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Introduction

Escherichia coli (E. coli) is a type of gram-negative, rod-shaped bacteria that has been used as a model organism in biology since it was discovered in 1885. The bacteria is named after German pediatrician Theodor Escherich, who discovered it in the stool of infants while looking for the cause of neonatal dysentery. E. coli has been put to good use since its discovery in 1885; discoveries involving E. coli have received eleven Nobel prizes and the bacteria has been used in countless experiments, making it one of the most important organisms in science. E. coli became one of the foremost model organisms owing to the fact that it is small, reproduces quickly, and can be grown and cultured easily. As a model organism, it has shaped knowledge in the fields of genetics and biology. Five groundbreaking experiments involving E. coli have been summarized below.

Experiment 1

Discovery of Genetic Code: In 1961, Marshall Nirenberg and Heinrich Matthaei made the groundbreaking discovery of the genetic code. While scientists understood that DNA served as a template for RNA and proteins, they didn’t fully understand how. Nirenberg and Matthaei mixed ribosomes, tRNA, and aminoacyl-tRNA synthetases from the E. coli with a synthetic RNA chain of uracil bases. Essentially, they put all the tools of protein synthesis in a test tube and created their own RNA to see what proteins might be created from it. When mixed, the E. coli components read the chain of uracil bases and created a protein chain of only phenylalanine. The conclusion they drew was that uracil coded for the protein phenylalanine. Having broken into one example of the genetic code, the duo then fed different sequences of RNA through the elements to see what proteins were created. This laid the foundation of discovering how DNA translated RNA into proteins. It allowed scientists to understand how the 64 combinations of RNA codons, based on three nucleotides each, can encode the twenty standard amino acids. These combinations make up the translation we now call ‘genetic code’. (CITE ACS) Because the proteins, such as ribosomes, could be easily harvested from the E. coli and used in vitro in the cell, E. coli once again proved itself as an effective model organism.

Experiment 2

Discovery of semi-conservative replication:

After Watson and Crick proved that DNA is shaped as a double helix, the next question many researchers had was how DNA replicated. There were three possible explanations for DNA replication: dispersive, semi-conservative, and conservative replication. The dispersive theory hypothesized that the original parent strand and new daughter strand would each be broken up into several chunks and re-synthesized in a DNA strand patchwork of parent-daughter DNA. The semi-conservative theory hypothesized that one strand of parent DNA and one strand of daughter DNA would combine with each other to form the DNA double helix. The conservative replication theory hypothesized that as DNA replicated, the parent strand would remain completely intact but serve as a template for the new daughter DNA strands. Thus, there would be one completely new daughter double-helix and the original parent double-helix. Meselson and Stahl used density-gradient centrifugation, a technique invented by Meselson, to settle this debate. The pair originally centrifuged DNA from a bacteriophage to try to figure out which replication theory was correct. However, the bacteriophage DNA broke apart during centrifugation and also replicated too quickly for the researchers to be able to tell which theory was correct. As a remedy, they turned to using trusty (E. coli) DNA. The researchers marked the parent strand of (E. coli) DNA with the isotope ¹⁵N and put ¹⁴N isotopes in the mixture surrounding the DNA. By weighing the density of the DNA strands, which now varied because of the isotopes, they could see which isotopes the new daughter double-helix contained and use that knowledge to elucidate what method of DNA replication was occurring. If the DNA contained both ¹⁵N and ¹⁴N isotopes, the parent and daughter strands did combine, disproving the conservative hypothesis. Whirling the (E. coli) DNA strands around on the order of 14,000 times the force of gravity, the denser DNA was pushed towards the bottom while the lighter strands floated towards the top. The result showed a band of ½ ¹⁴N strand DNA and ½ ¹⁵N isotope-labeled DNA. These bands with a mix of the two isotopes disproved conservative DNA replication theory. Researching further, Meselson and Stalk measured the density of the new DNA strands, The density of these strands was the exact density in between ¹⁵N and ¹⁴N strands of DNA. If DNA followed dispersive replication theory, the weight of the strands would have varied slightly. Each DNA strand being the same weight proved the semiconservative model of DNA replication. Thus, (E. coli) contributed to the knowledge of DNA replication because of its tightly bonded DNA and well-timed cycles of replication.

Select a topic about genetics or evolution in a specific organism or ecosystem.
Overall text length (all text sections) should be at least 1,000 words (before counting references), with at least 2 images.

The topic must include one section about microbes (bacteria, viruses, fungi, or protists). This is easy because all organisms and ecosystems have microbes.

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Type your exact title in the Search window, then press Go. The MicrobeWiki will invite you to create a new page with this title.

Open the BIOL 116 Class 2024 template page in "edit."
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Figure 1. Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.[1].

At right is a sample image insertion. It works for any image uploaded anywhere to MicrobeWiki. The insertion code consists of:
Double brackets: [[
Filename: PHIL_1181_lores.jpg
Thumbnail status: |thumb|
Pixel size: |300px|
Placement on page: |right|
Legend/credit: Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.
Closed double brackets: ]]

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Section 1 Genetics

Section titles are optional.
Include some current research, with at least one image. Call out each figure by number (Fig. 1).

Sample citations:

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Second citation of Ref 1:

Here we cite April Murphy's paper on microbiomes of the Kokosing river.

Section 2 Microbiome

Include some current research, with a second image.

Here we cite Murphy's microbiome research again.

Conclusion

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References

Edited by [Author Name], student of Joan Slonczewski for BIOL 116, 2024, Kenyon College.