Eukaryotic Genomes 11 November, 2005 Text Chapter 19 In eukaryotes, DNA is present as several different molecules. Each DNA molecule, along with its associated proteins is one chromosome. Chromosomes are in the extended conformation while they are being transcribed. They are at their most condensed during nuclear division. Chromatin Changing the level of active protein to suit cell type and environmental conditions is the goal of gene regulation. This regulation can occur at any of the steps in gene expression. Regulation of transcription is most common, important examples of each type of regulation exist. Regulation of Gene Expression Histone acetylation leads to derepressed chromatin. Eukaryotic genes differ form prokaryotic genes in several important respects. First, these genes are never in operons. Each is transcribed independently. Introns exist, and are spliced out of the primary transcript. Finally, control elements, the binding sites for transcription factors, may be close to or far from the promoter. How can transcription factors bound to distal control elements (activators) influence transcription? Cell-type-specific Transcription Proteins recognize specific DNA sequences by forming weak interactions between amino acids on the protein and bases in the DNA molecule. Alternative Splicing RNAi Protein Degradation These changes include abnormal activation of oncogenes and inactivation of tumor suppressor genes. Cancer results from genetic changes that affect the cell cycle. Usually, a cancerous transformation in any cell requires that several oncogenes be activated and several tumor suppressors be inactivated. The genetic changes that cause colon cancer are well understood. Eukaryotic DNA Replicative transposition and unequal crossing over can lead to gene duplication Multigene families The globin gene family arose by duplication and differentiation. Duplication can occur due to unequal crossing over. Exon shuffling can lead to new proteins with novel functions.