Origins of Life Flashcards
Formation of RNA ?
Long chainlike molecule that conveys genetic information and speeds up other chemical reactions
When did DNA take over as heritable genetic material ?
Only later in evolutionary time
Explain the RNA world - a very volatile place ?
- Volcanic eruptions shaped the early earth atmosphere:
- Thick with water vapour
- Intense UV light – no ozone
- Electrical storms
- Rich in nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen and hydrogen cyanide
Explain Stanley Miller volcanic spark discharge experiment (1953)?
- Volcanic spark discharge experiment
- Abiotic synthesis of amino acids
- Darwin’s ‘warm little pond’ feasible
Explain Stanley Miller 1953 experiment – repeated in 2008 ?
- After Miller’s death in 2007 several boxes found at University of Chicago
- Contained vials of dried residues from original experiments
- Re-analysed using more modern and sensitive detection systems
- 22 amino acids plus 5 amines (new amino acids underlined)
What are the Pyrimidines ?
Cytosine and Uracil
Explain the RNA formation - Pyrimidines ?
- Derived from precursor compounds acetylene and formaldehyde
- Undergo a sequence of chemical reactions
- Criticised because of precursor complexity
- Nucleic acid precursors created from hydrogen cyanide (HCN), hydrogen sulfide (H2S) and ultraviolet light (UV)
- Abundant on early earth – HCN from comets
- Also created natural amino acids and lipids
What are the Purines ?
Adenine and Guanine
Explain the RNA formation - Purines ?
- Previously found a molecule called formamidopyrimidine (FaPy) can react to form purines
- Investigated to see if FaPy could be formed in early earth conditions
- First step easy, HCN readily reacts with water to form aminopyrimidines
- Once highly reactive amines were controlled purines formed
Explain the RNA formation – Pyrimidines and purines together ?
- Starting material was a plausible early earth chemical: hydroxylamine
- Cycled through wet/dry, hot/cold and acidic/basic phases to mimic varying seasons on earth in presence of phosphate containing materials
- Two separate pools that dried, rehydrated and intermixed each other
- Purines and pyrimidines formed in one-pot reactions
RNA formation - Job done?
- Definitely not, much still to learn
- Not clear what reactions result in ribose sugar formation (RNA backbone)
- Not clear how A’s, G’s, C’ and U’s linked up together to form RNAs
- However, experiments at least show with the correct conditions life’s essential building blocks can be formed
RNA acts as instruction ?
- Complementary base pairing enables a single RNA strand to serve as template for an exact copy
- Uracil pairs with adenine, guanine with cytosine
- This can happen in vitro (outside of usual environment)
- In this way RNA serves as an ‘instruction’
RNA acts as as a catalyst ?
- Base pairing and ‘nonconventional’ hydrogen bonds allow RNA to fold into unique 3D shapes
- Determined by nucleotide sequence
- Folding permits RNA to act as an enzyme (ribozyme) for many types of chemical reactions
- Simple structure limits catalytic efficiency and scope compared with other enzymes
- Retains catalytic roles on fundamental processes in gene expression – RNA splicing/translation
Could RNA catalyse its own replication ?
- Has the properties – Instruction and catalyst
- No self-replicating RNA molecules discovered in nature - Scientists working on laboratory models
- Even if demonstrated would not prove they were the origins of life - simply shows such a scenario is possible
How does RNA predates DNA ?
- Ribose readily formed from formaldehyde – principle products formed from early-earth simulating experiments
- Sugar deoxyribose (DNA) harder to make
- Deoxyribose produced from ribose in reaction catalysed by enzymes
RNA compartmentalisation?
- ‘Cells’ can form in test tubes
- Just add phospholipids to oil/water mixture and shake!
- Highly fluid, self-sealing, self-repairing
Origins of protein synthesis?
- Now performed by complex interlocking system of protein and RNA molecules – Which came first proteins or translation apparatus?
- Experiments have shown RNA can bind amino acids
- Nucleotide sequences capable of ‘selecting’ amino acids
- E.g. RNA molecules binding Arginine = higher frequency of Arg codons
- Correlation not perfect but suggests a limited code possible with RNA serving as a crude template
- Any RNA that could guide synthesis of a useful polypeptide would have had competitive advantage
Arrival of DNA ?
- DNA arose later and more suitable as a permanent repository of genetic information
- Deoxyribose sugar makes DNA more chemically stable than RNA
- Greater lengths of DNA can be synthesized without breakage
- Addition of thymine (instead of uracil) makes repair easier
- Deamination (unwanted) easier to detect and repair in DNA – because already exists in RNA (uracil)
Scientists found that Escherichia coli engineered to incorporate ?
- 2 extra bases X and Y into DNA
- Successfully transcribed into RNA
- Extends range of amino acids to 152 from 20
Origins of life – Hydrothermal vents ?
- Mineral rich water superheated to 400 degrees Celsius followed by rapid cooling
- Abundance of chemical essentials for life
- Harsh environment but teeming with life (worms, crabs, chemosynthetic bacteria)
Complexity of life ?
- Enormous complexity and variation to life
- 99% of species that have ever existed now extinct – lucky to be here!
- All life has been driven by genetic mutations acted on by natural selection
What did Darwin find out again ?
On his famous trip to Galapagos islands on the Beagle Darwin noted the variation in the shape and size of finch beaks
Explain Genetic mutations ? give examples
- Many types of point mutation
- Generally heritable germ-line mutations
- Mutations can be favourable, unfavourable or neutral
- Outcome dictated by protein change and selection pressure
Some examples are:
- Silent mutation
- Missense mutation
- Nonsense mutation
- Frameshift insertion
- Frameshift deletion
Types of natural selection ?
- Stabilising selection - favours an average phenotype
- Directional selection - a single phenotype is favoured, causing the allele frequency to continuously shift in one direction
- Diversifying selection - two or more distinct phenotypes that each have their advantages
Explain Human evolution ?
- Human evolution probably not linear
- Evidence now that intermediates overlapped for considerable periods of time
- Modern dating techniques have put Homo erectus extinction around 100,000 years ago in Java, Indonesia
- Homo Sapiens arrived 200-300,000 years ago
RNA, DNA and proteins became membrane encapsulated to create ?
First self-replicating systems (early cells)
What led to evolutionary complexity ?
Rise in oxygen levels, acquisition of genetic mutations and natural selection pressures
