1. LEQ: How are information-storing biomolecules proposed to have evolved?
Reading: 12.3 (finish); 3.1, 3.3; test monday
Activator: enzymes
Biological catalysts are essential for cells to function as systems. Do you think cells could produce polymers by using uncatalyzed reactions? Why or why not?
Key terms – ribozyme

2. Several hypotheses propose how life began on Earth.
Origins of macromolecules:
Miller-Urey & variations (below); terrestrial
electrodes
heat source
amino acids
water
“atmosphere”
“ocean”
While scientists do not know how life began all hypotheses have some basic version of the following story:
The origin of life can be understood using chemistry
Complex chemicals evolved from simpler molecules
Chemical reactions were needed to generate many copies of biomolecules
Biological information of the type we see in nucleic acids and proteins was part of this evolution
There are three general ways of approaching this problem. Miller-Urey (above) is one particular take on “Primordial Soup.” The soup was meant to represent Earth’s primitive ocean rich with simple molecules evaporating and cooling, which sets up a basic water cycle through the atmosphere. The electrodes and heat provided the energy necessary to cause certain chemicals to react. Many different versions of this experiment have been conducted yielding some basic molecules necessary for life including amino acids.
Circumstantial evidence suggests that some of the basic monomers of life may have formed in meteorites and the ices of comets in space!
Extraterrestial origin: meteorite hypotheses

3. Did information or sustained chemical reactions evolve first?
“metabolism-first”:
Cells began as sustained series of chemical reactions
Simple carbon molecules
Uses transition metals
Efficient at managing heat
The second competing hypothesis emphasizes the need for a series of sustained chemical reactions that produced simple molecules used to make monomers. One particular take on this is the iron-sulfide world. In the iron-sulfide version of Earth’s early ocean, the heat and pressure near geological activity in Earth’s primitive oceans is a perfect environment for complex carbon molecules to form. Transition metals and Sulfur emanating from cracks in the Earth’s crust may have supplied the necessary means to move electrons around to create bonds and maybe some simple molecules similar to carbohydrates and amino acids. In this sense, a primitive, pre-cellular metabolism sustained by chemical reactions may have began and simple cell was built upon this foundational chemistry.

6. Ribozymes are RNA molecules that catalyze their own replication.
RNA probably was the first genetic material; information may have come first
Ribozymes are RNA molecules that catalyze their own replication.
Autocatalytic
Specificity
In the RNA world models – biological information was essential to the formation of life and formed early on. RNA is the most likely of all candidates to fill this role. Unlike DNA, RNA of certain types can catalyze chemical reactions. Thomas Cech’s work first described RNA molecules that catalyzed chemical reactions and subsequent work has shown that RNA molecules called ribozymes not only catalyze chemical reactions, but they can also replicate themselves without the need for a gene or an enzyme!
Such a chemistry is thought to be the foundation for early life and may have been used in primitive cells.

7. Why RNA? DNA needs protein enzymes to replicate.
DNA likely assumed its role after RNA began propagating genetic information. One clue that biochemists have is related to how DNA copies itself. All DNA requires the presence of proteins to copy itself, while ribozymes do not. This means that proteins of a complex nature would have had to evolved first followed by DNA, leaving RNA’s role a mystery in this implausible scenario. Keep in mind that there is yet to be a role established for DNA as genetic material as well.

8. Summary
Which do you think is more likely to have come first:
RNA: A collection of nucleotides that spontaneously reacted and assembled into RNA and began to catalyze their own replication.
Reactions (metabolism): A series of chemical reactions that are self sustaining and eventually lead to the formation of biological molecules like RNA.
Why? Explain your answer.
Journal prompt – If scientists announced tomorrow that life (maybe just a simple cell-like organism) was discovered in the solar system or elsewhere, how would you react?