The RNA world hypothesis suggests that life on Earth began with a simple RNA molecule that could copy itself without help from other molecules.
DNA, RNA, and proteins are central to life on Earth. DNA stores the instructions for building living things—from bacteria to bumble bees. And proteins drive the chemical reactions needed to keep cells alive and healthy. Until recently, RNA was thought of as little more than a messenger between DNA and proteins, carrying instructions as messenger RNA (mRNA) to build proteins. However, RNA can do far more. It can drive chemical reactions, like proteins, and carries genetic information, like DNA. And because RNA can do both these jobs, most scientists think life as we know it began in an RNA world, without DNA and proteins.
The first RNAs
So how did RNA evolve on Earth? Scientists think RNA building blocks (nucleotides) emerged in a chaotic soup of molecules on early Earth. These nucleotides bonded together to make the first RNAs. No sooner were they made than they broke down; however, new ones were made in their place. Some RNAs turned out to be more stable than others. These RNA strands grew longer and bonded nucleotides more quickly. Eventually, RNA strands grew faster than they broke down. This was RNA’s chance to start life.
All living things reproduce. They copy their genetic information and pass it onto their offspring. And for RNAs to start life, they needed to reproduce too. This is why scientists think that the RNA world took off when an RNA emerged that could make copies of itself. As it did, new self-copying RNAs emerged. Some were better at copying themselves than others. The RNAs competed against each other, and the most successful won out. Over millions of years, these RNAs multiplied and evolved to create an array of RNA machines. At some stage, DNA and proteins evolved. Proteins began to drive chemical reaction in cells, and DNA—which is more stable than RNA—took the job of storing genetic information.
Maintaining enough RNA building blocks (nucleotides) would have been a top priority in the RNA world. Scientists think nucleotide-building RNAs evolved on early Earth to provide nucleotides for building new RNAs.
Supplying the RNA world
According to the RNA world theory, the first RNAs were made using free-floating nucleotides that emerged in a primordial soup of molecules. They bonded together to make strands of RNA that weren’t very stable and degraded quickly. But some were more stable than others; these RNAs grew longer and bonded nucleotides more quickly. Eventually, RNA strands grew faster than they broke down—and this was RNA’s foot in the door. Over millions of years, these RNAs multiplied and evolved to create an array of RNA machines that are the basis of life as we know it today. But for RNA molecules to take hold, they would have needed an abundant supply of nucleotides. And scientists think nucleotide-building RNAs evolved to provide these RNA building blocks.
Test tube evolution
Scientists attempting to re-create the conditions of early Earth in a test tube have managed to evolve a number of RNA machines that can drive chemical reactions to make some parts of a nucleotide. This proves that RNA can drive nucleotide-building chemical reactions. But researchers have yet to create an RNA machine that can create whole nucleotides using ingredients that would have been available on primitive Earth.
In an RNA world, scientists think that simple RNAs grabbed onto other RNAs or molecules to form complexes that could change, or enhance, their function. This was a step towards more complex life.
Ribosomes, which are a cell’s protein-assembly machines, are made of ribosomal RNAs (rRNAs) and proteins. But the rRNAs in a ribosome evolved long before ribosomal proteins. Back in the RNA world, it is possible that one RNA may have grabbed onto another RNA to create an RNA machine that—for the first time ever—linked amino acids together to make a protein. Thus, the first version of a ribosome emerged.
Some messenger RNAs (mRNAs) in bacteria and some plants contain a section of code called a riboswitch that can grab onto a specific molecule. Binding this molecule controls whether the mRNA is translated to make a protein. The molecule could be a nutrient that binds to an mRNA riboswitch and triggers the mRNA to be translated to make a protein that breaks this nutrient down. So mRNAs that contain riboswitches can regulate themselves in response to specific molecules. It was previously thought that only proteins regulated the production of protein from mRNA; however, riboswitches hint of a regulation system that may have existed in an RNA world long before proteins existed.
Our cells contain a protein-directing machine, made of RNA and proteins, that sends newly made proteins to where they’re needed in a cell. This machine is called the signal recognition particle (SRP).
The SRP is on the lookout for proteins being made in a cell’s protein assembly machine (ribosome). When it spots the beginning of a protein poking out of the ribosome, it binds to it. The ribosome halts protein production while the SRP brings the ribosome and its partly-built protein to where it’s needed in the cell. On arrival, the SRP is released and protein synthesis starts up again.
