Types of Gene Expression - Essay Example

INDEX Introduction Gene expression in Drosophila melanogaster Gene expression in vertebrates Comparison Introduction Before we start to Compare and contrast transgenic methods for manipulating gene expression between Drosophila and vertebrate animals let us discuss in brief about what is gene expression and function of genes Gene expression Genes exert their effects on the phenotype. The biological information is contained in the base sequence of DNA. Gene expression is the process by which this information is made available to the cell. This has been described by the central dogma. This states that information is transferred from DNA to RNA and RNA to a protein. During expression gene synthesis from mRNA molecule, into which information has been transferred in the form of genetic code. This synthesis of mRNA from of the strands of the DNA molecule is called transcription. The first step in gene expression is called transcription. Two strands of DNA separate, only one them act as template for the transcription into mRNA in 5'3' direction. This strand is non coding (antisense) strand and the other stand of DNA is coding strand (sense strand). This synthesis of polypeptide chain or a protein from mRNA is called translation. The sequence of amino acids into polypeptide is determined by the nucleotide sequence of Mrna in accordance with the rules of the genetic code. mRNA contains codons and these codons are decoded to respective amino acids in a process of translation to a polypeptide. Translation occurs in the ribosome's. The amino acids in the polypeptides and the nitrogen bases in DNA of the respective gene are related in a direct linear fashion. So, these two are colinear. Benzer 1955, proposed the term cistron, muton and recon for the gene Cistron: cistron is a segment of DNA specifying one polypeptide chain in protein synthesis Muton: muton is the smaller segment of DNA that can be changed in a mutation. It can be as small as one complimental nucleotide pair. Recon: the smallest segment of DNA that undergoes recombination. It is also as small as One complimental nucleotide pair. Functions of genes Many genes express whenever the product is required and the expression is regulated. These genes are called regulated genes. Certain genes are expressed as a function of the interaction between RNA polymerase with promoter without any regulation. Such genes are called constitutive genes. They are constantly needed for cellular activities. Flow of genetic information from DNA to protein through RNA was described by crick as central dogma of molecular biology. The studies of bacterial genetics indicate that all genes not only specify the structure of an enzyme but some of them also regulate the expression of other genes. These genes are called regulator genes. This concept of gene regulation has been studied by Franois Jacob and Jacques monod in 1961.the relationship between the nucleotide sequence of mRNA and amino acid sequence that constitute a polypeptide chain is called as genetic code. Gene expression in Drosophila melanogaster Introduction Heterochromatin was originally defined visually as the heavily staining region seen in both the meiotic and interphase nucleus (hietz 1928). The contrast between the openness of euchromantin and the compactness of the heterochromatic implied that the euchromatic is the location of most gene function and led to the neglect of heterochromatin. However, it has become increasingly clear that the nuclear genome is more than a collection of autonomous DNA sequence and that a deeper knowledge of the structure of the nucleus and the chromatin, including the heterochromatin, will contribute to our understanding of the gene expression. In Drosophila melanogaster heterochromatin accounts for approximately 30 % of the genome. Its properties have been studied since the advent of drosophila's use as an experimental organism. Heterochromatin in this species is found surrounding the centromeres of the X chromosomes and the two large autosomes. Additionally, the entirety of the Y chromosome is heterochromatic, as well as much of the small fourth chromosome. In the diploid metaphase chromosomes preparation heterochromatin can be mapped using differential staining pattern. However, heterochromatin is exceptional in that it is mostly underrepresented in polytene nuclei. Furthermore, the heterochromatic regions of polytene nuclei associate into a single chromocentre, so that when squashed, the euchromatin arms of the chromosomes appear to radiate out from a single point. A phenomenon in Drosophila called position effect variegation (PEV) has been intensively studies with the anticipation of gaining insight into the proteins that interact with heterochromatin and are responsible for its distinctive properties. Classical PEV is observed when a euchromatin gene is placed next to heterochromatin by a chromosomal rearrangement. The aberrant proximity of heterochromatin leads to the silencing of the gene. This silencing is thought to be an all-or-nothing event so that ones see patches of tissue where the gene is expressed as its normal levels interspersed with tissue where it is totally off, hence variegation. A classic example is Inversion (I) white-mottled-4, an allele of the gene that allows the deposition of eye pigment, whose absence leads to a white eye. In this overview we will describe the types of sequences and structure of Drosophila heterochromatin and point out the connection between repetitiveness and heterochromatin behavior. The heterochromatin of D. melanogaster harbors at least 40 conventional genetic loci, some nestled deep in centric heterochromatin. The heterochromatin genes that have been cloned contain numerous insertion of middle repetitive DNA surrounding the gene and within the introns. Interestingly these genes can show a heterochromatin dependent distance effect that can be thought of as an inverse of standard euchromatin PEV. Rearrangements that move a heterochromatin gene from its normal position to a more distal position in euchromatin have been found to decrease gene expression. Gene expression in vertebrate Earlier gene was regarded as a hypothetical particle. Chromosomal theory of Sutton and boveri explained that chromosomes are the carriers of genes and are the basis for mendelian segregation and independent assortment. Independently they discovered that chromosomes segregate during gametogenesis due to meiosis, like mendelian factor. These gametes become haploid and diploid nature is natured is restored in zygote due to random fertilization. Then, which is genetic material-either protein or nucleic acids This was solved with the transformation experiments conducted by Griffith. F. Griffith 1928 worked on Diplococcus pneumonia (now named as Streptococcus pneumonia). He used type II R and type III S types