Unit 7: Molecular Genetics 7.5 Gene Regulation and Expression

Remember A gene is a series of DNA bases that are the instructions to make a protein. Only a small percentage of the genome makes up our genes-called exons The majority of our genetic sequence acts as “switches”-called introns All somatic cells contain the entire genome but do not need to transcribe all of their genes at the same time! The environment as well as internal signals tell the cell what genes to transcribe.

Prokaryotic Gene Regulation

Prokaryotic Gene Regulation Prokaryotic cells turn genes on and off by controlling transcription. Some of these regulatory proteins help switch genes on, while others turn genes off The genes in bacteria are organized into operons-a group of genes that are regulated together and usually have related functions.

Operon Structure On each side of the operon’s genes is a regulatory region. A promotor is a DNA segment that allows a gene to be transcribed by allowing RNA polymerase to bind. An operator is part of DNA that turns a gene “on” and “off”. An operon includes a promoter, an operator, and one or more structural genes that code for all the proteins needed to do a job. The lac operon was one of the first examples of gene regulation to be discovered The lac operon has three genes that code for enzymes that break down lactose.

Example The lac operon acts like a switch The lac operon is “off” when lactose is not present The lac operon is “on” when lactose is present.

Eukaryotic Gene Regulation

Eukaryotic Gene Expression Eukaryotes regulate gene expression at many points Different sets of genes are expressed in different types of cells. Transcription is controlled by regulatory DNA sequences and protein transcription factors

Regulatory DNA Sequences Most eukaryotes have a TATA box promoter-this is a DNA sequence that is found just before a gene. This box binds a protein that helps position RNA polymerase

Transcription Factors Transcription Factors are proteins that bind to the DNA at the regulatory region (TATA Box) Transcription factors either encourage the gene to be expressed OR they keep the gene from being expressed Gene promoters (TATA Box) have multiple binding sites for transcription factors Some factors activate many genes all at once, dramatically changing patterns of gene expression in the cell Only form a response to chemical signals Other factors: mRNA leaving the nucleus, stability of the mRNA strand, breakdown of a gene’s protein, uncoiling tightly packed chromatin, Multiple transcription factors must bind before RNA polymerase can bind to the promoter region-they create the binding site!

mRNA Processing RNA processing is also an important part of gene regulation in eukaryotes. mRNA processing includes three major steps. Introns are removed and exons are spliced together A cap is added A tail is added

RNA Interference Most cells contain small RNA molecules that interfere with mRNA A “Dicer” enzyme cuts the interfering RNA into strands about 20 base pairs long. These small strands of RNA bind to proteins to form a silencing complex The silencing complex binds to the complementary mRNA to destroy it to keep them from making a protein

Genetic Control of Development

Regulating Gene Expression Regulating gene expression is especially important during the development of an organisms-each cell in the adult originates from the same single cell! Gene regulation helps cells undergo differentiation-becoming specialized cells As development occurs, different sets of genes are regulated by transcription factors and repressors.

Edward B. Lewis’ Fruit Fly Experiment First to show that a specific group of genes controls the identities of body parts in the embryo of the common fruit fly A mutation in one of these genes resulted in a leg instead of an antenna! The result: Homeotic genes: a set of master control genes that regulates organs that develop in specific parts of the body. Master control genes are like switches that trigger particular patterns of development and differentiation in cells and tissues. Two important types: Homeobox genes and HOX genes

Homeobox Genes Molecular studies of the DNA show that the homoeotic genes share a very similar 180 base DNA sequence-named Homeobox genes. Homeobox genes code for transcription factors that activate other genes that are important in cell development and differentiation. When these genes are activated in certain regions of the body, they encourage growth of the organs in that region.

HOX Genes Appear in a cluster HOX genes determines the identities of each segment of a fly’s body. Arranged in the exact order they are expressed from head to foot All living organisms have HOX genes that function in the same way

Environmental Factors Environmental Factors like temperature, salinity, and nutrient availability can influence gene expression. The environmental factors can promote/repress transcription factors that will function at the molecular level. Examples: Lac operon in E. Coli is switched on only when lactose is present Metamorphosis