The RNA in the SRP is found in all living things, which suggests that it evolved in very early life-forms. When proteins first emerged on Earth, an early version of this protein-directing RNA may have helped organize proteins in a cell. It could have enhanced primordial cells by directing proteins to form a cytoskeleton. A cytoskeleton helps a cell keep its shape and is like a highway system for transporting molecules around a cell.
One messenger RNA (mRNA) can be remixed in different ways so that its genetic code can be translated to make lots of different proteins. The ability to make more than one protein from one mRNA sped up the evolution of multicellular life.
Newly made mRNA is spliced by a molecular machine called the spliceosome that is like the scissors and glue of the cell. Made of RNAs and proteins, this machine chops out unwanted sections of mRNA code and sticks the remaining mRNA back together again, to create mature mRNA that can be translated to make a protein.
One gene and lots of proteins
Back in the 1970s, scientists thought that one gene coded for one mRNA, which in turn coded for one protein. This is largely true for bacteria and other single-celled life; however, for multicellular life one gene codes for one mRNA that can be spliced in different ways to create many different proteins. This is called alternative splicing.
Alternative splicing is an ingenious way of creating a diverse range of proteins from a relatively small number of genes. One of the surprises of the Human Genome Project was that the human genome codes for so few genes. Scientists predicted that there would be about 100,000 human genes, but the number is closer to 20,000. These genes are spliced in different ways to create a huge number of human proteins.
Speeding up evolution
The arrival of alternative splicing in multicellular life-forms likely sped up evolution. It meant that an organism could create new proteins without going through the lengthy process of evolving new genes. Chance mutations would have arisen that caused existing mRNAs to be spliced in different ways. These alternatively spliced mRNAs coded for brand new proteins that may have driven brand-new cellular processes, driving forward the evolution of complex life.
RNA machines were likely central to the evolutionary leap from single-celled to multicellular life forms.
Multicellular life starts with a single fertilized egg. This cell divides into two cells, which divide again… and so it goes on. Soon, the cells in this developing life-form start carrying out different jobs. In a plant, they could become leaf cells or root cells. In an animal, they could become blood cells or nerve cells. There are about 200 different types of cells in a human, and it is essential that each is made in the right place at the right time in a human embryo. What a cell becomes is determined by what molecular machines—RNAs and proteins—are operating in that cell. And those RNAs and proteins that are present in a cell are determined by transcription factors that switch genes on and off.
Telling heads from tails
We get clues about the molecular machines that could have driven the leap to multicellular life by studying how embryos develop in organisms now. A favorite creature for developmental biologists is the fruit fly. And an important RNA in fruit fly development is called bicoid. It plays a vital role in organizing the body plan of a developing fruit fly. In an unfertilized fruit fly egg, bicoid RNA is found in the end of the egg that will become the fly’s head. Once the egg is fertilized, the bicoid mRNA is translated to make a protein. Bicoid protein switches on genes that make head-making proteins and switches off genes that make tail-making proteins. So, bicoid tells fruit fly embryos exactly where to make the head.
Want to join the conversation?
- What makes the fruit fly so suitable for research?(15 votes)
- So they say that the first organisms were RNA? Or not?(3 votes)
- Yes, scientists claim that RNA formed first (possibly at the bottom of the sea) and then fused into double-stranded DNA organisms. That's what the RNA Life Theory is. Hope that helped(8 votes)
- "The ribosome halts protein production while the SRP brings the ribosome and its partly-built protein to where it’s needed in the cell. On arrival, the SRP is released and protein synthesis starts up again."
In plants, the ribosome can be within the Mitochondria and Plastids. The ribosome in plants is usually made of three molecules of rRNA and about 50 types of proteins that associate and form into subunits. While most plants synthesize few proteins and have few ribosomes, some others produce more like the protein-rich seeds of legumes. Each molecule of mRNA is long enough for 6 to 10 ribosomes to attach to it and read it simultaneously. When they are bound together by mRNA, they form a polysome.
Are signal recognition particles (SRPs) found in only animals, or are they also in plants? If they are found in both, would there be a difference between SRPs in prokaryotic and eukaryotic cells in plants? Also, is the ribosome (which is inside the protoplasm), the area where protein synthesizers should look at if they were to create new proteins and thereby change or modify the pre-existing proteins of plants? Or, can other organelles like Mitochondria and Plastids also be looked at for information regarding RNA synthesis?(5 votes)
- What makes the fruit fly so suitable for research?(4 votes)
- at least one of the traits that make it so suitable for research is that you can breed a lot of individuals with very little expenditure of time and money.(1 vote)