Tuesday, March 9, 2010

DNA and RNA as the Basis of Evolution

DNA is familiar to most people as our genetic code, the stuff that makes us who we are. Although this is true, it does not nearly encompass all that DNA is responsible for.

In humans, you must travel into any organ, such as the heart, find a tissue, such as heart tissue, and find cells within that tissue. The cells, on their own, are as complex as some organisms. But within the cell, we must magnify even further, and find the blob in the center that is known as the nucleus. Inside this nucleus, we encounter DNA wrapped and doubled up upon itself many times to form X-shaped structures called chromosomes. The human body contains 46 of these chromosomes in each cell, and each one has a much longer DNA strand (n fact, uncoiled, the DNA in a chromosome stretches 10,000 times longer). To fully understand of what importance DNA is, one must understand the structure of DNA.

DNA, short for deoxyribonucleic acid, is in the form of a double helix, which resembles a twisted ladder. The sides of this twisted ladder are made of deoxyribose, which is a sugar. The "rungs" of this ladder are called base pairs and there are four types. These types are cytosine, adenine, guanine, and thymine. These are abbreviated C, A, G, and T, respectively. These four are DNA bases. The bases A and T, and the bases C and G, always bond at a molecular level to form each "rung".

A diagram of DNA labeling each of the base pairs.

DNA is present in all known living organisms, which suggests that DNA would be behind the origin of life. However, another component of the cell is needed to aid DNA for life to continue. This component is the all-important enzyme. Most enzymes are proteins, and they are responsible for many of the chemical reactions that occur in the body. Remarkably, the combination, breaking down, and transforming of chemicals within the body is controlled by enzymes. For example, the breaking down of food in the stomach is attributed to acids, but the breaking down of chemicals at a molecular level is handled by the enzymes. In fact, DNA is dependent on enzymes for its very survival! Enzymes help to split DNA and later replicate it and even "check" for errors in the DNA strand, which lowers the number of mutations and changes that occur, which, if left alone, have the potential to cause disease and genetic disorders. In conclusion, enzymes are, along with DNA, a huge success in evolution that has developed remarkably over the last few billion years.

Despite the fame and omnipresence of this remarkable pair, DNA and enzymes are not believed to have been the basis of life on Earth. The most widely believed hypothesis has RNA present before DNA and enzymes, and all the simple life forms that only had RNA have been extinct for several billion years. But first, a brief (or possibly not so brief) word about RNA.

RNA is another acronym, standing for ribonucleic acid. This name is the same as deoxyribonucleic acid, except without the "deoxy". As explained previously, the sides of the double stranded DNA ladder are made of deoxyribose. This differs from RNA, because RNA is single stranded, with the strand made of ribose. Due to the absence of a second strand, the base pairs of RNA are just hanging off of it, which makes RNA more unstable, but it also has its advantages. The final major difference is that one of the base pairs, thymine, is absent, and replaced by uracil (U). The new base uracil still bonds with adenine (A) to make a base pair. The unique structure of some types of RNA allows the strand to fold over on itself, forming a double helix shape with a "hairpin" on the end.

In this image, a section of RNA is starting to fold up upon itself.

However, the single stranded nature of the RNA molecule provides many more benefits that to simply fold up upon itself. It can also attach to other RNA strands, and that is where the different types of RNA come in. There are three main types of RNA: messenger, transfer, and ribosomal RNA. Although there are other types, the three above will be sufficient to explain the main purpose of RNA.

Messenger RNA, abbreviated mRNA, contains the genetic code needed for synthesizing chemicals, mainly amino acids, which make up the center of a protein. The genetic code is a sequence of base pairs, just like in DNA. Transfer RNA, abbreviated tRNA, "reads" the genetic code from the mRNA by attaching itself to the mRNA and matching up the base pairs. Together, tRNA and mRNA create the many different types of amino acids using their genetic code to cause chemical reactions. The amino acids are transported by the tRNA, hence their name, to the site of protein formation, the ribosome. The ribosome (one of the main parts of a cell) contains ribosomal RNA, abbreviated rRNA, which develops the "meat" of a protein, the amino acid being a main component. The tRNA carries the amino acid into the rRNA's construction area by attaching to the rRNA, therefore creating a finished protein.

From the above information one can conclude that although DNA and enzymes are better at their individual jobs, RNA is more flexible, capable of both carrying a genetic code in a double helix form and starting chemical reactions that produce important parts of a living thing, amino acids, and completed proteins. Therefore, RNA is hypothesized to be present without DNA and enzymes in the simplest forms of life, some of them, without a cell. We define "living things" as things with cells, which isn't necessarily an accurate definition. Although there is no absolute information from the era of the first living things (which was from 3-4 billion years ago), we can speculate on events that lead to the first single celled organisms, some of which survive today.

The only thing surviving from the era that could be a living thing but isn't considered one is the virus. The virus isn't an organism by our standards because they don't contain cells. They do reproduce, which is another requirement for being a living thing, but it cannot reproduce on its own; it needs a host cell to infect with its genetic material. The encoded genetic message tells the cell to make for viruses, therefore spreading the virus. Viruses, on their own, are mere balls of genetic material, some types have DNA and some types have RNA, surrounded by protein. My personal opinion is that life can exist without cells, and that life, if started on other planets besides ours, could lack cells as well.

The moral of this story is that for all terminology, including "living thing" "mammal" (the main debate here is the platypus, which lays eggs, but is still considered a mammal) and even "planet" (Pluto. The rest is history) does not draw a distinct line classifying things. Things that possess RNA could be considered living, some (not many, but some), consider everything with atoms "living" in a sense. At this point, "living" is not an appropriate word, and should be replaced by "has the potential for living when grouped together with enough of its own kind". Nothing in this realm is clear, and it cannot be solved. By dropping terminology though, we can say that RNA is the basis of the first organisms that have set the stage for all bacteria, protists, plants, fungi, animals, and ultimately, us.

Sources: http://dl.clackamas.edu/ch106-08/enzymes.htm, http://www.uic.edu/classes/phys/phys461/phys450/ANJUM04/RNA_sstrand.jpg (image), http://www.elmhurst.edu/~chm/vchembook/583rnatypes.html, The Ancestor's Tale by Richard Dawkins.

No comments: