Tuesday, January 20, 2009

Origins 5, DNA



The Three Domains of Life are divided as to cell types. The Archaea and Bacteria do not have the well-defined nucleus of the Eukaryotic (true) cell or many other organelles. They are therefore often lumped into the category called Prokaryotes (from Greek meaning "kernel"). The first picture shows a sketch of a Prokaryote. The DNA is organized in structures called Chromosomes . In Prokaryotes, it sits loosely in the middle and is usually Circular DNA. It is reproduced by proteins which copy it from one point to the another along the single strand.

The Eukaryote has a nuclear membrane which holds its DNA. The DNA is reproduced in a different way, called Mitosis. The second picture shows DNA in pairs which are lined up and divide along lines made of specialized tubules. This is a much more complex set-up than the Prokaryotes. So, one would think that the group of proteins which copy the DNA in the Prokaryotes, Bacteria and Archaea, would be similar to each other and be different in Eukaryotes.


However, this is not the case! The proteins were found to be similar in Archaea and Eukaryotes, and different in Bacteria! This is reported by Edgell and Doolittle in "Archaea and the Origin(s) of DNA Replication Proteins," Cell, June 27, 1997 (in the link, it is easier to see the chart which compares proteins if you bring up the PDF file option shown above the title). There is almost no primary sequence similarity between the bacteria and archaea/eukaryotes. I have pictured two proteins which copy DNA, one each from Archaea and Bacteria, in a previous post. As it tells you there, they each are made of hundreds of amino acids in specific orders to give them the shape needed for their jobs. Though various combinations of amino acids can produce similar protein functions, there are more combinations which do different jobs within the cell and vastly more which do not function at all.

This dramatic finding shows a lack of logical progression from supposedly simple organisms to complex. In the next post I will talk about one similarity of all three domains before going into the chemistry of pre-life origins.

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