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DNA Organization

DNA Organization. Eukaryotes vs. Prokaryotes. Prokaryotes. small size of genome circular molecule of naked DNA called a PLASMID DNA is readily available to RNA polymerase control of transcription by regulatory proteins (operon) most of DNA codes for protein or RNA

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DNA Organization

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  1. DNA Organization

  2. Eukaryotes vs. Prokaryotes

  3. Prokaryotes • small size of genome • circular molecule of naked DNA called a PLASMID • DNA is readily available to RNA polymerase • control of transcription by regulatory proteins (operon) • most of DNA codes for protein or RNA • no introns, small amount of non-coding DNA • regulatory sequences: promoters, operators Plasmid

  4. Prokaryote

  5. Eukaryotes • much greater size of genome located in nucleus • how does all that DNA fit into nucleus? • DNA packaged into chromatin fibers • regulates access to DNA by RNA polymerase • most of DNA does not code for protein • 97% “junk DNA” in humans

  6. Remember… • The control of gene expression can occur at any step in the pathway from gene to functional protein • Today we will talk about regulation of gene expression • DNA packing/unpacking • RNA processing (pre- mRNA  mRNA) • Degredation of mRNA

  7. DNA Packing How do you fit all that DNA into nucleus of a eukaryotic cell? DNA coiling & folding double helix nucleosomes chromatin fiber looped domains chromosome from DNA double helix to condensed chromosome 2005-2006 7

  8. Nucleosomes “Beads on a string” 1st level of DNA packing histone proteins 8 protein molecules many positively charged amino acids arginine & lysine DNA backbone has a negative charge histones bind to DNA due to a positive charge 8 histone molecules 2005-2006 8

  9. 30 nm fibre (Solenoid Fibre) nucleosomes are organized in a stacked spiral structure the solenoid fibre is known as the 30 nm fibre

  10. Chromatin Packing Euchromatin Heterochromatin • eu – true • loosely packed DNA regions which allows transcription to readily occur • hetero – different • tightly packed DNA regions with little transcription

  11. DNA packing and transcription Degree of packing of DNA regulates transcription tightly packed = no transcription = genes turned off darker DNA (Heterochromatin) = tightly packed lighter DNA (Euchromatin) = loosely packed 2005-2006

  12. DNA Methylation attachment of methyl groups (–CH3) to cytosine Methylation of DNA blocks transcription factors no transcription = genes turned off nearly permanent inactivation of genes 12

  13. Histone Acetylation • attachment of acetyl groups (–COCH3) to histones • Acetylation of histones unwinds DNA • loosely packed = transcription • = genes turned on • conformational change in histone proteins • transcription factors have easier access to genes 2005-2006 13

  14. RNA processing Alternative RNA splicing variable processing of exons creates a family of proteins 2005-2006

  15. Regulation of mRNA degradation Life span of mRNA determines pattern of protein synthesis Eukaryotic mRNA can last from hours to weeks Prokaryotic mRNA is usually degraded within a few minutes of their synthesis Prokaryotes are therefore better able to respond quickly to environmental changes

  16. Protein Degradation by Proteosomes • Protein degradation • ubiquitin tagging • proteosome degradation

  17. Other DNA Regions

  18. Chromosomal Sections centromere • region where sister chromatids are connected • made up of repetitive sequences telomere • ends of chromosomes • made up of repetitive sequences

  19. Chromosome Structure • centromeres split chromosomes • p arm – petit arm • q arm – long arm

  20. VNTRs (microsatellites) variable number tandem repeats (VNTRs) – repetitive DNA sequences in coding and regulatory regions • repeating sequences can be of any length • usually 2 – 6 NTs • sequence repeated a different amount of times Huntington’s Disease

  21. Huntington’s disease Huntington’s Disease Mutation on chromosome 4 CAG repeats 40-100+ copies normal = 11-30 CAG repeats CAG codes for glutamine amino acid Abnormal (huntingtin) protein produced chain of charged glutamines in protein bonds tightly to brain protein, HAP-1 21

  22. Pseudogenes pseudogenes– NT sequence similar to that of another functional gene • not transcribed to RNA or make protein Thought to have been mRNA which were reverse transcribed to DNA and inserted into the genome.

  23. Classwork/Homework • Section 5.7 Pg. 265 #2 • Section 5.8 Pg. 267 #1,3-5 • Homework is being checked and taken up next class… so have your questions ready! • Section 5.6 (mutations) pg. 263 #1-8 • Section 5.5 (control mechanisms) pg. 258 #1-6 • Section 5.7 (Prokaryotes vs. Eukaryotes) pg. 265 #2 • Section 5.8 (Genome organization) pg. 267 #1,3-5 • Chapter 5 Quest Date: Thursday, March 1

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