Bio 2.4.PDF

Chapter 2: Gene Expression Lesson 2"4 Regulation of Gene Expression in Eukaryotes Introduction Each cell in an organism contains a copy of that organism's entire genome, which includes all coding and noncoding DNA. However, it would not be favorable to express every gene simultaneously. Whether a...

Chapter 2: Gene Expression Lesson 2"4 Regulation of Gene Expression in Eukaryotes Introduction Each cell in an organism contains a copy of that organism's entire genome, which includes all coding and noncoding DNA. However, it would not be favorable to express every gene simultaneously. Whether an organism is unicellular or multicellular, regulation of gene expression allows cells to respond appropriately and differentially to external and internalsignals. While there are many similarities between prokaryotic and eukaryotic gene regulation, this lesson focuses m eukaryotic gene regulation, and prokaryotic gene regulation is covered in more detail in Concept 6.3.05. 2.4.01 Differential Gene Expression Wtrile all cells in a multicellular organism contain the same set of genes, only some genes are expressed *any given time. A subset of genes, called housekeeping genes, are constitutively (ie, always) Wressed, and are required for the maintenance of basic cellular functions. The remaining genes are ergressed only when they are required for a specific function. Tte difference between two individual cell types is ultimately a result of which specific genes are elpressed in each. For example, during embryonic development, genes specific to neuronal fferentiation (eg, genes that specify neurotransmitter types) are expressed in cells that will become tEurons, whereas genes specific to muscle differentiation (eg, genes that specify muscle fiber types) are ecpressed in cells that will become muscle cells (Figure 2.30).  Chapter 2: Gene Expression Neuron ranscription activator Gene Iu'' expression ,**4 w Neuron phenotype gene (eg, neurotransmitter type) Muscle cells ranscription activator Gene t expression '"**** Muscle phenotype (eg, fiber type) Figure 2.30 Differential gene expression leads to cell specialization. In multicellular organisms, differential gene regulation is the basis for the initiation and maintenance of cell specialization. During embryogenesis, gene expression is regulated in a differential manner to give rise to all varieties of cell types found in the mature organism (see Lesson 10.3 for more information)' As discussed in Lesson 1.4, eukaryotic DNA is packaged with histone proteins into chromatin, which is condensed to fit inside the nucleus of a cell. The packaging of chromatin inside the nucleus affects gene expression on a large scale (ie, large regions of a chromosome encompassing many genes) and at the individual gene level. Chromatin exists in two ditferent conformations: a closed form known as heterochromatin and an open form called euchromatin (Figure 2.31). Heterochromatin is more densely arranged, and genes present within regions of heterochromatin are rarely expressed. Euchromatin is more openly arranged and is associated with higher levels of gene expression. Histone proteins in chromatin have tails that are accessible to modifying enzymes, which can add or remove chemical groups to change chromatin structure, resulting in induced or repressed gene expression. For example, the aijrlitiorr of acetyl groups to histone tails is catalyzed by the enzyme histone acetylase. 5B  Chapter 2: Gene Fxpressior"r Heterochromatin Euchromatin Histone Acetyl nrouP\a mRNA Histone i acetylation \- Histone deacetylation Transcriptionally inactive genes Transcriptionally active genes (not expressed) (expressed) =igure 2.31 Histone acetylation. - stone acetylation influences gene transcription by prorioting the formation of more openly arranged :-:hromatin, making that region of DNA more readily accessible to transcription machinery. Conversely, ristone deacetylase enzymes downregulate gene expression by removing acetyl groups from histones, ,', - ch promotes a denser heterochromatin conformation and restricts access of transcriptional machinery =:e Figure 2.31). - -