Homeotic Mutation
Homeotic mutations are changes in the genetic makeup of an organism which result in an incorrect positional placement of certain organs or structures. Genetic mutations such as these can lead to the development of bizarre organisms that have misplaced structures on their bodies.
Homeotic mutations were first described by Edward Lewis and his colleagues during the 1950's. Lewis dedicated much of his academic career to investigating the genetic makeup of the fruit fly. One facet of his studies involved finding the cause for a strange mutation in a fruit fly that resulted in legs growing from the head in place of antennae. During his investigation, he found that specific genes, he called homeotic genes, were responsible for the development of each region of the fly's body. He also discovered that these genes were arranged on the chromosome in a linear sequence. They were even arranged in the order corresponding to the order of the body region that they control. For example, the homeotic genes for the head were located at the beginning of the chromosome while those for the posterior were located at the end. The importance of Lewis' work to biology was recognized in 1995, when he was awarded the Nobel prize in physiology.
Homeotic genes are regulatory genes that dictate the type and placement of specialized structures on an organism. These genes even determine how body parts are shaped. For example, human toes and fingers are both composed of bone and muscle, but the shape and details are different. Toes and fingers develop differently because of the action of the homeotic genes. The proteins that these genes code for are regulatory proteins that bind to DNA. This binding action effectively activates or inactivates certain structural genes, thereby controlling which ones are expressed. A genetic mutation in a homeotic gene can effect the development of the organism.
Genetic mutations involve any change in the genetic code of a cell. They are rare events that typically occur during DNA replication. They may also be caused by mutagenic agents or radioactivity. A variety of point mutations can occur in which a single nucleotide in a gene is changed. Substitution mutations are those in which the wrong nucleotide is copied into the gene. Insertion mutations involve the addition of an extra nucleotide, and deletion mutations involve the deletion of a nucleotide from a gene. Any of these mutations can result in the expression of a malfunctioning protein. When a mutation occurs in a homeotic gene, the structural gene that it controls may be expressed inappropriately, creating a deformed organism.
In an organism like the fruit fly, a single homeotic gene can be responsible for the development of a whole structure. If a mutation occurs in this gene, a dramatic structural change can result. In more complex organisms, such as mammals, homeotic genes are typically found in multiple copies. For example, mice have four copies of their homeotic genes. These extra copies make it difficult to observe drastic changes as a result of mutations in homeotic genes. To some extent, the extra copies of the homeotic gene help compensate for the mutated one.
Since homeotic mutations lead to a change in body shape but are not necessarily lethal, it is thought that they may be responsible for evolutionary changes. This would occur if a change in body shape lead to a significant advantage for the organism. For example, a homeotic mutation could allow an animal to obtain food more effectively, or it could make the animal more attractive. This would lead to greater reproductive fitness of the animal and thus propagate the mutation to future generations.
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