Eukaryotic Cells

BIO 224

Intro to Molecular and Cell Biology

Cellular Origins

• First life appeared nearly 4 billion years ago

• Mechanism of origin is speculation, events have not been reproduced in a lab

• Experiments provide possible clues to processes of life

• Spontaneous polymerization of organic molecules was first suggested in the 1920s

Prebiotic Conditions

• Earth’s early atmosphere was thought to contain N2, CO2, H2, H2S, CO, and little or no free oxygen

• Conditions were favorable to spontaneous formation of organic molecules by addition of energy source

• Energy from sunlight or electrical discharge could promote molecule formation

Stanley Miller’s Experiments

• In the 1950s Stanley Miller produced organic molecules from other molecules in vitro

• Atmospheric conditions were meant to mirror those of prebiotic Earth

• A mixture of H2, CH4, NH3, and H2O was combined in the presence of electrical sparks and molecules including amino acids were produced

Spontaneous formation of organic molecules

Increasing Complexity

• Basic monomers may be spontaneously polymerized into macromolecules in the laboratory– Heating dry amino acid mixtures will result in

polypeptide formation

• Macromolecules are necessary for living cells• Self-replication is an essential characteristic

that life-producing macromolecules must possess

Self-Directed Replication

• For reproduction to have occurred, molecules capable of self-directed replication would have been necessary

• Nucleic acids are the only modern-day informational molecules capable of self-directed replication

• In the 1980s RNA was discovered to be capable of serving as both a template for and able to catalyze its own replication

Self-replication of RNA

The RNA World

• RNA was thought to be the initial genetic system

• Early stages of chemical evolution are thought to have been based on self-replicating RNA molecules

• Interactions between RNA and AAs were thought to have evolved into the genetic code, and DNA replaced RNA

Origin of Cells

• First cells thought to have come from enclosure of self-replicating RNA in a phospholipid membrane

• Enclosing RNA in a membrane maintained a self-reproducing unit

• RNA directed protein synthesis may have concurrently evolved, allowing RNA to encode proteins for the first cell

Enclosure of self-replicating RNA in a phospholipid membrane

Evolution of Cells

• Cells evolved mechanisms for energy generation and synthesis of necessary molecules

• ATP became the primary source of metabolic energy

• Metabolic pathway development is thought to have evolved in stages corresponding to glycolysis, photosynthesis, and oxidative metabolism

Generation of metabolic energy

Evolution of Metabolism

• Glycolysis is carried out in all organisms, consistent with its being the earliest– Allowed use of organic molecules for energy

in absence of O2• Development of photosynthesis allowed

cells to use sunlight to produce energy– Release of free O2 contributed to changes in

the atmosphere• Changed atmosphere allowed for

development of oxidative metabolism

Present-Day Prokaryotes

• Archaebacteria and eubacteria diverged early in evolution

• Archaebacteria live in extreme environments similar to what existed on primitive earth

• Eubacteria include common bacteria that live in wide range of environments

Origin of Eukaryotes

• The acquisition of membrane-bound organelles led to the evolution of eukaryotic cells

• Eukaryotic cells are thought to have evolved through endosymbiosis– Prokaryote cells living inside eukaryote precursors

• Genome thought to have arisen from a fusion of archaebacterial and eubacterial genomes

• Theory is supported by studies of mitochondria and chloroplasts, thought to have come from eubacteria living in larger cells

Endosymbiosis

• Theory of eukaryotic cell origin widely accepted• Mitochondria are thought to have evolved from

aerobic eubacteria– Their entry into cells allowed anaerobic organisms to

become aerobic• Chloroplasts are thought to have evolved from

photosynthetic eubacteria– Their entry into cells allowed some organisms to

become photosynthetic• Most of the genes of these symbiotic partners

became incorporated into the nuclear genome, leaving only fragments behind

Evolution of Eukaryotes

• Exact origin of cells is not settled in evolutionary studies• DNA sequence studies indicate that archaebacteria and

eubacteria are very different from each other• Both are completely different from present eukaryotes• Data indicate that two lines diverged from a common

prokaryote ancestor, leading to present archaebacteria and eubacteria

• Data have been inconclusive in determining eukaryote lineage

• Some genes favor those of eubacteria, where others share more similarity with archaebacterial genes

