Thursday, March 2, 2017 GOALS: Finish Ghost in your Genes Explain regulation of transcription and translation for eukaryotes RNAi HOMEWORK: Watch the RNAi video and answer the questions. You must have this done on Monday for discussion. Get ready for the Tuesday/Wednesday Exam
Chapter 18: DNA Packaging and Eukaryotic Gene Regulation Eukaryotic DNA is linear Chromosomes of DNA packaged during interphase as chromatin Chromatin condenses (further coils and folds) into chromosomes before mitosis On average, chromosomes contain 2 x 108 nucleotide pairs An ‘unpackaged’ chromosome of this size would be 6cm long! Detailed packaging info: see figure 19.1 Compare/contrast DNA packaging for eukaryotes vs. prokaryotes!
Histone proteins contain large amounts of positively- charged amino acids. Why? + DNA called a nucleosome! Distinguish between heterochromatin & euchromatin
DNA: Make a protein or not! 97% of DNA in humans does NOT code for proteins What does non-protein coding DNA do? Introns Tandem Repeats: may cause disorders such as Fragile X Syndrome and Huntington’s Chorea Regulatory: enhance or prevent transcription Make rRNA or tRNA or microRNA for RNAi!!! Interspersed, Repetitive DNA: Alu elements Interspersed, Repetitive DNA segments occur throughout the genome. Caused by transposons - moveable genetic elements. Each repeat is 100’s to 1,000’s of base pairs long. Copies are not completely identical to each other. In humans, 5% of DNA composed of repeats called Alu elements (300 bp long). Alu elements ARE transcribed, but function of RNA is unknown.
Regulation of Eukaryotic Gene Expression Not all genes are transcribed & translated in every cell The genes that are active at one time in the life of a cell will NOT be active during a different developmental time!
There are several places where regulation occurs in the process of gene expression: (figure 19.7) DNA packaging: methylation and acetylation Transcription: transcription factors, lipid hormones, RNA editing: alternative splicing Translation: RNAi (post-transcriptional silencing) Post-Translational Modification of Protein: protein cleavage, protein modification in ER or golgi apparatus (add lipids or oligosaccharides)
Opportunities for control of gene expression in eukaryotic cells Figure 18.6: Opportunities for control of gene expression in eukaryotic cells KNOW THIS
DNA Packaging Affects Expression Heterochromatin (highly condensed) DNA is usually NOT expressed because transcription proteins cannot access DNA. Genes around histones or attached to internal nuclear scaffolds are also NOT expressed Some nucleotides, specifically cytosine, are methylated (add -CH3 group). If DNA is methylated, it is usually NOT expressed.
Histone acetylation: add -COCH3 group to amino acids of histone proteins. Addition of group changes shape of histone, so they grip DNA less tightly, therefore DNA is expressed
Epigenetics What kind of rat mother are you? Go back to the Ghost in Your Genes Worksheet or the Lick your Rat website
Structure of a Eukaryotic Gene Control Elements: Upstream of gene (on DNA) that bind to transcription factors and therefore facilitate transcription Transcription factors are ESSENTIAL for RNA polymerase to bind DNA and begin transcription of gene sequence Proximal Control Elements: Near gene/promoter Distal Control Elements: far from promoter, upstream OR downstream
Promoter: Sequence of DNA where transcription begins; TATA box sequence Exon: sequence of nucleotides used to make a protein Intron: sequence of nucleotides not used to make protein; excised by spliceosomes Terminator: sequence of nucleotides downstream from last exon (signals RNA polymerase to cease transcription)
Eukaryotic Gene Structure and Primary Transcript
1. Activators bind to enhancers on DNA Model for enhancer action 1. Activators bind to enhancers on DNA 2. DNA bends 3. Other transcription factors bind to both activator & promoter for complete transcription initiation complex