A2.1: Origins Of Cells Flashcards
Explain the formation of the Earth
4.5 billion years ago:
Gravity piled gas+dust to make earth
During accretion (growth/increase by adding layers) ->
proto-earth compressed by gravity ->
increase temp ->
hot molten mass ->
Cooled to form layers of earth
Explain the origins of water on earth
As early as 4.4 billion years ago
From extra-planetary objects
Explain the conditions of prebiotic earth (natural ‘disasters’)
Frequent volcanic eruptions/meteorites/lightning/high temps
High energy/UV radiation (why its called Hadean Eon)
Lack of free oxygen -> no ozone -> +UV
Maybe helped catalyse formation of polymers
Explain the composition/orgin of the prebiotic atmosphere (1 and 2)
First atmosphere:
Start as region of escaping hydrogen and helium (main gases in the disk around sun where planets formed) -> all escape to space
Second atmosphere:
Ammonia
Methane
CO2, Water vapor (volcanoes)
Little oxygen -> reacted very quickly
High temps because of greenhouse gases
Explain Oparin and haldane’s idea of formation of early life (reducing)
Simple molecules -> complex organic molecules (lots of energy)
-> today most get energy to synthesize molecules by oxidizing nutrients in respiration/photosynthesis
Theory:
Early earth provided necessary conditions for formation of carbon compounds
- molecules present -> chemically reducing (reactive)
- reducing atmosphere -> energy from volcanic/lighting catalyze to create amino acids
After form could’ve spontaneously formed ploymers via anabolic reaction
- amino acids linked to form protein
- fatty acid concentrated enough -> membranes
- RNA nucleotides link when exposed to catalyst in clay or water
What are the requirements of life?
Has:
Physical structures of life (cells, DNA)
Physiological processes (MR H GREN)
Living things are composed of cells
Cell is the basic unit of life (small unit that is capable of using its own energy to sustain itself/stable state)
Why are viruses not considered life?
Not made of cells
Cannot keep itself in a stable state
Cannot grow or self-replicate
- viruses -> simpler subunits -> BUT can only be produced inside the host cells that they have infected
Cannot preform independent metabolism
What is the problem with cell theory (spontaneous origin of cells)?
Cell theory -> all cells are formed from pre-existing cells
Problems:
1. Cells are complex and no mechanism has been found to make cells from simpler subunits
- partially permeable membrane, genetic material, metabolic processes
- All known examples of growth are a result of cell division
- Genetic code is universal
- each of 64 codons (combin. of 3 DNA bases) produce same amino acids in translation regardless of organism
- meaning cells have arisen from division of a single common ancestor
Explain the oparin and haldane theory
- Formation of small organisms from small, inorganic, non living molecules
- Somehow monomers become ploymers (anabolic reaction)
- Somehow way of replicating genetic material to allow inheritance (formation of nucleic acid polymer (RNA))
- Formation of membranes to create different conditions inside and outside the cell
Very difficult to test hypothesis
What is the proto-cell first theory? (Divide)
Proposes that a cell-like compartment capable of basic metabolic functions arose spontaneously
-> called protocells
Initially lacked genetic material but would be able to grow/divide into daughter ‘cells’
Would eventually get genetic material (mostly likely RNA)
What is the gene-first theory? (Selection)
Proposes spontaneous development of a nucleic acid (most likely RNA) that had the ability to self replicate
Evolution by natural selection -> genetic variants -> develop into cell membrane/basic metabolic functions
What is the metabolism-first theory?
Proposes that life originated as a system of chemical reactions cable of sustaining itself
System would evolve to form cells and genetic material
Favored by scientist -> most life processes essential for existence of cell require energy from metabolic reactions
What elements do all amino acids have?
Nitrogen
Carbon
Oxygen
Hydrogen
(Occasionally sulfur too)
What were the gases in the early earths atmosphere?
Hydrogen
Nitrogen
Water vapor
Methane
Ammonia
Hydrogen sulphide
(All from volcanic activity)
No oxygen gas -> reducing atmosphere -> no oxygen gas until photosynthesizing plants
Explain/draw the setup of the miller-Urey experiment
Use diagram to check
How did miller and Urey recreate the conditions of pre-biotic earth? What were the results of the experiment?
