Chapter 19 The Organization & Control of Eukaryotic Genomes

Gene expression in eukaryotes has two main differences from the same process in prokaryotes. First, the typical multicellular eukaryotic genome is much larger than that of a bacterium. Second, cell specialization limits the expression of many genes to specific cells. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

control determines cellular differentiation during development eukaryotic genes needing simultaneous transcription are often scattered throughout genome -coordinated control is thought to be mediated by specific nucleotide sequences common to all genes of that group eukaryotic genes clustered in same location & related tend to be of common evolutionary origin, not metabolic pathway

Major goal of developmental biology is to explain how divergence of cells along different pathways results from different expression of identical genomes by specific mechanisms

Control of expression: coarse controls structural organization of chromatin sets coarse controls on gene expression -euk. DNA is managed by multilevel folding -chromatin consists of DNA & histone proteins compacted into nucleosomes -chromatin fibers fold in loops, which makeup compacted chromosomes -in interphase, heterochromatin-not actively transcribed

Control of expression: coarse controls -euchromatin-less condensed & actively transcribed; becomes highly condensed during mitosis (unfolded form of chromatin) Much of the DNA is noncoding Some DNA sequences are present in 100’s or 1000’s of copies, some is highly repetitive Repetitive sequences at telomeres help conserve chromosome tips Introns are part of noncoding DNA

multigene family – collection of genes similar or identical in nucleotide sequence

Levels of control: 1) structural – organization of chromatin into a compact form to fit into a cell’s nucleus is important in helping control which regions of the DNA are available for transcription 2) availability for transcription is the first level of control in euk. Cells; additional levels of control allow cells to alter gene expression in response to changing conditions & needs

Levels of control Transcriptional: 3) depends on regulatory proteins that bind selectively to DNA & other proteins Post transcriptional: 4) 5’ cap & polyA tail to mRNA transcript, introns removed & exons spliced together (any of these steps are places where regulation can occur)

Levels of control 5) proteins may bind to mRNA to block degradation by enzymes & block translation Translational: 6) usually block initiation phase of translation (regulatory proteins bind to specific sequences or structures at 5’ end) 7) inactivation of initiation factors

Levels of control: Post-translational: 8) modification of polypeptide by addition of chemical groups 9) selective degradation of particular proteins

Regulators hormones – chemicals that help control expression of genes - may activate certain genes -bind to cellular proteins to activate & bind to enhancers -bind to proteins to change conformation

Regulators transposons – can activate/inactivate genes by moving from 1 location to another methylation of DNA – (adding methyl groups) may decrease level of transcription

Control errors proto-oncogenes – key in cell growth & differentiation control -when these escape normal control, they release cells from constraints of contact inhibition, allowing formation of benign tumors *may be activated by environmental factors