How did the activities of protocells become inheritable?

Question

I have learned that first a protocell came to exist and its characteristics came to be passed on by genetic material. So how come genes for all the activities come to incorporate into genetic material? Did genetic material come first and random sequences made random proteins that were used to make cells ?

Answer 1

Well.. This is a tough question (Something that has bugged me for years and still doing so). I don't fully agree that replicative potential came first. This is just my opinion based on some reasoning and I dont really have any reference.

If you compare a nucleotide and a protein, the likelihood of formation of protein is much higher because:-

1. Amino acids are simpler molecules and simplest of them did appear in the famous Urey-Miller experiment. Nucleotides on the other hand are much more complex
2. Peptide/amide bond formation is a well known phenomenon even in non-biological chemistries whereas Phosporic esters cant be called common compounds.

In my opinion function evolved first and then its codification. Initially the cell division must have been just like a huge drop dividing into two. I would like to differ from mgkrebbs on this. Lipids are simple molecules and they can spontaneously form compartments such as miscelles based on simple principles of thermodynamics. If miscelles get huge they can divide.

I still cannot understand whether it is chemical reproduction that came fist or sustained multispecies chemical reactions. I tend to believe the second with relatively more confidence.

The popular RNA world hypothesis is justified by certain observations and fits nicely in the Darwinian scheme of biology that we are so used to believe in all the time. But it doesn't answer the previously mentioned thermodynamic perspective. Also I fail to understand why an ancient RNA would code a protein. What advantage would it have to code a protein in the first place. So to conclude I don't have a doubt that proteins came first. The initial soup and the basic functionalities were provided by them. Then nucleotides evolved parallely and they became catalysts of protein synthesis. So the reactants came before the catalysts and the better catalyst got selected.

I again iterate that this is just my opinion which you may consider pondering over along with the answers that others have provided.

Answer 2

The sequence is believed to be opposite of that implied by the phrasing of the question: the inheritance of structures supporting inheritance came before protocells. The important point is that you need a system of reproduction before building more complicated structures (such as a cell). Without replication (reproduction), a cell would be little more than a soap bubble, and would disappear without a trace before long.

The initial (or very early) form of replication is thought to be based on a simple form of RNA. The reason for this is that (1) RNA does encode genetic information in a way that is replicatable, and (2) RNA can take on shapes and chemical structures that act as an enzyme (called a ribozyme) that catalyze chemical reactions. The hypothesis is that an RNA molecule formed that catalyzed assembly of copies of RNA chains, including itself.

After such replication was established, probably some kind of enclosure like the cell membrane evolved to contain the chemicals, making the reactions more efficient. After (or maybe before) this, RNA catalyzing assembly of simple protein-like molecules whose sequence was encoded by RNA sequences evolved, leading to protein-based cellular machinery. This mechanism is still evident in life today in that protein assembly uses a ribozyme-based molecule assembly (the ribosome) to translate messenger-RNA into proteins.

Answer 3

The general theory is RNA came first, then DNA. Both are able to replicate with simple changes in temperature (see polymerase chain reaction). So if we imagine a world with nucleotides making RNA, the idea is the survival of the fittest RNA. RNA for example that also had enzymatic activity was "fitter" than RNA that did not. Then go further down the line where RNA that made a certain protein was more "fitter" as thus it's concentration increased. Keep going down the line as this continues, whatever was favourable in the surrounding environment increased until we have what we have today.

Of course this is widely disputed.

Answer 4

AndroidPenguin and mgkrebbs both gave excellent answers to this. There are currently several ideas about the first life on Earth. Data for these methods are pretty useless because we can argue for what is the most probable and what can be modeled in the lab, but we can't (as yet) look for any fossil remains of molecules to see what really happened. One of the most satisfying hypotheses is that protocells and self-replicating molecules (like RNA) probably came about independently, but in the same location. If the self-replicating molecule found a way to hide out in the protocell, this might be the beginning of something like what we call life. A good talk on this topic is Martin Hanczyc's TED talk.