Regulation of gene expression 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?

Selective gene expression 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) muscle cells liver cells

DNA Coiling Affects Gene Expression Chromosome Supercoils Coils Nucleosome Histones DNA double helix 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 Single chromosome is about 4 cm in length (1000s of times diameter of nucleus!) Wraps around histones (1/2 the mass of euk. chromosomes) Nucleosome - like a bead on a string (wrapped around 8 histones) Nucleosomes wrapped into tight helical fiber Coiled further into a supercoil Loops and folds further during mitosis to form a chromosome

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. Acetylation 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.

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 Different final mRNA from the same RNA transcript One gene = more than one polypeptide??

Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide