BIOL214 TOPIC 5/CHAPTER 25

Two things required for origin of cellular life

• continual source of energy (sun)

• temp range in which liquid water can form

• earliest events uncertain

  • probably include the development of protocells

• geological and chemical evidence: cellular organisms present in archaean geological eon

  • 3.4 bil yrs ago

Photosynthetic prokaryotes

• means oxygen enriched atmosphere

• may have been necessary for the development of the first eukaryotic cells

• eukaryotic cells appeared about 1.7 bil yrs ago

• animals about 1 bil yrs after that

Characteristics of Present Day Living Cells

• boundary membrane separating cell interior from exterior

• one or more nucleic acid molecules whose nucleotide sequences make up genetic information that determines properties of cell

• system for reading genetic information to make RNA molecules, proteins, and other biological molecules

• metabolic system to provide energy for these activities

Start of Natural Selection

• evolution of structures as complex as even the simplest living cells could not have occurred without natural selection

  • selection for efficient reproduction

  • acted on single molecules or complexes of molecules

Two potential origins

• Extraterrestial

• Abiogenesis

Abiogenesis

• most scientists under assumption that origin of life came from non living matter

  • chemical processes would be the same as those operating today

  • chemical environment different

• testable = can mimic conditions in lab

Extraterrestial Origin

• Panspermia

  • hypothesis that suggests life exists throughout the universe and it can travel between planets and star systems

  • life got carried to earth from somewhere else

• analysis of meteorites shown they contain characteristic of living organisms

• largely unstable

  • doesn’t really explain beginning of life

Early Earth Conditions

• Formation of Earth led to planet with three layers

  • core surrounded by mantel = rich in iron

  • crust = mainly silicates (mineral and chemical compound made mostly of silicon and oxygen)

• First atmosphere formed partially from volcanic activity

• released CO2, H2, N2, CH4, NH3, H20

• atmosphere then diff from now (mostly N2 and O2)

• Natural sources of energy caused chemical bonds to break and reform

  • through sunlight, electrical discharge, volcanic activity

  • created variety of molecules in atmosphere and oceans

Liquid water and life building blocks

• slow down of volcanic activity and extraterrestrial impacts

  • decrease in surface temp

  • water vapor becomes liquids (form oceans, lakes, river)

  • 2 important considerations

    • gravity

    • distance from sun

• one or more sources of energy must have enabled inorganic molecules to react to form simple organic molecules (building blocks for life)

  • synthesis of organic molceuls = steady release of energy and right mix of inoranic molecules

  • molecules synthesized faster than they were broken down

  • implication is that organic molecules built up

Oparin Haldane Hypothesis

• assumed Earth’s early atmosphere very diff from today’s atmosphere (important for hypothesis)

  • strongly reducing atmosphere

  • substances mostly single covalent bonds

    • bonds weaker, higher potential, more reaction w molecules

• Oparin and haldane said molecules of early atmosphere reacted w each other to create organic molecules

  • energy from solar energy and other resources (lightning)

• reaction produced vast quantities of organic molecules

  • two main reasons of accumulation

    • lack of chemical attack by oxygen

    • negligible decay by microorganisms

  • so breakdown decreased

• build up → primordial soup

Support for Oparin Haldane Hypothesis

• Miller Urey Experiment

  • Lab experiment of reducing atmosphere H2, CH4, NH3, H2O

  • conditions assumed by hypothesis

• in 1 week, 15% of carbon was organic molecule

  • included amino acids

• showed that organic molecules could be synthesized abiotically

  • didn’t need biological enzymes

  • supported hypothesis

• but later research showed that Earth’s early atmosphere was neutral (not reducing or oxidizing)

  • made up of CO2, N2, CO, H2S

New Hypothesis about First Synthesis Location

• “Black Smoker” Hydrothermal Vents

  • bursts of mineral-rich water of temps 400°C by submarine volcanoes

  • produced energy rich iron sulfur compounds

  • chem conditions necessary for life to originate

• Oceanic Hydrothermal Vents

  • reducing conditions

  • lots of chemicals essential for life

  • complex ecosystems associated

• Hydrothermal vents produce more organic material than miller urey experiment

  • but critics argue temp too high, molecules unstable and destroyed as soon as they are made

  • but if it is in approximate area of the hydrothermal vent, conditions might allow for some stability in organic molecules.

Alkaline Hydrothermal Vents

• very promising candidate for origin of life

  • found in yr 2000

• temps of only 30 - 90 °C, pretty alkalinei ph levels

  • contrasts with hot and acidic black smokers

  • characteristics compatible with biological metabolism

• thought to produce steady release of energy

  • longer than black smoker hydrothermal vents

Origin of Cells

• Dehydration synthesis: turn organic building blocks → macromolecules

  • subunits → larger molecule by removing water molecule

  • evaporation + condensation reaction can produce

    • polypeptide chains from amino acids

    • polysaccharides from glucose and other monosaccharides

    • nucleotides and nucleic acids from nitrogenous bases, ribose, and phosphates

Reproduction of Genetic Information

• smallest bacterial genoms have hundreds of genes

  • earlier forms of life much simpler

  • needs to be able to reproduce

• earliest genetic material prob not DNA

  • maybe another polymer (RNA)

  • molecular replicators (store and reproduce)

• natural selection at play early on

  • the better mechanism/material for reproducing genetic material will be chosen

RNA World Model

• says first genes and enzymes were RNA

  • only molecule that could store genetic info and catalyze chemical reactions

  • bc it can do both, allows for life in much simpler systems

  • complexity added over time (DNA)

• DNA creation could be bc of random removal of oxygen atom from ribose subunit of RNA nucleotides

  • found that DNA favored by selection (form longer polymers and more stability)