of Diplococcus for his experimentation. II R is a rough (non capsulated) and nonvirulent form causing no death when injected to mice. III S is smooth and virulent form causing death when injected to mice. These traits are genetically determined. When mice are injected with heat killed III S strain, mice were alive. When a mixture of heat killed III S strain and fresh II R strain was injected, the mice died. II R, the non virulent was transformed to virulent strain due to the heat killed III S strain. He concluded that transforming principle might be polysaccharide capsule or a compound required for capsule synthesis. O. T. Avery, MacLeod and McCarty, 1944 identified the nature of transforming principle in Diplococcus pneumonia. They treated the extract of transformed III S strain with different enzymes like protease, amylase, lipase, RNase, and DNase and conducted transformation assay tests. When the III S filtrate was treated with DNase the transformation did not occur. If filtrate was treated with any other enzyme, transformation occurred. They concluded that DNA was the genetic material (a transformation principle or active factor). A. D. Hershey and M. Chase 1952 also confirmed that the DNA is genetic material with their experiments using radioactive isotopes Comparison of transgenic method of gene expression between Drosophila and vertebrate Comparisons of invertebrate and vertebrate crystallin gene expression suggest convergent evolution for lens promoter activity in these distantly groups of animals. The first inkling for the mechanism of expression of crystallin genes between invertebrates and vertebrates came from studies on the glutathione S-transferase-derived S-crystallin genes of squid. The minimal promoters of the lens specific SL20-I-and SL11-crystallin genes contain an overlapping, protein binding AP-1/antioxidant responsive (ARE) element that is required for promoter activity in transfected vertebrate cells.Pax6 may also be connected to the regulation of squid crystallin gene as it is in vertebrate crystallin gene, although a direct connection has not been shown yet. The Pax6 gene is expressed in the developing squid eye and can induce ectopic eyes in Drosophila. These facts suggest resemblance between crystallin gene expression in squid and vertebrate. It is essential for an organism to develop normally, that genes be expressed at the proper time and in the proper cells. Therefore understanding the mechanism of gene regulation during development is one essential aspect of this study. The use of transgenic mice is invaluable to fulfill this goal; indeed, transgenes expression can be studied in all cell types and at any desired time, in the context of developing organism where all the possible regulatory circuits are at work. Innumerable individual reports addressing the question of gene regulation have been published during the past years. As already mentioned these genes have been shown to be evolutionary homologs of drosophila HOM-C genes. It comprises of 8 adjacent genes, the mutation of which results in homeotic transformation. Homeobox sequences are 183-bp exonic sequence present in multiple copies in Drosophila and vertebrate genomes. In Drosophila, homeobox sequences have been localized mainly in known developmental genes. In those cases in Drosophila homeobox genes where mutations are not available, expression pattern suggest that the homeobox genes are involved in embryonic development or differentiation. Based on the high levels of homology between Drosophila and vertebrate homeobox sequence, it was proposed that these genes may also play a role in embryonic development or cellular differentiation of vertebrates. Comparison of genomic organization of the Drosophila and vertebrate homeobox gene has shown that many of them are organized in clusters. The vertebrate clusters are homologous to fly clusters, the main difference being that in vertebrate the ancestral cluster apparently underwent several duplication. In addition to the homeobox genes clusters, in Drosophila there are a number of isolated homeobox genes scattered through the genome. Examples are caudal, H 2.0 and msh. For some of the Drosophila nonclustered homeobox genes, vertebrate homologs have been isolated such as Cdx 1 and CHox-cad for caudal, CHox E for H 2.0 and Hox 7.1 for msh. It has been observed that certain gene families include four members in a typical vertebrate but only one in Drosophila or other invertebrate, a pattern that has been cited as evidence for the 2R hypothesis. Despite talk of a 'four-to-one rule' in comparison of human to Drosophila family sizes, the number of gene families for which a ratio 4:1 is observed is in fact quite low. In a recent survey of all gene families, it was found that only 4.9% showed a ration of 4:1 between the number of members in human and the number of members in Drosophila. Furthermore, it is important to realize that a pattern of 4 genes in vertebrates and one in drosophila does not support the 2R hypothesis unless certain conditions are met. In Drosophila, genetic approaches have facilitated the identification and subsequent biochemical characterization of tissue-specific proteins that alter splicing patterns. However, tissue specific regulators of splicing, similar to those identified in Drosophila system, have not yet been identified in vertebrates. In the Drosophila genes of sex determination pathway, P-element transposase, and suppressor-of-white-apricot, alternative splicing is subject to regulation by factors that either inhibit or activate the use of alternative 5' or 3' splice sites. Based on this kind of regulation, functional or nonfunctional gene products are made. There are numerous characterized genes that utilize alternative RNA splicing for the generation of protein isoform diversity, and many of these genes have been catalogued previously. Some examples include genes encoding contractile proteins, immunoglobins, neuropeptides, and extra cellular matrix proteins. In Drosophila, genetic approaches have facilitated the identification of developmentally regulated proteins that specifically alter splicing patterns. In contrast to studies in Drosophila, much less is known about the cellular factors that mediate alternative splicing in vertebrate system. References 1. Louis Marie houdebine, (1997) transgenic animals: generation and use, 372-374 2. Lawrence, P.A. and morata, G. (1994) homeobox genes: their function in drosophila segmentation and pattern formation. Cell 78, 181-189 3. Robert J. Etches, (1993) manipulation of avian genome, 151-152 4. Bruce Albert's (1989) Molecular biology of cell, 554 5. Simon, J. (1995) locking in stable states of gene expression: transcriptional control during Drosophila development, 376-385 6. Benjamin lewin (1977) gene expression, 51-52 7. Joram piatigorsky (2007) gene sharing and evolution, 91-92 Read More