Chapter 11 Gene Expression

Role of Gene Expression Chapter 11 Role of Gene Expression Gene expression is the activation of a gene that results in transcription and the production of mRNA…and eventually the production of a PROTEIN Only a fraction of any cell’s genes are expressed at any one time Remember, EVERY CELL IN YOUR BODY (except red blood cells) has the same DNA But your kidney cells don’t need to make liver proteins and vice versa So kidney cells will transcribe KIDNEY proteins, and liver cells will transcribe LIVER proteins

Gene Expression in Prokaryotes Chapter 11 Gene Expression in Prokaryotes An operon is a series of genes that code for specific products and the regulatory elements that control these genes. In prokaryotes, the structural genes, the promoter, and the operator collectively form an operon.

Gene Expression in Prokaryotes, continued Chapter 11 Gene Expression in Prokaryotes, continued A promoter is the segment of DNA that is recognized by the enzyme RNA polymerase, which then initiates transcription Structural genes code for the actual protein An operator is the segment of DNA that acts as a “switch” by controlling the access of RNA polymerase to the promoter A repressor may bind to it, thus preventing transcription

Gene Expression in Prokaryotes, continued Chapter 11 Mechanism of lac operon The prokaryotic lac operon codes for the lactase enzyme In bacteria, if there is NO lactose present, the lac operon is REPRESSED (no transcription to make lactase) No point in making a product (in this case, lactase) if it is NOT NEEDED (that would be a waste of energy)

Gene Expression in Prokaryotes, continued Chapter 11 Mechanism of lac operon Again, the prokaryotic lac operon codes for the lactase enzyme In bacteria, if lactose IS present, the lac operon is ACTIVE (need to transcribe lactase in order to break down the lactose) Repressor is removed in the presence of the lactose, which acts as an inducer

Gene Expression in Eukaryotes Chapter 11 Gene Expression in Eukaryotes Structure of a Eukaryotic Gene Eukaryotes DO NOT HAVE operons The genomes of eukaryotes are larger and more complex than those of prokaryotes Eukaryotic genes are organized into noncoding sections, called introns, and coding sections, called exons Control After Transcription In eukaryotes, gene expression can be controlled after transcription—through the removal of introns from pre-mRNA Remember…introns stay IN the nucleus, but exons EXIT the nucleus and are EXpressed into proteins!

Removal of Introns After Transcription

Eukaryotic Gene Expression Many eukaryotic genes have a sequence called the TATA box The TATA box seems to help position RNA polymerase Eukaryotic promoters are usually found just before the TATA box, and consist of short DNA sequences

Heterochromatin vs. Euchromatin How tightly the DNA (chromatin) is coiled around the proteins (histones) also affects gene expression Euchromatin: Uncoiled; certain regions of DNA loosely associated with histones Genes are easily transcribed Heterochromatin: Tightly coiled; regions of DNA wrapped tightly around histones These genes are not easily transcribed

Heterochromatin vs. Euchromatin Because the DNA is not tightly wound up in euchromatin, the “machinery” needed to transcribe genes can fit onto the genes between the histones = TRANSCRIPTION In heterochromatin, the histones are too close together to allow the transcription “machinery” to position itself on genes ≠ TRANSCRIPTION NO transcription Transcription proceeds

Gene Expression in Embryonic Development Chapter 11 Gene Expression in Embryonic Development The development of cells with specialized functions is called cell differentiation Examples: Liver cells, muscle cells, white blood cells As an organism grows and develops, organs and tissues develop to create a characteristic form. This process is called morphogenesis Both cell differentiation and morphogenesis are governed by gene expression

Homeotic Genes Homeotic genes: Regulatory genes that determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis The “master genes of development” Homeotic genes control the expression of other genes, thus regulating many aspects of development

Hox Genes Hox genes initially discovered in Drosophila (fruit flies) Hox genes control the segmentation pattern of embryonic fruit flies, which eventually results in the normal development of the flies’ appendages in the correct location Each Hox gene controls the development of a particular body section

Hox Genes Gone Wrong Mutant with legs growing out of head Normal No legs, but plenty of eyes

Hox Genes In Other Organisms Hox genes seen in Drosophila appear to be conserved (evolutionarily similar) to regulatory genes in other organisms These similar genes allow for similarity in patterns of body development