  • RNA became intermediate step

Central Dogma

• Flow of genetic material from DNA → RNA → Protein

Membrane Evolution

• All life today made up of cells surrounded by lipid membrane

• most organisms have two layers of molecules

  • ie phospholipids

    • hydrophobic and hydrophillic end

• probably evolved very early

Useful membrane adaptations

• cells hold useful molecules, like genetic info

• cells maintain chemically distinct intracellular environment

• protect cells from parasitic genetic information and other unwanted molecules

Protocells

• evolution of increased genetic complexity likely would have involved several ribozymes specialized for different functions to work together and depend on each other

  • membrane enclosures help to keep community of RNA molecules together in one functional organism

• above describes protocell formation

  • primitive cell like structures that might have been precursor of cells

Early Membranes

• mix of fatty acids and other lipids from abiotic synthesis reactions

  • if in high concentrations, fatty acids form bilayer membranes spontaneously

• more permeable to other molecules

  • includes amino acids and other molecules

  • advantageous because it allows for the spread of genetic material into the cell

Geochemical Activity and Early Life

• energy required to produce and maintain organized structures

• contemporary life gets almost all energy from sun

  • but photosynthesis wasn’t around during early life

  • so earliest living organisms likely evolved close to geochemical activity

• back to alkaline hydrothermal vents

  • free energy

Last Universal Common Ancestor (LUCA)

• Two defining features

  • DNA based genome, to replicate DNA and transcribe into RNA

  • mechanism so nucleotides in DNA could code for amino acid sequences of polypeptides

• Coding mechanism included ribosomes, tRNA

• LUCA cytoplasm had oxidative system

  • supply chemical energy for protein synthesis and other mateirals

• mechanism of cell division

  • replicated DNA evenly split between daughter cells

• Enclosed by membrane

Few ways to determine Age of Life

• isotope ratios and radiometric dating

• fossil stromatolites

  • layers of carbonate or silicate rock that resemble present day stromatolites

  • formed as layers of photosynthetic prokaryotes grew and died

  • about 3.4 bya

• microfossils

  • remains of cell that has decayed and then filled in by calcium carbonate or silica

  • about 3.5 and 3.8 bya

Three Domains of Life

• Eukarya

  • includes fungi, plantae animalia, protists

• Archaea

• Bacteria

• archaea and bacteria prokayotes

Separating and Placing Archaea

• archaea membranes not phospholipids, but amiphiphilic lipids (hydrophobic + hydrophilic parts)

• resembles eukarya in some ways

  • genes often has errors

  • DNA wrapped around proteins that look like eukaryotic histones

  • RNA polymerase enzymes resemble eukaryotic RNA polymerases

  • attach amino acids to tRNA using aminoacyltrna synthease enzymes

    • more similar to enzymes in eukaryotes than bacteria

• but archaeal chromosome circular, genes arranged in operons

  • similar to bacteria

Challenges to Three domain Classification

• shows Eukaryotes are not sister group to Archaea

  • but evolved from within the Archaea Domain

• evidence that eukaryotes may be related to the common ancestor of Crenarchaeota, Thaumarchaeota, and Korarchaeota phyla

• Discovery of new superphylum of Archaea called Asgard

  • DNA sequences more similar to eukaryote DNA sequences than eukaryote DNA sequences are to other organisms

  • Suggests that eukaryotes evolved from within the Asgard superphylum

• Tree of life likely branched into two groups, bacteria and archaea

  • eukarya came much later

  • bacteria and archaea now primary domains

Endosymbiosis

• Mitochondria and Chloroplasts entered early eukaryotic cells by endosymbiosis

• Mitochondria descended from Rickettsia parasite

  • incorporated into cells early in the evolution of eukaryotes

• chloroplasts derived from cyanobacteria

  • after mitochondria

  • non photosynthetic eukaryote ingested by phagocytosis phootosynthetic bacteria

Eukaryotic Endomembrane System

• Evolved from Plasma Membrane

• organelles don’t have their own DNA and don’t replicate by binary fission

• Membrane composed of the same phospholipids

  • exchanged through exocytosis and endocytosis

  • hypothesized initial formation due to endocytosis

Eukarya as second domain

• eukaryotees evolved from group of archaea that fed by predation

  • acquired mitochondria through endosymbiosis

• comparisons of DNA show many eukaryotic genes more closely related to bacterial genes than archaeal genes

  • eukaryotes not simply branch of archaea

  • union of genetic toolkit of bacteria and archaea

Genetic Distance Method

• genetic distance between closely related species is smaller than genetic distance between distantly related species

  • more closely related species have less time to acquire mutations

• trees are made by making multiple comparisons of genetic distance between pairs of species, and then between groups

• not as powerful as maximum likelihood method

  • doesn’t depend on assumptions like likelihood of mutations

  • less computing power

Molecular Clocks

• If mutations accumulate at reasonably constant rate, differences in DNA sequence of different organisms can serve as a molecular clock

• a technique for dating time of divergence of two species or lineages, depending on the amount of molecular sequence difference between them

• large distance = divergence in distant past, small distance = divergence recent

• different molecules have their own rates of evolutionary change, so every molecule is an independent clock

• look at degree of genetic difference between species in relation to their time of divergence from fossil record to calibrate clocks

Phylogenetic Trees and Comparative Method

• Compare characteristics of different species to assess homology of their similarities

Molecular Phylogenetic Analyses

• Used to pinpoint disease origins

Horizontal Gene Transfer (HGT)

• differs from vertical gene transfer (genetic material passed on through reproduction)

• 3 mechanisms in bacteria

  • conjugation

  • transformation

  • transduction

• recent work indicated HGT has occured often in history of life

  • even between species in different domains

  • ~20% or more of genes found in contemporary bacteria entered through HGT