1.5 origin of cells

Question 1

what is the evidence from pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on earth

Answer 1

• 2 experimental setups were identical except that 1 had a swan neck while the other did not
• 1 was exposed to atmospheric microorganisms while the other was not
• showed that in a sterile solution without any cells (microorganisms) present, no new cells can arise
• shows that cells must be present for new cells to form
• disproves theory of spontaneous generation of life

Question 2

cells can only be formed by division of pre-existing cells

Answer 2

• cells are observed to be formed only from pre-existing cells via cell division (and this is built into cell theory)
• pasteur’s experiment proves that cells can arise only from existing living cells

Question 3

how does universality of genetic code prove origin of cells?

Answer 3

• genetic code of a possible permutation of 64 codons built from 3 DNA bases is universal amongst all life on earth, meaning that each unique codon will be translated into the same amino acid regardless of the species of the cell
• gives evidence that all life on earth could have arisen from one single ancestral cell

Question 4

the 1st cells must have arisen from non-living material. but how?

Answer 4

• if scientific assumption that there was a time period before the first cells appeared, then the first cell must have arisen from non-living material instead of cell division
• 4 key problems with this assumption:
  1. non living synthesis of simple organic molecules
  2. assembly of these organic molecules into polymers
  3. formation of polymers that can self replicate (enabling inheritance)
  4. formation of membranes to package these organic molecules

Question 5

how did miller and urey investigate the origin of cells back in primordial days?

Answer 5

• in primordial days (early days near the beginning of the formation of the earth), environment was rich in compounds with many that are strongly reducing
  • hydrogen
  • nitrogen
  • water vapour
  • methane
  • ammonia
  • hydrogen sulphide
• believed that most of these gases were produced and
  released during volcanic eruptions
• early earth conditions were also known as the ”primordial soup” or “primeval soup”
• miller and urey tried to recreate an estimate of the conditions in 1953
• trying to demonstrate “chemical evolution”, where complex biomolecules can form spontaneously from the simpler molecules in the “soup”
• combined in a closed gas vessel the molecules (from above) to simulate the atmospheric conditions, heated the water to simulate volcanic activity and sparked electricity through the vessel to simulate lightning
• after 1 week of experimentation, they discovered that the vessel now contained 13 of the 20 naturally occurring amino acids within

Question 6

how did miller and urey conquer problem 1? (non living synthesis of simple organic molecules)

Answer 6

• experiment showed that it is possible for cells to have arisen from non-living material found in the “primordial soup”
• conditions then are believed to be highly reducing (low oxygen), high in radiation levels, in temperature and with frequent electric storms
• water was boiled into the form of water vapour and was mixed with methane, ammonia and hydrogen gases
• electric sparks simulated lightning and contributed to initiate chemical reactions by the component gases
• upon cooling, the amino acids were discovered in the liquids formed

Question 7

how did miller and urey conquer problem 2? (assembly of these organic molecules into polymers)

Answer 7

• conditions found in the vessels in Miller and Urey’s experiment tends to inhibit the formation polymers by hydrolysing any bonds formed
• however, scientists discovered in deep sea thermal vents, there is spontaneous assembly of marine dissolved organic matter into polymer gels (Nature, 5 Feb 1998)
• hence in these places, which are in volcanically active areas that have high heat energy and a steady supply of reduce inorganic chemicals, polymer gels can form

Question 8

how was problem 3 conquered? (formation of polymers that can self-replicate, enabling inheritance)

Answer 8

• DNA are very stable compounds used to store genetic information, but are not capable of self replication but are dependent on enzymes (DNA polymerases) to do so
• RNA however, can store genetic information, and some are observed to be capable of catalysing the formation of copies of itself
• ribosomal RNA are an example of a ribozyme that is able to catalyse chemical reactions
• acts as a basis for the RNA World Hypothesis where it is hypothesised that early life forms used RNA to store genetic information and also catalyse reactions as enzymes, but later evolved to split the two roles to be taken up by DNA and proteins separately

Question 9

how did was problem 4 conquered? (formation of membranes to package the organic molecules)

Answer 9

• experiments have shown that phospholipids can naturally assemble into bilayers under the right conditions, and these can form a bubble that can isolate and create an internal environment
• gives rise to the compartmentalisation of an entire cell, or in eukaryotes the compartmentalisation using membrane bound organelles

Question 10

what is the endosymbiont / endosymbiotic theory?

Answer 10

• explains how some of the organelles found in eukaryotes came about
• states that such organelles like mitochondria and chloroplasts originated as prokaryotes that entered a cell and developed a symbiotic relationship with the host eukaryotic cell

Question 11

how does nucleus develop in cell?

Answer 11

• prokaryote grows in size and the cell surface membrane starts to develop folds inwards to increase the surface area to volume ratio of the cell
• infoldings pinch off and form an internal double membrane system
• nucleoid region is now encased within this internal membrane and becomes the nucleus

Question 12

how does the endosymbiont / endosymbiotic theory work? how did modern day eukaryote cells come around

Answer 12

• aerobic prokaryote enters a larger anaerobic prokaryote (possible due to phagocytosis and a predator-prey relationship, or as a parasite)
• ingested prokaryote was somehow not digested
• host anaerobic prokaryote now has a symbiotic relationship with the aerobic prokaryote within, with the host providing nutrients and protection and the aerobic prokaryote being able to release a lot of ATP to the host cell under aerobic conditions
• host cell has more ATP for use and outcompetes the competitors
• over time, the relationship evolves to a more stable form
• for chloroplasts, a similar event might have occurred, except that the prokaryote within the host is a photosynthetic one instead of being an aerobic prokaryote

Question 13

what is the evidence for endosymbiont / endosymbiotic theory?

Answer 13

• mitochondria and chloroplasts have their own DNA which replicates independently from the chromosomal DNA of the cell
• have DNA that are circular, just like prokaryotes
• contain ribosomes within them, and are of the 70S variety similar to prokaryotes and unlike the 80S variety found in eukaryotes
• have double membranes, and the inner membranes contains proteins that are more similar to prokaryotes than eukaryotes
• transcribe their own mRNA and synthesise their own proteins
• can arise only from the division of pre-existing mitochondria or chloroplasts, and produced by the cell itself