Control of Gene Expression Mechanisms of Gene Control Gene Control in Eukaryotes Master Genes Gene Control In Prokaryotes Epigenetics
Gene Expression The overall process by which information flows from genes to proteins (from genotype to phenotype) Control of gene expression: Ø Allows cells to produce specific kinds of proteins when and where they are needed Ø Cell Differentiation and development
Control of Gene Expression The switches that turn a gene on or off are molecules or processes that trigger or inhibit the individual steps of gene expression
At the chromosome level: DNA packing DNA packing in eukaryotic chromosomes regulates gene expression by preventing RNA polymerase and other transcription factors from getting in contact with the genes
Chemical Modification of the chromosomes DNA regions unwound from the histones are accessible to RNA polymerase Addition of chemical groups to the histone proteins can change the way they interact with the DNA Acetylation (-COCH 3 ) (loosens DNA) or methylation (CH 3 ) (tightens DNA)
Transcription Transcription factors Proteins that act in initiating or regulating transcription Repressors: Regulatory proteins that block the binding of RNA polymerase to the DNA Activators: Regulatory proteins that promote the binding of RNA polymerase to the DNA
Post-Translational Control Ø Many newly synthesized polypeptide chains must be modified before they become functional Ø Post-translational modifications inhibit, activate, or stabilize many molecules
Cell Differentiation in Eukaryotes Embryonic development involves: cell division, cell differentiation and morphogenesis Zygote: fertilized egg cell that results from the union of a female gamete (egg, or ovum) with a male gamete (sperm) Mitosis alone will produce a large ball of identical cells Cells have to differentiate to produce an organism
Cell Differentiation in Eukaryotes All of the cells within a complex multicellular organism contain the same DNA/ no loss or gain of DNA during cell differentiation (Genetic Equivalency) The body is composed of many types of cells What makes a liver cell different from a skin or muscle cell? The answer lies in the combination of genes that are turned on (expressed) or off (repressed)
Cell Differentiation in Eukaryotes Blood cells Bone cells
Cell Differentiation in Eukaryotes Ø As an animal embryo develops, its cells differentiate and form tissues, organs, and body parts Ø Driven by cascades of master genes expression Ø Master gene: Gene encoding a product that causes other genes to be expressed Ø Master genes define the overall plan of a body Ø Final outcome is the completion of an intricate task such as the formation of an eye
A homeotic gene is a master gene that governs the formation of a body part
X Chromosome Inactivation in Female Mammals/ Barr Body Initiated in early embryonic development (200 cells)/ Heavy methylation leads to inactivation of most of the genes on one of the X chromosomes / non coding RNA sticking to one of the X chromosomes Males and females have the same effective dose of the genes found on the X chromosome/ Dosage compensation Females are a patchwork mosaic: clumps of cells with the X chromosome inherited from mom being inactivated, clumps of cells with the X chromosome inherited from dad being inactivated
Tortoiseshell pattern on a female cat
Gene regulation in prokaryotes Ø Turns genes on and off Ø Prokaryotes do not undergo development, so these cells have no need for master genes Ø Helps prokaryotes respond to environmental changes and adjust their metabolism
Prokaryotes control gene expression mainly by adjusting the rate of transcription Operon: Cluster of genes with related functions along with some DNA control sequences Get transcribed together
The lac Operon
Epigenetic refers to heritable changes in gene expression that are not the result of changes in DNA sequence