Evolution of Eukaryotes

• A new hypothesis proposes a fusion of archaebacterial and eubacterial genomes that led to the mosaic nature of the eukaryote genome

• The proposed mechanism is endosymbiosis between a eubacterium and archaebacterium followed by fusion of the genomes

Eukaryotic cells

• Surrounded by a plasma membrane• Have a variety of membrane-enclosed organelles• Contain ribosomes for protein synthesis• Have a nucleus enclosing their genetic material• DNA molecules are linear and bind with proteins to

create chromatin: building blocks of chromosomes• Have mitochondria for oxidative metabolism

Eukaryotic Cells

• Some have chloroplasts for photosynthesis• Peroxisomes are the site of oxidative reactions• Lysosomes provide compartments for digestion

of macromolecules• Have vacuoles for digestion and storage• Have a membrane-derived endoplasmic

reticulum for protein processing, transport and synthesis of lipids, and metabolism of various substances

Eukaryotic Cells

• Have a Golgi apparatus to sort and transport proteins destined for secretion, serve as a site of lipid synthesis, and (in plant cells) as the site of synthesis of some of the polysaccharides that compose the cell wall

• Are organized with an internal cytoskeleton of protein filaments responsible for cellular movement and intracellular transport and positioning of organelles and chromosomes

Mitochondria and Chloroplasts

• Similar in size to bacteria• Reproduce by fission (splitting)• Both have their own DNA to encode a portion of

their components• Their DNA is replicated every time they divide• Their genes are transcribed in the organelle and

translated on organelle ribosomes• Their ribosomes and rRNAs are more like those

of bacteria than those of eukaryotes

Simple Eukaryotes

• Some eukaryotes are single celled

• Yeasts are the simplest eukaryotes– Smaller and simpler than

plant or animal cells

• Other unicellular eukaryotes are more complex – As much DNA as human cells– Consume other organisms,

photosynthesize

Evolution of Multicellular Organisms

• Evolved from unicellular organisms over 1BYA

• Colonial organisms may represent a transition state from unicellular to multicellular organisms

• Possible precursors of modern-day plants• Cell specialization and division of labor

eventually led to transition from unicellular colonies to multicellular organisms

Specialized Plant Cells• Ground tissue

– Contains parenchyma cells• Carry out most metabolic reactions

– Contains collenchyma and sclerenchyma cells• Have thick cell walls to provide support

• Dermal tissue– Contains epidermal cells

• Covers plant surface• Forms a protective coat and allows nutrient absorption

• Vascular Tissue– Contains xylem cells

• Transport water throughout the plant– Contains phloem cells

• Transports nutrients throughout the plant

Specialized Animal Cells

• More diverse than plant cells• Humans have over 200 cell types making

up 4 general tissue types• Epithelial tissue, connective tissue,

nervous tissue, and muscle tissue• Cells making up the particular tissues are

specialized to perform various functions of those tissues

Animal Cells

• Epithelial tissue– Cells form sheets to cover body surfaces and

internal organs– Specialized for many functions: protection,

absorption, secretion• Connective tissue

– Composed of many cell types• Bone, cartilage, adipose, blood

– Provide structure, support, transport, insulation, and protection

Animal Cells

• Nervous tissue– Made of neurons and supporting cells

• Specialized for transport of signals through the body

– Sensory cells receive external signals• Muscle tissue

– Responsible for movement and production of force

Evolution of Diversity

• Animal evolution involved development of much cellular diversity and specialization

• Cell growth and differentiation from a single fertilized egg to a complex organism involves many factors

• Cell biology aims to understand the processes involved

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