GENE REGULATION RESULTS IN DIFFERENTIAL GENE EXPRESSION, LEADING TO CELL SPECIALIZATION Eukaryotic DNA

Epigenetics – Ghost in Your Genes  Watch to see how your environment and your choices influence inheritance…  feature=share&list=PLA2E1F3FFBFAE1CB6 feature=share&list=PLA2E1F3FFBFAE1CB6

Differential Gene Expression  Nucleosome Packing: DNA wraps around histone proteins to form a structure called a nucleosome. Nucleosomes help pack DNA into eukaryotic chromosomes.  When acetyl groups attach to the histone proteins the DNA in chromosomes loosens to allow for transcription.  The addition of methyl groups to histone proteins can cause DNA to condense thus preventing transcription.  In Genomic Imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development.

Fig Enhancer (distal control elements) Proximal control elements Poly-A signal sequence Termination region Downstream Promoter Upstream DNA Exon Intron Exon Intron Cleaved 3 end of primary transcript Primary RNA transcript Poly-A signal Transcription 5 RNA processing Intron RNA Coding segment mRNA 5 Cap 5 UTR Start codon Stop codon 3 UTR Poly-A tail 3 Organization of Typical Eukaryotic Genes

The Roles of Transcription Factors  Regulatory Proteins, repressors and activators, operate similarly to those in prokaryotes, influencing how readily RNA polymerase will attach to a promoter region. In many cases, numerous activators are acting in concert to influence transcription.

Fig Enhancer TATA box Promoter Activators DNA Gene Distal control element Group of mediator proteins DNA-bending protein General transcription factors RNA polymerase II RNA polymerase II Transcription initiation complex RNA synthesis

Coordinately controlled eukaryotic genes  A particular combination of control elements can activate transcription only when the appropriate activator proteins are present.  All cells of an organism have all chromosomes/genes but certain genes are only active in certain cells. The transcription factors present in the cell determine which genes will be active and which won’t (but they are both still present)

Fig Control elements Enhancer Available activators Albumin gene (b) Lens cell Crystallin gene expressed Available activators LENS CELL NUCLEUS LIVER CELL NUCLEUS Crystallin gene Promoter (a) Liver cell Crystallin gene not expressed Albumin gene expressed Albumin gene not expressed

Post Transcriptional Regulation  Alternate Gene Splicing - different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns

Fig or RNA splicing mRNA Primary RNA transcript Troponin T gene Exons DNA

Noncoding RNAs role in gene expression  RNA Interference, noncoding RNAs play multiple roles in controlling gene expression. MicroRNAs (miRNAs) and Small inserting RNAs (siRNAs) are small single-stranded RNA molecules that can bind to mRNA. These can degrade mRNA or block its translation. The difference between the two is that they form from different RNA precursors.

Fig miRNA- protein complex (a) Primary miRNA transcript Translation blocked Hydrogen bond (b) Generation and function of miRNAs Hairpin miRNA Dicer 3 mRNA degraded 5