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TR056/PG1005 Lecture 14 Chromosomal Organisation (Composition and Packaging of DNA) Dr Neil Docherty Dr. My Teaching Objectives •Revise R i th the content t t off th the nucleus l and d th the organisation of DNA into chromosomes •Introduce Introduce the structure of DNA and its organisation into functional units known as genes •Describe double helical and nucleosomal DNA f ldi and folding d packaging k i The Nucleus Revision Nucleus is the largest organelle (>6 micron diameter) Surface Nuclear envelope (double membrane), ribosome studded. N l Nucleoporin i fformed d channels. h l Transmission Electron Microscopy The Nucleus Inside Discernable Features -Heterochromatin -Euchromatin E h i -Nucleolus Constituents DNA, RNA Nucleoprotein p ((histone/non-histone)) Active cell -Prominent Prominent nucleoli -euchromatin -Dispersed basophilia Resting cell -Small absent nucleoli, -Heterochromatin H t h ti -Dense basophilia The Fundamental Role of The Hereditary Material Required q for •Replication of cell Tissue integrity/function Required for Function of cell DNA Required for •Gene transcription Exception: What about Red Blood Cells and Platelets?? Chromosomes Human DNA-46 units of linear double stranded DNA polymer -22 22 pairs of autosomes (Ch1 (Ch1-22) 22) -2 sex chromosomes (XX / XY) Total Length of Human DNA Per chromosome=1.5cm-8.5cm =Estimate of around 2m per cell total. The Human Karyotype N.B. DNA compaction of 10,000 fold in mitotic nuclei Chromosomal Landmarks Where are the Genes? H Human Ch Chromosome 22 Vital Statistics of Human Chromosome 22 and the Entire Human Genome DNA length g Number of genes Smallest protein-coding gene Largest gene M Mean gene size i Smallest number of exons per gene Largest number of exons per gene Mean number of exons per gene Smallest exon size Largest exon size Mean exon size Number of pseudogenes** Percentage of DNA sequence in exons (protein coding sequences) Percentage of DNA in highcopy repetitive elements Percentage of total human genome CHROMOSOME 22 pairs* 48 × 106 nucleotide p approximately 700 1000 nucleotide pairs 583,000 nucleotide pairs 19 000 nucleotide 19,000 l tid pairs i HUMAN GENOME 3.2 × 109 approximately 30,000 not analyzed 2.4 × 106 nucleotide pairs 27 000 nucleotide 27,000 l tid pairs i 1 1 54 178 5.4 8.8 8 nucleotide pairs 7600 nucleotide pairs 266 nucleotide pairs more than 134 not analyzed 17,106 nucleotide pairs 145 nucleotide pairs not analyzed 3% 1.5% 42% approximately 50% 1.5% 100% DNA Structure Basics Deoxyribonucelic y acid duplex p Each strand is a linear polymer of nucleotides 1. Deoxyribose sugar unit 2. Phosphate p Purine (Adenine,Guanine) 3 Nitrogenous base 3. Pyrimidine (Cytosine, Thymine) Linkage in The Single Strand Polymer N1-C1 link PYRMIDINE C and T Above plane -glycosidic linkages N9-C1 N9 C1 link PURINE A and G Forming The Double Helix Bases on two separate single strand polymers forming specific base pairs which allow for the formation of a helix. Watson-Crick Model -Two Two anti anti-parallel parallel helical chains coil around a common axis axis. -Sugar-phosphate Sugar phosphate “backbone” backbone runs on outside,bases inside. -Bases lie perpendicular to common to axis. -A regular double helix is formed due to the base pairs occupying the same internal space (A-T=G-C in space across helix). Helical stacking promoted by intra-base pair hydrogen bonding And stack-stack Van der Waal’s attraction. The Double Helix Hydrophobic interaction Length equivalency -Right handed helix -10 bases per 360˚ base-base hydrogen bonding The Nucleosome The double helix is packaged by proteins called histones. Histones H2A,H2B,H3,H4 (core histones) H1 (linker histone) 25% arginine or lysine residues (basic amino acids) Histones form a complex around which DNA can be; -supercoiled stac ed -stacked The Histone Octamer Basic constituent of chromatin -2 2 molecules of each H2 histone for unit unit. (H3)2(H4)2 tetramer (H2A-H2B dimer)2 -146bp 146bp wound on per unit in 1.8 turns (left hand supercoil). -80bp linker attached to H1 (nucleosome seal). -Repeat. Solenoid www.mun.ca/.../Histone_ Protein_Structure.htm Solenoid Helices Nuclesome formation compact DNA seven fold Helical stacking of solenoids provides up to 104 fold condensation. condensation Now DNA resembles packets=chromosomes. Most prominent in duplicated form during mitotic metaphase. p Message g On Packing g Clear implication is that packaging allows accommodation of a large amount of genetic information To access information (i.e. DNA, chromatin must be opened up) See subsequent lectures for; REPLICATION GENE EXPRESSION Your Learning Objectives Your learning from today should focus on being able to; •Define D fi and d articulate ti l t what h t genes and d chromosomes h are as structural t t l and functional units of heritable material. •Describe the basic chemical components of DNA •Explain the biochemical basis for double helix formation •Appreciate the necessityy for efficient packaging g g of DNA and comprehend how amino-acid sequence matches function in the nucleosome.