GENE PART-

5

Gene structure of drosophila melanogaster

DROSOPHILA MELANOGASTER:-

Drosophila melanogaster is a species of Fly (the taxonomic order Diptera) in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting with Charles W. Woodworth's proposal of the use of this species as a model organism, D. melanogaster continues to be widely used for biological research in studies of genetics, physiology, microbial pathogenesis and life history evolution. It is typically used because it is an animal species that is easy to care for, has four pairs of chromosomes, breeds quickly, and lays many Drosophila melanogaster eggs. D.melanogaster is a common pest in homes, restaurants, and other occupied places where food is served.

Flies belonging to the family Tephritidae are also called fruit flies, which can lead to confusion, especially in Australia and South Africa, where the term fruit fly refers to members of the Tephritidae that are economic pests in fruit production, such as Ceratitis capitata, the Mediterranean fruit fly or "Medfly".

Genome of drosophila:-

The genome of D. melanogaster (sequenced in 2000, and curated at the Fly Base database contains four pairs of chromosomes: an X/Y pair, and three autosomes labeled 2, 3, and 4. The fourth chromosome is so tiny that it is often ignored, aside from its important eyeless gene. The D. melanogaster sequenced genome of 139.5 million base pairs has been annotated and contains approximately 15,682 genes according to Ensemble release 73. More than 60% of the genome appears to be functional non-protein- coding DNA involved in gene expression control. Determination of sex in Drosophila occurs by the X:A ratio of X chromosomes to autosomes, not because of the presence of a Y chromosome as in human sex determination. Although the Y chromosome is entirely heterochromatic, it contains at least 16 genes, many of which are thought to have male-related functions.

Similarity to humans:-

About 75% of known human disease genes have a recognizable match in the genome of fruit flies, and 50% of fly protein sequences have mammalian homologs. An online database called Homophila is available to search for human disease gene homologues in flies and vice versa. Drosophila is being used as a genetic model for several human diseases including the neurodegenerative disorders Parkinson's, Huntington's, spinocerebellar ataxia and Alzheimer's disease. The fly is also being used to study mechanisms underlying aging and oxidative stress, immunity, diabetes, and cancer, as well as drug abuse.

HOX GENE:-Hox genes (also known as homeotic genes) are a group of related genes that control the body plan of an embryo along the anterior-posterior (head-tail) axis. After the embryonic segments have formed, the Hox proteins determine the type of segment structures(e.g. leg, antennae, and wings in fruit flies or the different types of vertebrae in humans) that will form on a given segment. Hox proteins thus confer segmental identity, but do not form the actual segments themselves.Hox genes are defined as having the following properties:

•their protein product is a transcription factor•they contain a DNA sequence known as the homeohox•in many animals, the organization of the Hox genes on the chromosome is the same as the order of their expression along the anterior-posterior axis of the developing animal, and are thus said to display colinearity

Hox gene function in Drosophila

Drosophila melanogaster is an important model for understanding body plan generation and evolution.The general principles of Hox gene function and logic elucidated in flies will apply to all Bilaterian organisms, including humans.Drosophila, like all insects,has 8 Hox genes.These are clustered into two complexes, both of which are located on chromosome 3.The Antennapedia complex (not to be confused with the Antp gene) consists of 5 genes: labial (lab), proboscipedia (pb), Deformed (Dfd) Sex combs reduced (Ser) and Antennapedia (Antp). The Bithorax Homeohox gene expression inDrosophi la melanogaster complex, named after the Ultrabithorax gene, consists of the remaining 3 genes:Ultrabithorax (Ubx),abdominal-A (abd-A) and Abdominal-B (abd-B).

lab (labial)

The lab gene is the most anteriorly expressed gene. It is expressed in the head, primarily in the intercalary segment (an appendage-less segment between the antenna and mandible), and also in the midgut. Loss of function of lab results in the failure of the Drosophila embryo to internalize the mouth and head structures that initially develop on the outside of its body (a process called head involution). Failure of head involution disrupts or deletes the salivary glands, pharynx. The lab gene was initially so named because it disrupted the labial appendage; however, the lab gene is not expressed in the labial segment, and the labial appendage phenotype is likely a result of the broad disorganization resulting from the failure of head involution.

pb (proboscipedia)

The pb gene is responsible for the formation of the labial and maxillary palps. There is evidence that pb interacts with Scr.

Dfd (Deformed)The Dfd gene is responsible for the formation of the maxillary and mandibular segments in the larval head. The mutant phenotypes of Deformed are similar to those of labial. Loss of function of Deformed in the embryo results in a failure of head involution (see labial gene), with a loss of larval head structures. Mutations in the adult have either deletions of parts of the head or transformations of head to thoracic identity.

Ser (Sex combs reduced)The Ser gene is responsible for cephalic and thoracic development in Drosophila embryo and adult.

Antp (Antennapedia)The 2nd thoracic segment, or T2, develops a pair of legs and a pair of wings. The Antp gene specifies this identity by promoting leg formation and allowing (but not directly activating) wing formation. A dominant Antp mutation, caused by a chromosomal inversion, causes Antp to be expressed in the antenna! imaginal disc, so that, instead of forming an antenna, the disc makes a leg, resulting in a leg coming out of the fly's head

Ubx (Ultrabithorax)

The third thoracic segment, or T3, bears a pair of legs and a pair of halteres (highly reduced wings that function in balancing during flight). Ubx patterns T3 largely by repressing genes involved in wing formation. The wing blade is composed of two layers of cells that adhere tightly to one another, and are supplied with nutrient by several wing veins. One of the many genes that Ubx represses is blistered, which activates proteins involved in cell-cell adhesion, and spalt, which patterns the placement of wing veins. In Ubx loss-of-function mutants, Ubx no longer represses wing genes, and the halteres develop as a second pair of wings, resulting in the famous four-winged flies. When Ubx is misexpressed in the 2nd thoracic segment, such as occurs in flies with the "Cbx" enhancer mutation, it represses wing genes, and the wings develop as halteres, resulting in a four-haltered fly.

abd-A (abdominal-A)

In Drosophila, abdominal-A (abd-A) is expressed along most of the abdomen, from abdominal segment 1 (Al) to A8. Expression of abdominal-A is necessary to specify the identity of most of the abdominal segments. A major function of abd-A in insects is to repress limb formation. In abd-A loss-of-function mutants, abdominal segments A2 through A8 are transformed into an identity more like Al. When abd A is ectopically expressed throughout the embryo, all segments anterior of A4 are transformed to an A4- like abdominal identity The abd-A gene also affects the pattern of cuticle generation in the ectoderm, and pattern of muscle generation in the mesoderm.

Abd-B (Abdominal-B)

Abd-B is transcribed in two different forms, a regulatory protein, and a morphogenic protein. Regulatory Abd-B suppress embryonic ventral epidermal structures in the 8th and 9th segment of the Drosophila abdomen. Both the regulatory protein, and the morphogenic protein are involved in the development of the tail segment.