(Step 1: Inorganic compound -> organic compound)
reducing atmosphere (low oxygen)
- water boiled to form vapor
- methane, ammonia, hydrogen
High radiation
- no oxygen -> no ozone
High temps
- result of greenhouse gases
Electrical storms
- mix of gases exposed to electrical discharge (lightning)
Results:
Mixture cooled/condensed after 1 week -> 13/20 naturally occurring simple amino acids and complete oily hydrocarbons
- 15% of carbon in organic compounds
Under hypothesized conditions -> organic molecules can be formed
What is abiogenesis?
The formation of living cells from nonliving materials
(Inorganic compounds -> organic compounds -> polymers -> self replication -> formation of cell)
What were some criticisms of the miller-Urey experiment? (Monomers)
Only worked in the absence of oxygen
Opposer believe that oxygen was present before time organic compounds were generally accepted to appear -> if true miller-Urey experiment would be invalid
May have been presence of other gases
Not continuous lightning
Different atmospheric conditions
- ex: less methane
Didn’t take into account nuclear and UV radiation
Amino acids stayed monomers in the presence of water
No nucleotides were generated
Explain step 2: assembly of organic molecules
Miller-Urey created amino acids -> but conditions generally discouraged polymerization
BUT on earth small organic monomers spontaneously formed polymers
DEEP SEA THERMAL VENTS:
Fissures in planets surface where geothermally heated water escapes
Common in volcanically active areas
Heat+reduced inorganic chemicals+pressure (underwater) = right conditions for organic polymer formation
What is polymerization?
The process in which relatively small particles combine chemically to produce a large, chain-like molecule (monomer->polymer)
Anabolic reaction (complex molecules from simple molecules by condensation reaction)
Explain step 3: formation of ploymers that can self-replicate (enabling inheritance)
First genetic material -> RNA
Most likely because:
DNA is stable+effective at storing info BUT cannot self-replicate (need enzymes which most likely didn’t exist)
RNA molecules can act as catalyst to their own replication
Can store info and self-replicate (form copies without using enzymes)
-> now seen in ribosomes
Step 3: what are the properties of RNA that makes it most likely to be the first genetic material?
What evidence is there that supports this?
Properties:
Can assemble spontaneously from nucleotides
Able to self-replicate
can control rate of chemical reactions
-> modern cells have ribosomes which contain ribozymes (forms peptide bonds between amino acids during translation) to catalyze
Evidence:
Ribose (R in RNA) can be formed from methanal (product of miller-Urey experiment)
-> deoxyribose (D in DNA) is produced from ribose in enzyme catalyzed reaction
Ribozymes able to join amino acids together to form RNA template
Explain step 4: formation of membranes to package the organic molecule and create different conditions to surroundings
Early cell membranes
All cells have membrane barrier
Can be: vesicle, micelle, bilayers (look in notes for image)
Happened in volcanic pools or meteorite
early cell membrane maybe formed from fatty acids (simpler than phospholipids)
-> fatty acid - simple amphipathic lipid molecules (hydrocarbon chain (hydrophobic tail) and carboxyl group (hydrophilic head))
when fatty acid in water solution - naturally attracted - spontaneously form micelles - polar parts in water, non-polar parts stick out
-> high concentrations - form vesicles
Explain step 4: formation of membranes to package the organic molecule and create different conditions to surroundings
Later/modern cell membranes
Experiments show that phospholipids naturally assemble into bilayers
-> and that membranes would trap amino and nucleic acids
Formation of blilayer -> isolated internal environment with optimal conditions
Phospholipids -> form spontaneously from gases
Like fatty acids will form micelles in most conditions
-> then vesicles/bilayer/membranes by arranging themselves into rows back to back
What is LUCA? (+ little things about first cell)
Last Universal Common Ancestor
Most likely single celled prokaryote (earliest domains -> bacteria and archaea)
Eukaryotes later
(3.8-4.5 billion years ago)
No standard model of the origin of life -> challenges in explaining spontaneous origin of cells
BUT there is evidence for the origin of carbon compounds + spontaneous generation of cell-like structures (with lipid bilayers)
first cell -> membrane surrounding self replicating molecule (RNA)
What evidence exist for LUCA?
