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 no introns, small amount of non-coding DNA regulatory sequences: promoters, operators Plasmid

Prokaryote

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

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

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 nucleosomes “beads on a string” 1st level of DNA packing histone proteins have high proportion of positively charged amino acids (arginine & lysine) bind tightly to negatively charged DNA from DNA double helix to condensed chromosome 2005-2006 7 7

Nucleosomes “Beads on a string” 1st level of DNA packing 8 histone molecules 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 2005-2006 8 8

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

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

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 11

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 12

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 Acetylation – neutralizes positive charge so it can no longer bind to neighbouring nucleosomes 2005-2006 13 13

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

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 Prokaryote mRNA is usually degraded within a few minutes of their synthesis, so they quickly respond to environmental changes. 15

Protein Degradation by Proteosomes ubiquitin tagging proteosome degradation

Other DNA Regions

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

Chromosome Structure centromeres split chromosomes p arm – petit arm q arm – long arm

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 Repeats Disease < 27 - 27 – 35 36 – 39 + / - > 39 +

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 21

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.

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