Abiogenesis and Biogenesis.pptx

notes on abiogenesis and biogenesis

1. SPONTANEOUS GENERATION (ABIOGENESIS) OR BIOGENESIS Men of ancient times (Thales, 624-548 B.C.; Anaximender, 611-547 B.C.; Anaximenes, 588-524 B.C.; Empedocles, 504-433B.C.; Aristotle, 384-322 B.C.; Epicurus, 341-270 B.C.; and Socretius 99-55 B.C.). They believed that all living organisms could spring forth spontaneously from non-living matter. This belief has been referred to as Doctrine of Spontaneous Generation or Abiogenesis (a= not; bios= life; genesis= origin) They believed that frog, snakes and mice could be born of moist soil, that could emerge from manure, and that maggots could arise from decaying materials.

2. Van Helmont (1577-1644) devised a method for manufacturing mice. He recommended putting some wheat grains with soiled linen and cheese into an appropriate receptacle and leaving it undisturbed for some time. Mice would then appear. However, the idea of spontaneous generation continued until the mid 19 th century with great oppositions against it.

3. Controversy over Spontaneous Generation Actually, it was the discovery of microorganisms and improvements in microscopy that enabled scientists to think seriously about the origin of life. Francesco Redi (1626-1679), an Italian physician, demonstrated during 17 th century by simple experiment that spontaneous generation (abiogenesis) does not exist. He took rotting meat pieces and placed then in jars and sealed some of these jars tightly and left other open.

4. In a few days, maggots appeared in open jars in which the flies went freely in and out and laid their eggs on meat. Contrary to it, the sealed jars in which the flies could not enter did not show any maggots. From these observation Redi concluded that the maggots arise from eggs laid down by parent flies and that the maggots can not appear spontaneously.

5. LOUIS JABLOT In 1670 Jablot conducted an experiment in which he divided a hay infusion that had been boiled into two containers: a heated container that was closed to the air and a heated container that was freely open to the air . Only the open vessel developed microorganisms. This further helped to disprove abiogenesis .

6. REDI’S and JABLOT’S EXPERIMENTS

7. Still, the supporters of abiogenesis did not agree with Redi and argued that the free air, which was considered as “vital force” necessary for spontaneous origin of life, was not allowed to reach the meat placed in sealed jars. Redi set up new set of experiment in which he covered jars with fine muslin cloth or gauze instead of sealing them tightly and thus allowed free air to go in and out of the jars. Even after doing so the maggots appeared only in those jars in which flies were allowed free to go in and lay their eggs on the meat.

8. Francesco Redi Experiments

9. Even after Redi’s convincing demonstration, abiogenesis versus biogenesis controversy continued. John Needham (1745) advocated that even after he heated chicken broth and corn infusion (nutrient fluids) before pouring them into covered flasks, the cooled solution showed existence of tiny organisms in them and thus he claimed that the organisms originated spontaneously from nutrient fluids. Later, that this result was due to insufficient heating which failed to kill heat resistant forms of bacteria containing endospores but nothing was known about endospores at that time.

10. In the year 1765, twenty years later Lazzaro Spallanzani demonstrated that nutrient fluids of Needham did not contain microorganisms when they were subjected to prolong heating after being sealed in flasks. He explained that the microorganisms from air probably had entered Needham’s solutions after they were boiled. Needham responded to it and said that the free air the “vital force”, present inside Spallanzani’s sealed flasks had been destroyed by heating and therefore, microorganisms did not appear in nutrient fluids in absence of the “vital force”.

11. LOUIS PASTEUR (1822 - 1895) Disproved spontaneous generation of microbes by preventing “dust particles” from reaching the sterile broth In 1861 completes experiments that lays to rest spontaneous generation. Showed microbes caused fermentation and spoilage

12. PASTEUR’S EXPERIMENT trapped airborne organisms in cotton; he also heated the necks of flasks, drawing them out into long curves, sterilized the media, and left the flasks open to the air. In this way Pasteur disproved the theory of spontaneous generation

13. Disproved by: Schwann, Friedrich Schroder and von Dusch (1830s) – Air allowed to enter flask but only after passing through a heated tube or sterile wool John Tyndall (1820-1893) – Omission of dust 🡪 no growth. Demonstrated heat resistant forms of bacteria ( endospores )

14. John Tyndall, an English physicist, in the year 1877. He explained that bacteria exist in two forms: Heat labile forms (thermolabile) which could be killed by exposure to high temperatures, and heat- resistant forms which could not be killed by continuous boiling of the broth and after the broth has cooled, they resulted in microbial growth in such broths. He further stated that if such broths are subjected to intermittent boiling (discontinuous boiling) on successive occasions, a process now popular as tyndallization, the heat- resistant forms of bacteria will be killed and the broths become completely free of them, and do not show any microbial growth.

15. It so happens because the first boiling kills vegetative cells of bacteria but endospores remains as such. The endospores now germinated in cooled broth and produce new bacterial cells which are killed during further boiling and so on. In this way, Tyndall validated Pasteur’s results and helped ending the debate on abiogenesis versus biogenesis.