- Universality of genetic code
DNA codons (triplets) code amino acids -> proteins
Every living thing -> same code/codon make same amino acids - Conservation of key molecules
Certain chemicals -> common to all + same function
Ex:
cytochrome enzyme (all aerobic organisms)
ATP -> universal energy molecule
Ubiquitin -> used as ‘tag’ protein that needs to be broken down - Highly conserved genes
Certain genes -> same across all cellular organisms
Ex: enzymes needed for DNA replication/transcription/translation
Summary:
Same biochemistry
Same DNA base/genetic code/amino acids
What did Madeline Weiss’ studies show us about LUCA?
Studied the genomes of living bacteria+archaea (B+A)
-> only 355 genes coded for homologous protein
MEANING:
B+A have been diverging for a long time
Any genes shared -> inherited from LUCA (didn’t appear independently)
355 homologous genes showed how LUCA lived:
Anaerobe (no oxygen)
Chemoautotroph (got energy from hydrogen and converted CO2 and nitrogen into essential organic compounds)
Able to live in extreme heat (conditions found in hydrothermal sea vents)
Explain the chemical evidence for LUCA
Earliest evidence of life:
Rock have fossil like structures with isotopes -> suggest organisms remains
-> has organic compounds with carbon isotopic signature that could be result of biological processes
Evidence examined:
Analyzed using ratio of isotopes
C12 to C13 ratio suggest living processes -> autotrophs used C12 over C13 in photosynthesis
What are biomarkers?
Molecular fossils of lipids and other organic compounds that are incased in sedimentary rock
-> can remain intact for millions of years
Explain the fossil evidence of LUCA (stromatolites)
Stromatolites:
-> structures that are created by photosynthetic bacteria that live in shallow water
Covered in clay -> bacteria move up to light (needed for photo)
Movement -> traps and binds sediment into layers
Material precipitates into layers -> fossilized stromatolites (even if microbes die off)
Fossil studied to learn about origin of life
Explain the fossil evidence of LUCA (vents)
Hydrothermal vents on sea floor -> earliest habitable environments -> proposed environment for LUCA+early life may have been present there (3.8 billi years ago)
Fossilized evidence from old vents
Slice rock paper thin then study the small fossils
Explain the genetic evidence for LUCA
Molecular clock -> compare genome of modern organism to infer the time-frame in which LUCA existed
Use difference in genome of 2 species proportional to time since they diverged from a common ancestor
-> DNA changes as mutations occur and accumulate
-> estimating average times for DNA mutations -> general date when LUCA was can be determined
-> includes changes in amino acid compositions (change in DNA-> change in translation -> change in amino acid/protein)
What do the fossils found in Canada show in terms of the similarities between modern and early organisms?
Conditions provide opportunity for organisms to generated energy by chemosynthesis
Fossilized structure found by vent near Quebec, Canada
- similar to those made by modern prokaryotes near vents
- at least 3.77 billion years old (but could be more) -> one of the oldest forms of life ever found
- small tubes made of hematitekey (mineral form of iron (III) oxide)
-> presence of carbonate/carbonaceous material - suggest oxidation/biological activities
-> suggest ancient bacteria similar biochemistry to similar modern bacteria
What characteristic were deduced about LUCA from modern analysis?
Analysis of modern species near vents -> suggest they had common ancestor
- based on properties/functions of amino acid sequence data LUCA might’ve had these characteristics:
-> anaerobic (can survive with x oxygen)
-> converted CO2 -> glucose
-> hydrogen -> energy source (no sun)
-> nitrogen -> ammonia -> synthesis of amino acids
-> tolerant high temps (thermophilic)
-> autotrophic extremophile
-> lived in hydrothermal vents
-> environment with lots of hydrogen, CO2, iron
(NOT ONLY HYPOTHESIS FOR ORIGIN OF LIFE -> scientist continually finding data)
