Download Cell differentiation and gene ACTION As the fertilized eggs begin to

CELL DIFFERENTIATION AND GENE ACTION
As the fertilized eggs begin to cleave, the biparental chromosomes duplicate themselves
and all daughter cells usually receive identical sets of chromosomes. Yet a complex organism
with several organs is formed from these genetically identical cells mass. The key event,
underlying such a development is cellular differentiation, a process by which the descendants of
the single celled zygote come to differ from one another and to form tissues and organs
performing specialized functions. The embryonic cells, during their differentiation into their final
specialized conditions, are influenced by successive inducing stimuli and thus become more
determined to differentiate along specific pathways.
The earliest step for cells to undergo differentiation is to stop proliferation.
Differentiation and proliferation are inversely related, i.e. differentiation programmes are
initiated as cell proliferation decreases. In molecular terms, the initiation of differentiation
programme that results from transcriptional activation of defined genes is closely linked to the
molecular mechanism that lead to the cell cycle arrest.
Mechanism of Gene Action
Gene expression is the process by which information from a gene is used in the synthesis
of a functional gene product. These products are often proteins, but in non-protein coding genes
such as ribosomal RNA (rRNA), transfer RNA (tRNA) or small nuclear RNA (snRNA) genes,
the product is a functional RNA. The process of gene expression is used by all known life eukaryotes (including multicellular organisms), prokaryotes (bacteria and archaea), possibly
induced by viruses - to generate the macromolecular machinery for life. Several steps in the gene
expression process may be modulated, including the transcription, RNA splicing, translation, and
post-translational modification of a protein. Gene regulation gives the cell control over structure
and function, and is the basis for cellular differentiation, morphogenesis and the versatility and
adaptability of any organism.
Gene expression is the most fundamental level at which the genotype gives rise to the
phenotype. The genetic code stored in DNA is "interpreted" by gene expression, and the
properties of the expression give rise to the organism's phenotype.
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Regulation of gene expression (or gene regulation) includes the processes that cells and
viruses use to regulate the way that the information in genes is turned into gene products.
Although a functional gene product can be an RNA, the majority of known mechanisms regulate
protein coding genes. Any step of the gene's expression may be modulated, from DNA-RNA
transcription to the post-translational modification of a protein.
Gene regulation is essential for viruses, prokaryotes and eukaryotes as it increases the
versatility and adaptability of an organism by allowing the cell to express protein when needed.
Although as early as 1951 Barbara McClintock showed interaction between two genetic loci,
Activator (Ac) and Dissociator (Ds), in the color formation of maize seeds, the first discovery of
a gene regulation system is widely considered to be the identification in 1961 of the lac operon,
discovered by Jacques Monod, in which some enzymes involved in lactose metabolism are
expressed by the genome of E. coli only in the presence of lactose and absence of glucose.
In multicellular organisms gene regulation drives the processes of cellular differentiation
and morphogenesis, leading to the creation of different cell types that possess different gene
expression profiles though they all possess the same genome sequence.
Differentiated eukaryotic cells possess the capacity for the selective expression of genes.
The selective binding of cell specific proteins called transcription factors to promoters and
enhancers in the genes modulates the rate of initiation of transcription, and thereby gene
expression.
It is now well established that differential gene expression results in the cellular
differentiation during embryogenesis. The environment in which the genes lie appears to be
responsible for activating or repressing certain genes. The cytoplasmic factor to induce gene
expression need not always be present in the respective cell cytoplasm, but can be derived by
cellular induction from the neighboring tissues to the responding tissue.
Factors influencing cellular differentiation
There are two major ways by which cell differentiation during embryogenesis takes
place. The first involves the cytoplasmic segregation of determinative molecules (morphogens)
during embryonic cleavage, wherein the cleavage planes separate qualitatively different regions
of the zygote cytoplasm into different daughter cells. The second mode of differentiation is
embryonic induction, which involves interaction of cells or tissues to restrict the fates of one or
both of the participants.
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