Origins of cells

Question 1

formation of sun and earth (when?) and when did the pre-biotic period in the Earth’s development end?

Answer 1

4.5 billion years ago
not for another 0.5 billion years (after that life formed)

Question 2

conditions in the per-biotic period

Answer 2

• atmosphere composition: O2 lower, CO2 and CH4 much higher
• high temperature (because of the greenhouse gases)
• high UV radiation (no ozone layer)
• intense bombardment by meteorites (E that enabled chem. reactions)
• frequent storms with lightning (high temperatures)

Question 3

possible spots for accumulation of complex C-compounds and life formation

Answer 3

• hot springs (terrestrial)
• hydrothermal vents (ocean)

Question 4

changes made by the 1st organisms to the Earth’s conditions

Answer 4

• O2 increased
• UV radiation and CO2 levels decreased
  -> these conditions made it possible for Life to evolve again

Question 5

Life is a…

Answer 5

…self-sustaining chemical system capable of Darwinian evolution.

Question 6

exceptions to the general definition of life:

Answer 6

• mature erythrocytes (no nucleus, no DNA)
• aseptate fungal hyphae (continuous rows of fused cells)
• mule (sterile, reproduction impossible)

Question 7

challenges explaining spontaneous origin of cells

Answer 7

• under current conditions on Earth, cell can be made from pre-existing cells only but on pre-biotic Earth it should have been made spontaneously from non-living matter

Question 8

steps in life (1st cell) formation:

Answer 8

1) inorganic into simple organic compounds
2) simple organic compounds (monomers) into polymers
3) plasma membrane surrounds cell content
4) self-replicating molecules - basis of inheritance

Question 9

AD 1)

Answer 9

• Miller-Urey experiment
• result: after few weeks, all a-a, nucleotides, monosaccharides, ATP in a primordial soup

Question 10

AD 2)

Answer 10

• dropping the mixture of monomers on hot clay, rocks, sand -> polymers

Question 11

AD 3)

Answer 11

• spontaneous formation of vesicles by coalescence of phospholipid molecules in primordial soup -> likely how membranes were made

Question 12

AD 4)

Answer 12

• RNA is the presumed first genetic material because it is capable of both storing genes and self-replication without enzymes (unlike DNA)
• some types of RNA can act as catalysts

Question 13

benefits vs weaknesses of RNA as a genetic material

Answer 13

benefits:
- capable of self-replication (no enzymes needed)
- high mutation rate (?)
weaknesses:
- unstable
- low capacity for gene storage
- high mutation rate (?)

Question 14

LUCA

Answer 14

Last Universal Common Ancestor
- the most recent population from which all organisms on Earth share a common descent
- all organisms are derived from a common source (sharing certain traits)

Question 15

evidence for the existence of LUCA

Answer 15

• the universality of genetic code
• all life shares a common mechanism of transcription and translation
• certain genes are distributed across all organisms (e.g. instructions for DNA replication)

Question 16

horizontal gene transfer

Answer 16

due to HGT, the genome of the LUCA is likely present in some ancient community of organisms (that evolved independently from FUCA) but became extinct due to competition with LUCA and its descendants
- LUCA is not the first cellular organism
- other organisms probably coexisted with it

Question 17

approaches used to estimate the dates of the 1st cells and LUCA

Answer 17

• microbes don’t leave fossils
  1) biochemical evidence
  2) phylogenetic comparisons

Question 18

1) biochemical evidence

Answer 18

• chemicals produced by cellular processes that provide evidence of past or present life

Question 19

the earliest evidence of life - when and in what form

Answer 19

• dates to 3.42 billion years ago in the form of stromatolites (layers of sediment laid down my microbes, Australia)

Question 20

2) phylogenetic comparisons

Answer 20

• comparing the genome (genomic sequences) of different species to develop a timeline for evolutionary divergence
• the more differences, the longer the time since they shared a common ancestor

Question 21

how did scientists establish the original LUCA genome?

Answer 21

• they searched for genes present in both prokaryotic domains (bacteria and archaea) - 355 genes are proposed to have originated directly from LUCA

Question 22

molecular clock

Answer 22

• estimated the time of evolutionary divergence using genomic comparisons
• using the mutation grade of DNA/RNA/proteins
• based on the assumption that there is a direct correlation between number of sequence differences and the time since two species diverged

Question 23

example of a molecular clock

Answer 23

if a gene mutates at 1 base pair per 100 000 years rate and 2 species have 6 differences in gene sequence, divergence occurred 600 000 years ago

Question 24

when did LUCA’s evolution into archaea and bacteria happen and how do we know this

Answer 24

• at any point between 2 to 4 billion years ago
• based on phylogenetic comparisons and the molecular clock

Question 25

“white smokers”

Answer 25

• alkaline hydrothermal vents (cracks on Earth’s surface with hot water and reduced chemicals (FeS)
• contain CH4, H2, NH3 and CO2
• optimal conditions for the emergence of the protocell

Question 26

fossil evidence for the evolution of LUCA near hydrothermal vents

Answer 26

• fossilized evidence of bacteria discovered in hydrothermal vent precipitates dated at around 3.7 bya

Question 27

genomic evidence for the evolution of LUCA near hydrothermal vents

Answer 27

• genomic analysis supports the idea that the LUCA developed near hydrothermal vents
• genes proposed to belong to LUCA are involved in the use of H2 as an E source

Question 28

the number of differences between…

Answer 28

…the genomes of two species is proportional to the time since they diverged from a common ancestor.