The Remarkable Unity in Diversity: Common Features of All Life Forms, Study notes of Biology

Download The Remarkable Unity in Diversity: Common Features of All Life Forms and more Study notes Biology in PDF only on Docsity! The Great Truth Slide 2 You’re probably familiar with the saying that some see the glass as half full while others see the glass as half empty? It refers to how two seemingly opposite conclusions can be made about a single observation. The same situation arises when observing the biological world. Slide 3 One cannot help but be impressed by the diversity of life forms on the planet. They vary in every imaginable way – shape, size, color – flying and not flying – 2 legs, six legs, and no legs – single celled, microscopic protists to blue whales – feathers, fur, and bald – and so many more ways. Even within a single kind of organism there is exceptionally impressive diversity. There are 900,000 species of insects that have already been identified, and probably 2 million species in total. In the United States alone, there are 11,500 species of moths and butterflies.1 But high levels of diversity are not found only in insects. There are 200,000 species are leafy plants like trees, bushes, and ferns that have been identified, and even 4000 species of mammals!2 There are so many different kinds of organisms on the planet that a person could conclude that organisms are mostly different from one another. However, a quite different conclusion would be reached if one considered the differences and similarities that exist at the cellular level in various species. While there are certainly differences among cells from different species, and from different tissues found in a single species, there are also many features that are remarkably conserved, and are common to all forms of life no matter their size, shape, or other characteristics. Slide 4 So what are some of these features that are so highly common? Well, some of these common features you have already learned about. All cells are made of the same kinds of macromolecules – nucleic acids, lipids, proteins, and so on. And remarkably, these are all made from the same kinds of monomers – there are only 4 nucleotides in DNA, only 4 nucleotides in RNA, only 20 common amino acids in proteins, and the basic structure of lipids is always the same. No matter what species you choose to examine – from the smallest bacterium to the largest mammal – their composition and structure will be remarkably similar at the cellular level. 1 http://www.si.edu/resource/faq/nmnh/buginfo/bugnos.htm 2 http://www.cotf.edu/ete/modules/msese/earthsysflr/species.html Slide 5 This remarkable similarity suggests that cells from all species must have many catabolic and anabolic functions in common. As it turns out, this is actually the case. All cells must metabolize substrates and synthesize the monomers that are then used to make the macromolecules found in cells. This could be done by a myriad of different pathways that differ among species – but this is not the case. There are very few pathways for the synthesis of monomers that exist and they are common to all living organisms. How can this be true? Of course the answer to this question is unknown, but the existence of common pathways that lead to the synthesis of common monomers, that lead to the synthesis of common macromolecules constitutes some of the strongest corroborating evidence that support Darwin’s theory of evolution, and the parsimonious conclusion that life on earth as we know it evolved from simpler life forms through diversification and adaptive evolution. In other words, certain biological processes evolved early in the history of life on earth, and have been conserved throughout evolutionary time. Slide 6 Let’s consider a simple example. The heritable, genetic information of cells is always encoded in deoxyribonucleic acid – DNA. The code exists as “triplets” – sequences of 3 nucleotides. Each triplet in DNA is always faithfully transcribed into a matching triplet found in messenger RNA. Each of these mRNA triplets – which are called codons – encode a specific amino acid. This is always true in all organisms. So perhaps when we look at the remarkable diversity of life forms on the planet we can see that the “glass is both half empty and half full” – there is an incredible diversity of life forms, but there are also many features that have been conserved throughout evolution and can be found in all them. Slide 7 Since the macromolecules and monomers that comprise cells are common to all life forms, you might guess that the biochemical pathways by which they are produced are common to all life forms – and you would be right! Of course, not all organisms can do everything. In some instances an organism cannot make some amino acids or vitamins that it requires and these must be supplied in its diet. For example, human cells cannot make the amino acid lysine, and it must be consumed in the form of lysine- containing proteins. Nonetheless, the cells of most organisms have most of the means to make what they need in order to grow and reproduce. So you’re probably thinking that since cells are so complex, there must be a large number of biochemical pathways that lead to the synthesis of a large number of basic cell components … and you’d be wrong. Here’s a great truth about cellular biochemistry. All the components of cells can be synthesized from 12 intermediates that are formed during the metabolism of substrates. In addition, during