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Eukaryotic Gene Expression Chapter 19 Why control gene expression To respond to their external environment To respond to their internal environment Direct the cellular differentiation necessary to create specialized cells Packing of DNA Each chromosome is long-all 46 chromosomes are about 2 m long Multilevel folding helps determine gene activity Histones are small positively charged proteins Histones bind to DNA to make up chromatin These form “beads” each of which consists of a nucleosome of DNA wound around a protein core of 8 histones Higher Levels of DNA packing 30 nm chromatin fiber is a repeating array of 6 nucleosomes organized around histone molecules (H1) Looped domain is a fold in the 30 nm chromatin fiber that may contain genes that are expressed in a coordinated fashion Looped domains may coil and fold, compacting the chromatin Highly compacted heterochromatin is not transcribed Open form of chromatin, euchromatin, is transcribed Role of DNA Packing and Methylation Barr bodies are examples of heterochromatin that is not transcribed DNA methylation of cytosine residues are not expressed Drugs that inhibit methylation can induce gene reactivation DNA methylation may cause the double helix to assume the left-hand Z form, which may function in gene regulation Transcriptional Control Chromosome puffs appear along polytene chromosomes in larvae These DNA loops make the DNA more accessible to RNA polymerase Loops are regions of active RNA synthesis Shifting locations of puffs indicated that genes are turned on and off Ecdysone, insect hormone that initiates molting, can induce changes in puff patterns Gene regulation is responsive to chemical signals Gene activation Most genes are inactive Gene regulatory proteins interact with the nucleotide sequences, or regulatory sites, to activate transcription of genes Promoter regions 2 types of promoter regions have been identified that are specifically bound by RNA polymerase There are 3 different RNA polymerases which transcribe rRNA, mRNA, & tRNA Enhancers are DNA sequences that greatly increase the activity of nearby genes Enhancers are recognition sites for proteins that increase DNA’s accessibility to RNA polymerase Transcriptional Control Summary DNA packing Polytene puffs Gene regulatory proteins Promoter regions Enhancers Posttranscriptional Control Gene expresseion is meausred by the number and types of proteins that are made Gene expression can be blocked or enhanced at any posttranscriptional step Translation is separated by nuclear membrane from transcription Posttranscriptional Control in nucleus Removal of intron DNA segments Splicing together of exons Poly U caps and poly A tails Posttranscriptional Control in cytoplasm Eukaryotic mRNA can last hours or even weeks Length of time before degraded by cellular enzymes is related to quantity of protein synthesis they can direct Translation of mRNA can be delayed until control signal initiates it Initiation factors may be required for translation Polypeptides are extensively cleaved to produce an active protein Selective degradation of protins and metabolic regulation of enzymes serves as control mechanism in cell Multigene families These are collections of similar or identical genes that code for RNA products Identical genes coding for the major rRNA molecules are arranged one after another hundreds or thousands of times, forming huge tandem arrays Tandem arrays enable cells that are actively synthesizing proteins to produce millions of ribosomes required Beta polypeptide chains of hemoglobin are an example of a multigene family Origin of multigene families Families of identical genes may have risen from repeated gene dupliation Nonidentical families probably arose from mutation in duplicated genes Pseudogenes, sequences of DNA that resemble real genes but lack signals for expression, may be present in gene families Globin pseudogenes lack introns and have poly A tails, may have been transcribed from RNA by reverse transcriptase and may have moved about the genome by transposition Highly repetitive sequences Highly repetitive short sequences may make up 10-25% of total DNA Called satellite DNA because of their base compositions may be sufficiently different from rest of the cell’s DNA to isolate them by ultracentrifugation This DNA is located at centromeres and may be structural rather than genetic role in cell Cell differentiation Regulation of gene expression leads to genes producing their particular set of proteins exclusively Zygote is totipotent Differentiation of cells is signaled by changes in cellular structure, appearance of characteristic proteins, or initially, by accumulation of specific mRNA molecules Environmental cues or chemical turn on the genes that allow cells to become specialists at making the particular proteins associated with their functions Genomic Equivalence All cells contain the full complement of genes Differentiation is reversible Determination affects the ability of a nucleus to express all of its genome Regeneration is replacement of lost parts in an organism Many vertebrates can regenerate wholes from fragments Cancer: Development and differentiation out of control No contact inhibition Tumor forms (the HeLa cell line has been reproducing in culture for 30 years!) Benign tumors remain at original site and may be completely removed by surgery Malignant tumors formed by cells that lose ability to recognize each other and lose sticking qualities to other cells Cells may then travel throughout body-metastasis Carcinogens: physical or chemical agents or viruses may induce cancer Carcinogens may activate protooncogens to become oncogenes If a gene has multiple copies, has been transposed, or translocated, it may become an oncogene Homeotic genes and homeoboxes Homeotic genes play major role in control of developental patterns in all organisms Homeoboxes are translated into amino acid sequences, called homeodomains, which are able to bind to DNA Proteins containing homeodomains are regulatory proteins Gene amplification Multiple copies of rRNA genes are present in the genomes of most eukaryotic cells – form nucleolus Gene amplification is the selective synthesis of extra DNA Selective Gene loss Chromosome diminution is elimination of whole or parts of chromosomes from cells early in development Occurs in some organisms (like flies) Rearrangements in the genome Rearrangements may activate or inactivate genes Much of the variation in antibodies arises from different combinations of variable and constant regions of 4 polypeptides that form the complete antibody molecule This is an example of genomic nonequivalence Aging Organisms change by growth and maturation, metamorphosis, wound healing, and regeneration of body parts Genes program aging and death of selected parts of organisms throughout development Webbing of fingers of human embryo, production of mature xylem, loss of tadpole tails Human fibroblasts in cell culture stop reproducing and degenerate and die after 50 divisions other cell types have different number Aging may be accumulation of mutations and other problems resulting in functional decline of cells Aging and death may be programmed Epigenesis versus preformation Preformation – egg has preformed mini me Epigenesis forma emerges gradually through progressive development