The fundamental similarity of all living things

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What is the point?

  1. To be able to describe the commonalities in central information processing,metabolism, nd structure in all living things
  2. To describe what can be inferred from this similarity with respect to the common ancestry of living things

Before spending the rest of this book describing diversity, i.e. how organisms are different from one another, it’s useful to remember and keep always in mind that all cells are fundamentally the same in almost every way. One way to view this is by walking through the flow of information in the cell - the "Central Dogma" - and point out the fundamental similarities common to all living things.

The Central Dogma  Cheezy hand drawing by Jim Brown
The Central Dogma
Cheezy hand drawing by Jim Brown

DNA

All cells encode information in the form of DNA. The DNA in all cells is composed of the same 4 bases (G, A, T and C), the same sugar (2´deoxy-D-ribose), assembled with the same chemical structure and steriochemistry. Information in DNA is stored using a universal 3-letter code (e.g. AAA encodes Lys in all cells), and DNA replication process is handled the same in all organisms (the replication fork complexes are all very much alike). The function of DNA is carried out via transcription into RNA, using RNA polymerases that are all essentially alike.

RNA

RNA is used primarily to direct protein synthesis based on information in DNA. RNA in all cells has the same structure, the same 4 bases (G, A, U and C), sugar (D-ribose), and the same steriochemistry. Furthermore, all cells have the same types of RNAs, e.g. ribosomal RNA, transfer RNA, messenger RNA, &c. These RNAs are very much alike in sequence and structure in all cells; for example, the ribosomal RNAs in all organisms share a conserved core of sequence, and are very similar in secondary structure.

The E. coli and H. sapiens small subunit ribosomal RNA secondary structures from the Comparative RNA Database, Robin GutellThe E. coli and H. sapiens small subunit ribosomal RNA secondary structures from the Comparative RNA Database, Robin Gutell
The E. coli and H. sapiens small subunit ribosomal RNA secondary structures
from the Comparative RNA Database, Robin Gutell

Protein

Polypeptides (proteins) direct most of the cells catalysis and structure. Proteins in all cells use the same 20 (or so) amino acids in the same stereochemical conformations, synthesized in the same way, and use the same post-translation modifications. Most of the reactions catalyzed by these proteins are the same (see below) and the enzymes that carry them out are similar in amino-acid sequence, cofactors, 3-dimensional structure, and mechanism of action.

Function

With few exceptions, all cells use the same metabolic pathways: the TCA cycle, glycolysis, amino acid biosynthesis, purine and pyrimidine biosynthesis, lipid metabolism, electron transport, and ATP synthesis via the electron gradient. There is perhaps more variation in metabolism that in the previous parts of the central dogma, but these are generally minor variations, such as the use of glycolysis verses the Entner-Deuteroff pathway.

Cells

In addition, all organisms are built of one or more cells, which are bound by a lipoprotein membrane that strictly controls what goes in and what comes out of the cell. This generally defines and separates inside & outside.

So, all organisms are mostly the same. What does this mean?

Primarily, it means that all organisms share a common ancestry. In other words, all known organisms can trace their past back to a single origin of life. This might not have been; other lineages, if they ever existed, seem to be extinct (or perhaps unrecognized?).

The fact that all organisms are very much alike also means that the last common ancestor of all known living things was a complex organism or population of organisms. Most of biochemical evolution predates the last common ancestor! The last common ancestor had all of the biochemistry that is now universal, which means nearly everything! Biochemical evolution occurred very early in the emergence of life. The diversity in extant life (known modern life) is in peripheral biochemistry - just the details!