1.5 (Origin of Cells)

Question 1

Pasteur’s experiment.

Answer 1

A- to test whether sterile nutrient broth could spontaneously generate microbial life
M- Two experiments were setup
- In both, Pasteur added nutrient broth to flasks and bent the necks of the flasks into S shapes
- Each flask was heated to boil the broth so that all existing microbes were killed
- After the broth had been sterilized, Pasteur broke off the swan necks from the flasks in Experiment 1, exposing the nutrient broth within them to air from above
- The flasks in Experiment 2 were left alone
R- Broth in experiment 1 turned cloudy whilst the broth in experiment 2 remained clear
- Indicates that mircobe growth only occurred in experiment 1
C- Pasteur rejected the hypothesis of spontaneous generation as for growth of microbes to occur a source of contamination was needed

Question 2

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

Answer 2

• Cells multiply through division
• Mitosis results in genetically identical diploid daughter cells
• Meiosis generates haploid gametes (sex cells)

Question 3

Evidence for cells can only be formed by division of pre-existing cells.

Answer 3

1. Cells are highly complex structures and no mechanism has been found for producing cells from simpler subunits
2. All known examples of growth be it of a tissue, an organism or a population, are all a result of cell division
3. Viruses are produced from simpler subunits, but they do not consist of cells, and they can only be produced inside the host cells that they have infected.
4. Genetic code is universal each of the 64 codons (a codon is a combination of 3 DNA bases) produces the same amino acid in translation, regardless of the organism

Question 4

The first cells must have arisen from non-living material.

Answer 4

1. Non-living synthesis of simple organic molecules: Miller and Urey recreated the conditions of pre-biotic Earth in a closed system.
   - Reducing atmosphere (low oxygen), high radiation levels, high temperatures and electrical storms
   - Water boiled to form vapour then mixed with methane, ammonia and hydrogen
   - Mixture of gases was exposed to electrical discharge (sparks) to simulate lightning
   - Mixture was allowed to cool and after one week was found to contain some simple amino acids and complex oily hydrocarbons
   - Concluded that under the hypothesised conditions of pre-biotic Earth, organic molecules could be formed
2. Assembly of these organic molecules into polymers: Deep-sea thermal vents
   - Fissures in a planet’s surface from which geothermally heated water issues. Vents commonly found near in volcanically active areas)
   - Vents issue a ready supply of reduced inorganic chemicals and heat
   - Vents provide the right conditions and chemicals to allow organic polymers to arise
3. Formation of polymers that can self-replicate (enabling inheritance):
   - DNA though very stable and effective at storing information is not able to self-replicate – enzymes are required
   - RNA can both store information and self-replicate - can catalyse the formation of copies of itself
   - In ribosomes RNA is found in the catalytic site and plays a role in peptide bond formation
4. Formation of membranes to package the organic molecules:
   - Experiments have shown that phospholipids natural assemble into bilayers, if conditions are correct
   - Formation of the bilayer creates an isolated internal environment
   - Formation of an internal environment means that optimal conditions, e.g. for replication or catalysis can be maintained

Question 5

What is the endosymbiotic theory in relation to the nucleus?

Answer 5

• Prokaryote grows in size and develops folds in it’s membrane to maintain an efficient SA:Vol
• Infoldings are pinched off forming an internal membrane
• Nucleoid region is enclosed in the internal membrane and hence becomes the nucelus

Question 6

What is the endosymbiotic theory in relation to the mitochondria?

Answer 6

• Aerobic proteobacterium enters a larger anaerobic prokaryote (possibly as prey or a parasite)
• Survives digestion to become a valuable endosymbiont
• Provides a rich source of ATP to it’s host enabling it to out-compete other anaerobic prokaryotes
• As the host cell grows and divides so does the aerobic proteobacterium; subsequent generations automatically contain aerobic proteobacterium
• Evolves and is assimilated and to become a mitochondrion

Question 7

Outline evidence for the endosymbiotic theory.

Answer 7

• Have their own DNA (which is naked and circular)
• Have ribosomes that are similar to prokaryotes (70S)
• Have a double membrane and the inner membrane has proteins similar to prokaryotes
• Roughly the same size as bacteria and are susceptible to the antibiotic chloramphenicol
• Transcribe their DNA and use the mRNA to synthesize some of their own proteins.
• Can only be produced by division of pre-existing mitochondria and chloroplasts