Meditations on a Cell

Escherichia coli
Escherichia coli – Scanning Electron Microscopy. Public domain image.

The K-12 strain of E. coli, the best studied prokaryotic (meaning: without a nucleus — cells that have one, such as animals and plants, are in the eukaryote category) bacteria, has a genome that contains 4,639,221 nucleotide pairs.

These 4.6 million nucleotide pairs, which are contained in a single circular molecule of DNA that floats inside the plasma membrane of the bacteria (remember, no nucleus) can code about 4,300 different kinds of proteins (the tools and building blocks used by life).

Proteins are made from amino acids (there are 20 kinds). It takes 3 nucleotide pairs to code one amino acid; since there are 4 “letters” in DNA, T,A,G,C, you get 4 x 4 x 4 = 64 possibilities with 3 pairs, so some triplets code for the same amino acids. The code for each protein, depending on the organism and how it is expressed, can produce more than one variant of a protein. For example, humans have genes to code about 24,000 proteins, but our bodies can contain up to a million different variants.

To complicate things further, different strains of E. coli can be very different from each other. Some can contain hundreds of genes that are absent from others, and two strains can have as little as 50% of genetic material in common. Some have a flagella and are mobile while others are not, some can cause serious illness in their host (they usually colonize the human digestive system within 40 hours of birth, via food and water) and some are fairly benign. That high variability in genetic makeup within a single species can be explained in good part by the non-sexual reproduction of E. coli.

So all of these 4.6 million DNA base pairs, and these 4,300+ proteins, are working to create life within a relatively simple single cell organism that is less than one micrometer – one millionth of a meter – thick and not much more than two micrometers long. Even the smallest and simplest forms of life are incredibly complex and endlessly fascinating.

Now think about how mind-boggling our human bodies are (3.2 billion DNA base pairs, though not all of them gene-coding).

Biology is the new frontier.

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