Explain the Eukaryotic Gene Expression ?
Eukaryotic cells regulate the transcription of individual genes, large parts of chromosomes, or even entire chromosomes. Gene expression in eukaryotes is mostly controlled at the transcription level, and involves multiple proteins and multiple binding sites. While in prokaryotes, protein synthesis begins from a strand of mRNA before transcription is even completed, in eukaryotes, transcription occurs in the nucleus and translation takes place later in the cytoplasm.
RNA synthesis depends upon the coordinated action of sections of the chromosome called enhancers, RNA polymerase, and other proteins that attach to DNA in a coordinated fashion to form a transcription complex. A particular enhancer is active only in cells that contain the proper stimulator protein, which may be a hormone-protein complex. Many proteins are specified not directly by sequence of bases in DNA, but after mRNA is formed.
Eukaryotic genes contain large segments of DNA that is never transcribed into protein. They are separated into sections called exons, that are expressed into proteins, interspersed with inert regions called introns. After transcription, RNA nucleotides bind at the beginning and end of the newly synthesized RNA. Then, enzymes cut the molecule apart, the intron sections are discarded, and the exons are spliced together before transport of the new mRNA molecule through the nuclear pores into the cytoplasm and to the ribosomes.
The order in which exons are spliced together can vary depending upon the environment in the cell. For example, in humans, one possible mRNA splicing arrangement results in formation of the hormone calcitonin in the thyroid, while another protein associated with communication between brain cells is synthesized in the brain. Another regulatory mechanism is mRNA editing, for example, in humans, there is evidence that in the small intestine of a certain mRNA nucleotide is deaminated, which produces a stop signal for protein synthesis. The same change does not occur in the liver. Just which proteins are synthesized from a particular DNA strand is determined not only by the sequence of bases in the gene, but also by modification of the mRNA after it has been transcribed.
Development of an organism from a zygote is controlled by clusters of genes called homeotic genes containing units called homeo boxes that code for proteins called homeodomains. The arrangement of homeo box genes has been very much conserved in evolution, so that the vertebrate genes are similar to those in the fruit fly. One homeodomain coded by a mouse gene differs from that coded by a fly by only one amino acid. Homeotic genes are arranged in the same sequence as their position in the body of the organism, from head to tail.
Hormones signal the cell to start producing proteins in two ways: 1.) by entering the cell and triggering the production of proteins; and 2.) hormone-type molecules that bind to receptors in the cell membrane and cause the production of proteins. Contact between cells can also trigger the expression of genes.