Pre-existing cells

• In 1852 Robert Remak made the conclusion that cells divided to form new cells, that is cells came from?pre-existing cells
• His conclusion was reached after studying cells from?chicken embryos
• This discovery is often attributed to Robert Virchow who in 1855 proposed the phrase?omnis cellula e cellula?(all cells come from cells)
• Prior to these announcements, it was believed that life arose?spontaneously?from non-living matter

NOS: Testing the general principles that underlie the natural world; the principle that cells only come from pre-existing cells needs to be verified

• Up until the 17th century, the consensus was that life was?spontaneously generated?(living organisms arose from non-living matter). This was believed due to:
  • The lack of technology - microscopes were not extensively used
  • Observations being made - Aristotle observing insects forming from dew or van Helmont observing a mouse appearing from a jar containing a sweaty shirt and wheat
  • The idea supporting the?cultural and religious beliefs?of the time
• From the 17th century scientists such as Francesco Redi with his maggot and rotting meat experiment began collecting evidence to test and verify that?life required life to exist
• However, it was Louis Pasteur's experiments involving swan-neck flasks that provided sufficient verification to convince scientists that cells could only come from pre-existing cells
• The universal acceptance that cells come from pre-existing cells also comes from the idea that :
  • The?highly complex ultrastructure?of cells has not been able to be synthesised by humans
  • All the known examples of growth are a result of?cells dividing?by mitosis or meiosis
  • Although viruses have a much simpler structure, they can only be produced inside a host cell
  • The?universality?of the?genetic code?suggests that all life evolved from the?same original cells
    • The translation of the 64 codons produces the?same amino acids?for nearly all organisms - although there are some rare minor variations that have likely arisen since the common origin of life

Experiments disproving spontaneous generation

Louis Pasteur's experiments

• Louis Pasteur's experiments were designed to verify the principle that cells can only come from pre-existing cells
• To demonstrate this Pasteur used swan neck flasks (flasks with S-shaped necks) which?trapped the microorganisms in the bend of the neck
• Pasteur added?nutrient broth?to the flasks then boiled them to?sterilise
• With some of the flasks, Pasteur broke off the necks (leaving no bend)
• After a long period of time, Pasteur observed that the broth in the flasks with the?snapped necks?had gone?cloudy?whereas the broth in the?swan neck?flasks remained?clear
• Thus Pasteur had shown that the swan necks prevented microorganisms in the air from entering the broth and that?no organisms appeared spontaneously

• The?Oparin-Haldine hypothesis?is that, to create the original?first?cells from non-living material, the following four stages occurred:
  1. Simple organic compounds?needed to be synthesised from?inorganic molecules?(this was demonstrated by Stanley?Miller?and Harold?Urey)
  2. Then?assembled?into?polymers
  3. Some of these polymers (it is thought to be RNA) developed the ability to?self-replicate?(which enables inheritance)
  4. Formation of?membranes?(by lipids) that surrounded the polymers creating packages with internal chemistry different from the surroundings

The key stages involved in life arising from non-living materials

Miller-Urey experiment

• Miller and Urey recreated the conditions thought to have existed on Earth prior to life, using a specific piece of apparatus
• The apparatus allowed them to:
  • Boil water to produce?steam?reflecting the early primordial soup?evaporating?in the?high temperatures?that existed on Earth
  • Mix the steam with a mixture of?gases?(including methane, hydrogen and ammonia) that recreated the?atmosphere
  • Add electrical discharges to the gases to stimulate lightning (one of the sources of energy available at the time)
  • Cool the mixture (representing the condensation of water in the atmosphere)
• After a week Miller and Urey analysed the condensed mixture and found?traces of simple organic molecules?including amino acids

The apparatus used by Miller and Urey