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The Code of Life: Topic 4 Regulation of gene expression What determines your phenotype? • Gene expression! • You have 23 pairs of chromosomes. • In each pair you get one from your mother and one from your father. • Each chromosome in a pair holds all the same genes as the other. • So what determines which gene is expressed when you develop? • ie how do you get your mother's eyes or your father's nose? Regulation of gene expression starts with cell differentiation during development. • Selective gene expression – All your cells contain ALL of your genes, they are not split up. – Depending on the function of the cell, some genes will be expressed and some will be permanently "turned off". • Can you think of a gene that is needed in red blood cells, but not in the pancreas? Neuron (nerve cell) liver cells muscle cells DNA Coiling Affects Gene Expression Nucleosome Chromosome DNA double helix Coils Supercoils Histones • • • • • DNA is negatively charged (phosphate groups) Histone proteins are positively charged This makes the DNA wrap around groups (8-9) of histones Each wrapped group is called a nucleosome The string then coils due to further charged-region interactions Levels of chromatin packing • • • • Chromatin modifications Remember… DNA is slightly negative Histone proteins have positive regions This results in coiling of the DNA, making it difficult for enzymes to reach some portions of the DNA for transcription and keeping other portions exposed. • This can be modified by two processes: • Methylation: addition of methyl functional groups to histone proteins. • Acetylation: addition of acetyl functional groups to histone proteins. – Acetyl groups are negatively charged. – They neutralize the histones they attach to, so DNA will not wrap as tightly and will uncoil. – This exposes regions of DNA to transcription enzymes. Acetylation Epigenetics • The study of how environmental factors can change gene expression without a change in DNA. • Examples: – Diet (during development and beyond) – Stress – Hormones – Chemicals – Inheritance of methylation patterns Extra slides Regulation of gene expression • Regulation occurs at many stages: – Chromatin coiling and modification – Transcription – Processing – Translation Regulation of gene expression Transcription Alternative RNA Splicing Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide