7.2(2) Regulation of Gene Expression

Important Terms Gene expression Gene regulation Epigenetics Genotype Phenotype Lac operon Nucleosomes Methylation Transcription factors

Gene Expression Virtually all of our cells have identical DNA yet they appear vastly different and have different functions

Gene Expression So the DNA of an individual is not the only thing that determines structure (also known as phenotype) The proteins made by a cell are influenced by other factors

Regulation of Gene Expression Gene expression (the degree to which genes are made into proteins) can occur at many different places

Transcriptional Regulation Sometimes gene expression is controlled by regulating the rate of transcription

Transcription Review As we learned last class, transcription begins when RNA polymerase binds to a promoter region on DNA Actually, it’s a little more complicated than that. RNA polymerase activity can be enhanced by a variety of proteins that bind to a part of the DNA called an enhancer region as well as to the RNA polymerase

The Lac Operon In prokaryotes, the transcription of genes that code for enzymes that digest lactose are regulated by lactose itself Initially, RNA polymerase cannot attach to the DNA because the site is blocked by a repressor protein When lactose builds up in the cell, a lactose molecule will bind to the repressor and remove it so the enzymes can be transcribed

Transcriptional Regulation of Gene Expression Only about 1% of our DNA actually codes for proteins. The rest of it has different functions. A great deal of non- coding DNA is there to control the rate that genes are transcribed at Many substances are known to affect the rate of transcription by binding to enhancer or repressor regions of DNA These substances are known as transcription factors

Nucleosome Regulation When DNA is stored in a cell, it is tightly wrapped around sets of proteins called histones. This combination of DNA and histones is called a nucleosome

Nucleosome Regulation Before DNA can be transcribed, it must loosen itself from the histones that it is wrapped around Sometimes a methyl group can attach to the tail of a histone and prevent the DNA from unwinding. This will inhibit transcription

Nucleosome Regulation The addition of an acetyl group (C 2 H 3 O) to histones can loosen the DNA and increase the rate of transcription

Environmental Influence There is still a lot to learn about gene regulation but there is much evidence to show that an individuals environment can influence the degree to which genes are expressed. Some factors, such as stress, can cause certain areas of someones DNA to be “tagged” with methyl groups. This would prevent certain genes from being expressed

Epigenetics Epigenetics is the study, in the field of genetics, of cellular and physiological phenotypic trait variations that are caused by external or environmental factors that switch genes on and off Different cells have different methylation and acetylation tags When sperm meets egg, this epigenome gets erased. However, ~1% of it does not so it appears tha a small number of these tags can be passed down from one generation to the next. This is contrary to Darwin’s theory which states that traits acquired in an individuals lifetime cannot be passed to offspring

Watch These! zWoobt_Q (transcription factors) zWoobt_Q xdI1zvk (the lac operon) xdI1zvk