The Molecular Origins of Life:
- DNA molecule’s primary function is replication and it carries out a variety of chemical reactions required for metabolism
- DNA encodes info
- The structure makes it easy to pass info to descendants
- In order to replicate, DNA needs proteins
- RNA is the original life molecule
- It is single stranded, so it can morph into a variety of sequence-specific structures
- It serves as both info molecule and a catalytic molecule
- RNA World Hypothesis:steps that led to evolution of life on earth
- 1. Organic molecules first accumulated on earth
- 2. Through chemical reactions, these chemicals assembled together to form ribonucleotides
- 3. Over time, ribonucleotides assembled into long chain polymers to form RNAs of varying sequences
- 4. Eventually, a RNA was able to replicate, creating an ‘explosion’ of the number of RNA
- 5. RNA molecules evolved, becoming better and faster replicators.
- 6. Certain sequences of ribonucleotides might have attracted and weakly bonded to specific amino acids
- 7. If RNA favored production of one protein, natural selection would occur and that RNA would prosper
- Created the early dynamic of gene expression
- DNA is more stable/better suited to store info, so it replaced the RNA
- Chemical reactions was the property of the initial life form, not information replication
- Iron-Sulfur World Hypothesis:
- Mineral catalysts present near deep sea hydrothermal vents are promoting a series of chemical reactions that could have promoted the evolution of life.
- The energy comes from the hydrothermal vents, when the redox difference between the reduced briny hot water emerges from the mantle into the cold oxidized ocean
- The surface of iron sulfide minerals catalyzes a series of chemical reactions that create a reverse Krebs cycle
- Takes in carbon monoxide or carbon dioxide and reducing it using the electrons provided from troilite to form more complex organic molecules that are abundant in many of today’s life forms
- Vents produce reduced organic molecules
- Complex organic molecules built up in large quantity in a small area of times
- Organic molecules will accumulate in the vicinity of the iron-sulfur catalyst. Some will aggregate, forming membranes. These membranes separate the iron-sulfur catalyst from the rest of the ocean, eventually forming a cell that encloses the metabolic life form.
- One group suggested that the first cells were encased in an iron-sulfur casing, not a lipid membrane. This allowed for the compartmentalization of a primitive cell, provided the catalyst needed for chemical reactions to occur, and allowed for the accumulation of chemicals in a closed system.
- While the RNA molecule consists of a select set of building blocks, many other closely-related analogs were probably present during the formation of life.
- There are more than just four nucleobases that can be created by chemical reactions.
- The RNA hypothesis has a difficult time explaining how RNAs formed using consistent building blocks in the presence of so much chemical clutter
- There is no way to encode the bond type between nucleotides. The RNA’s offspring will not be identical to the parent, and therefore the offspring will not have the replication ability. Since the ability to replicate is lost after one generation, there is no evolution
- The error rate of the best ribozyme isolated thus far is about five times higher than would be needed for an RNA of that size to really replicate itself.
- They don’t have high processivity.
- The copy is not expected to have the same catalytic function as the original molecule
- RNA is an unstable molecule
- It is a relatively new idea and has not been tested enough
- In the presence of carbon dioxide, these reactions cannot take place.
- The rate at which iron-sulfur catalyzes reactions is not fast enough to account for the emergence of life in the one billion years between the end of the heavy bombardment period and the appearance of cellular life
- At the temperatures found near hydrothermal vents, troilite does not easily give its electrons to carbon
- Therefore, it is unlikely that iron-sulfur can serve as a catalyst in the biochemical reactions
I agree with neither theories completely, but I agree with the Metabolism first hypothesis more. The replication theory has too many cons that it isn’t supported enough. RNA is so unstable that how can it be possible that it was the beginning of life? RNA also can’t evolve, so how could it evolve into DNA overtime? Metabolism is a more probable explanation for the beginning of organic life. There is less evidence against it, and with time and more experiments I am guessing that the theory will be supported more.