Saturday, March 14, 2009
Immortal Genes
On page 79 it says that "The genes these organisms all share have withstood more than 2 billion years of the steady bombardment of mutation and stand out as threads of text whose sequence and meaning have not changed significantly despite the bast differences among the species that carry them." How can genes stay the same while the species differ so drastically? The genes are immortal, as it says in the text, but what are some examples of such an organism that has many different species, but have many of the same strands of DNA? How does an organism diverge so far that it doesn't appear to be part of the same species, yet it contains the same DNA? What parts are therefore the same between those species if what we see is so different?
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ReplyDeleteOne of the main reasons that we find so many of these "immortal genes" amongst organisms of different species and even kingdoms is because of the similar functions we all need in order to survive. Carroll provides an example of a function that all living things need like, "the decoding of DNA and RNA and the making of proteins" (79). These sequences of genes that program our bodies to perform these functions are critical to our survival. These "immortal" genes NEED to stay the same amongst the species, because without them then life would cease to exist. That doesn't mean though that amongst every species these strands of immortal genes are exactly the same, because they are prone to mutation too. They serve the same purpose and can perform the same function because the body has been created to protect these genes. On page 59, Carroll lists the 4 facts about DNA, stating that mutations in DNA, although present, also occur in areas that may not be important to the organisms (not coded for). Also, species that do have mutations within these immortal genes still function because of our bodies ability to have several different codes for the same protein. That way, substitutions and the switching out of genes through mutation still produces the same desired effect (82).
ReplyDeleteOne gene that stays the same while species differ so drastically is the MC1R Gene or the melanocortin 1 receptor. On pg 60, Carroll uses this gene to explain the probability that mutations in the MC1R gene would cause the pigment in mice to change to black. However, this gene is not only present in mice. It is an immortal gene that holds the key to providing instructions for making the protein melanocortin 1 receptor, which plays an important role in normal pigmentation of not only mice, but humans, a range of mammals, etc. Nearly every organism ranging from species to species contains this gene, reason being that hair, fur, or skin color plays an adaptive/environmental role. Obviously, a mouse and a human have diverged so far that they would almost never though to be similar creatures, yet it contains the same code (MC1R) and many others that produce the same desired, pigmentation effect that best fits its social environment.
One gene that has stayed nearly the same in all species is the Pax-6 eye-building gene. This gene is the main gene that controls the development of the eyes, sensory organs, and neural tissues that relate to sight. Nearly every Bilateria has almost the exact same Pax-6 gene, as shown by Figure 8.2 on page 195. Only six amino acids differ between the Pax-6 gene in fruit flies and mammals, and the gene is the exactly the same in both mice and humans. The only difference in this gene stems from the fact that different animals have different kinds of eyes, including, “a camera-type eye with a single lens, a mirror eye with a lens and a reflecting mirror, and compound eyes made up of from ten to eight unit eyes” (194).
ReplyDeleteThe reason why this gene has been preserved between millions upon millions of species is that almost every species, especially terrestrial species, needs eyes. Eyes are vital in obtaining resources, finding mates, and avoiding predators. Also, eyes with trichromatic vision are especially important in animals with a high proportion of fossilized olfactory genes, such as humans. Some animals, such as a few burrowing animals and fish that live at very large depths of the ocean, don’t need eyes since they would be unnecessary in such dark environments. However, as Melissa said, some genes are completely necessary to most or all organisms, and the Pax-6 gene is one of them. Certain genes that code for such things as enzymes for replication, transcription factors, sodium-potassium pumps, and microtubules for other cell division are also vital to the life of all organisms.
The necessity for similar genes among most animals is shown by the huge amount of genes that diverse animals have. For example, as shown in Table 3.1 on page 77, even some of the least complex animals, such as fruit flies and nematodes, have humongous amounts of genes (13,468 and 20,275 respectfully). Although, humans have similar amounts of genes as nematodes, as they have 20-25,000 genes. This means that since very different animals have similar numbers of genes, then a lot of these genes kept by all these animals are the same. Genes for a wide variety of cellular processes are inevitably needed in all animals, and are represented in our genomes as immortal genes.