell From an intermediate kind of cell, the next advance was likely to have been a motile phagocyte. The first true eukaryotic cell possessed a cytoskeleton and a nuclear envelope; it also may have had an associated endoplasmic reticulum and Golgi apparatus and perhaps one or more flagella.

The Origin of the Eukaryotic Cell During the early stages of eukaryotic evolution, the O2 levels in the atmosphere were increasing as a result of the photosynthetic activities of the cyanobacteria. Most living things were unable to tolerate this new aerobic, oxidizing environment, but some prokaryotes and ancient phagocytes were able to survive. One hypothesis suggests that the key to the survival of the early phagocytes was the ingestion of a prokaryote that became symbiotic and evolved into the peroxisomes of today.

From Prokaryotic Cell to Eukaryotic Cell (Part 1) Dual membrane support endosymbiosis

The Origin of the Eukaryotic Cell