Essay 12 02/14/2004: Biology, Organic Chemistry, Psychology Self-replication It is easy to see that all living organisms on our planet today are replicators. Each plant, animal, fungus, protist and moneran is here now because its ancestors were able to escape death, and successfully pass their genetic material on to their offspring. Since the beginning of life on earth every member of each of the five kingdoms has been fine tuned to accomplish one ultimate objective- replication. Chemically, the genetic material used by all organisms on the earth is so uncannily similar that most scientists believe that we all came from a single ancestor. It is interesting to note that humans and bacteria share the same chemical systems for replication, in fact they even share many of the same genes. The very first replicators on our planet were most probably microscopic, organic molecules- strands of chemicals that attained the ability to copy themselves sequence by sequence. It is thought that the first chemicals to achieve this ability to copy themselves were strands of ribonucleic acids or RNA. The basic constituents of RNA, and DNA as well, are four different nucleotide molecules. The compounds that nucleotides form have chemical affinities for one another and they tend to group together creating long strands. The RNA world hypothesis, a well accepted theory for the origins of earth’s first replicators, suggests that these chemicals existed in small amounts near the bottom of the ocean and that, over time, the long sequences of nucleotides developed a way to make identical duplicates of themselves. Because the earth is such a dangerous place for large organic chemicals, any RNA strand that could not duplicate itself would eventually be worn down or destroyed. In a volatile physical world, rapid replication would have been the only path that allowed these RNA strands to perpetuate themselves. The way in which these first strands of nucleotides replicated was probably very similar to the way that our DNA replicates itself within our own bodies. DNA replication is achieved through the use of chemical catalysts called enzymes which travel down the length of the DNA or RNA strand and duplicate it nucleotide for nucleotide. The enzyme that our bodies employ to complete this task is called DNA polymerase. It is not clear what enzyme was used by our oldest ancestors, but it was surely very similar to DNA polymerase in structure and function. Luckily for us, the mistakes inherent in RNA replication, also known as mutations, allowed these early replicators to be modified from their original forms and to respond to the rigors of their environment. DNA polymerase makes occasional mistakes, and often these mistakes are not caught by the figurative spell checker, DNA repair enzyme. Such mistakes often have no consequences for the organism. Sometimes they can be debilitating or fatal and at other times they can be beneficial. When these mistakes prove useful they can help create genetic variability in a gene pool and thus allow organismal populations to change with time. Theorists believe that natural selection began to act on these RNA strands on the bottom of the ocean, weeding out the poor or slow replicators and allowing the robust ones to proliferate. After some time, competition for resources must have caused these original replicators to develop new survival strategies. It is thought that the environment drove these RNA strands to compete with one another. For instance, a scarcity of resources may have caused these replicators to use chemicals to destroy one another. If they were in fact successful at destroying other RNA strands then they not only kept other strands from taking scarce resources but they also had immediate access to a nucleotide corpse that they could use to create a replicate of themselves. It is thought that this “chemical warfare” caused them to develop protein enclosures to protect themselves from other strands and from the harsh environment. Today, we know these protein enclosures as cells, the smallest units of life. These simple cells changed and evolved to perform complex and meaningful functions, all of which came about to protect the RNA (and later DNA), and to increase the productivity of replication. Some of the earliest replicators used cells as vehicles to replicate more successfully, and it is obvious that we use our bodies (complex conglomerations of cooperating cells) in much the same way. Put simply, our bodies are vectors for replication which grew more and more complicated in response to intensely hostile and competitive environments. But it is important to point out that genetic or behavioral complexity is not necessary for successful replication. Not only are there far more individual bacteria on the earth than there are individual humans, but bacteria also make up a larger proportion of the earth’s biomass than all members of the human species combined. The evolutionary success of both humans and bacteria shows that nature can allow a large number of different reproductive strategies. As reproductive strategies go, there seem to be two very successful kinds. One, used by bacteria, is reliant on the transference of genes alone. The other, employed by humans, is reliant on the transference of genes as well as the transference of memes. Memes are units of cultural or social information and they are passed or taught from parent to child. The human condition is reliant on memes- without language, knowledge and skills humans can not survive on their own in a natural environment. Bacteria, plants and many other organisms however have no use for memes. All of the behavior that they need to reproduce is already contained in their genes and in their bodies at birth. It is interesting to note that memes can be thought of as another form of replicator. Just like genes they are passed from parent to child and usually only the ones that increase survival and reproductive success are able to persist. It turns out that there are two antagonistic strategies used by organisms to perpetuate themselves, one is the r strategy and it is reliant on genetic transference alone and the other is the K strategy which is reliant on both genetic and memetic transference. To learn more about the r and K strategy continuum click here. DNA Polymerase: noun Any of a variety of enzymes responsible for replicating a series of nucleotides by using a single stranded portion of DNA as a template. Meme: noun A unit of cultural or social information that is transmitted by behavior or by observation. A lesson that is transmitted from one mind to another. Nucleotide: noun The basic chemical constituent of DNA and RNA. A compound consisting of a phosphate group and a nucleoside. Parental Investment: noun The transference of resources, protection, memes or any form of care from a parent to its offspring. Replicator: noun Any construct that is able to reproduce or make copies of itself. Living organisms, memes, computer programs, cellular automata all can be thought of as replicators. |
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