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Chapter 12

Biology 1202B - Chapter 12 DNA Structure, Replication & Organization Notes

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Western University
Biology 1202B
Brenda Murphy

Biology 1202B Circular DNA  Mitochondria Chapter 12: DNA Structure & Organization Linear Chromosomes  Nucleus Biology 1202B: Chapter 12 – DNA Structure, Replication and Organization The Human Genome The Plant Genome 1952 Hershey & Chase – DNA inherited material  Mitochondrial DNA  Chloroplasts 1953 Watson, Crick & Franklin – Double helix DNA structure  Nucleoids contain genetic material 1956 Tjio & Levan – 46 chromosomes in human nucleus 1963 Margit and Sylvan Nass – mitochondrial DNA Hershey & Chase – Transformation Transformation: the conversion of DNA released by another cell’s breakdown  Studied bacteriophage T2; viruses infecting bacteria o Virus: infectious agent containing DNA/RNA surrounded by a protein coat; uses cell’s resources  Phage Replication Cycle o Phage attaches to bacterium’s surface and makes progeny (descendant) phages o T2 phage contained a DNA core encapsulated by proteins  Thus, proteins or DNA are genetic material  Protein (S-35) and DNA (P-32) were tagged to 2 bacteriophages o No radioactivity in bacterial cells = easily measure material  If phages contained radioactive DNA, bacteria was radioactive  Radioactivity was seen in progeny (replicated phage) phages (P-32)  Established transformation; identifying DNA was the hereditary molecule Watson & Crick – DNA Structure and Semiconservative Replication Semiconservative Replication: 2 parental strands separate and act as templates for new progeny synthesis of double-stranded DNA molecules  Used existing data from Rosalind Franklin o X-ray diffraction was used to study DNA structure o Molecules are arranged into a crystal form  Atoms bend/reflect into smaller beams determined by atom arrangement and produce exposed spots created a pattern o Indicated DNA were cylindrical and 2nm in diameter o Interpreted X-shaped distribution as a helical structure  2 polynucleotide chains twist right-handedly into a double-spiral staircase  “Old” or parent strand is a template for the addition of complementary bases (2 DNA helixes are exact copies of the parental DNA molecule with one “old” strand and one “new” strand)  Double-helix model: 2 sugar-phosphate backbones are separated by a regular distance o Purines & pyrimidines fill space b/w backbone chains in the double helix Jennifer Chiu, 1 Biology 1202B Chapter 12: DNA Structure & Organization o Complementary base-pairing: one strand is complementary to another; stabilized through hydrogen bonds  A=T and C ≡ G  Perpendicular along DNA helix; 10 base pairs in 1 full turn o Antiparallel: fit in a stable chemical way when in opposite directions  3’ end and 5’ end line up together o DNA must store & transmit genetic information throughout life  Coded into 4 nucleotides preserved by covalent bonds Meselson & Stahl – Semiconservative Replication  Used non-radioactive “heavy” Nitrogen isotope ( N) to normal 14N isotope o Distinguish parent strand from newly synthesized DNA strand 15 o Grew E. coli in N  heavy nitrogen became incorporated into nitrogen base of DNA 14 o Transferred E. coli into normal N isotope  all new DNA contained light isotope o Mixed solution with cesium chloride (CsCl) and centrifuged the mixture in high speed o D Models of Replication: NA Semiconservative Replication of  2 parent strands separate and serve as new templates for DNA progeny diff ere Conservative Replication Model nt  2 parent strands are templates for new den DNA strands siti  After 2 complementary copies separate, es they wind into a new helix Dispersive Replication Model sep arat ed into bands with denser (heavier) DNA settling closer to the bottom of the tube Jennifer Chiu, 2 Biology 1202B Chapter 12: DNA Structure & Organization 5’ (Phosphate)  3’ (Hydroxide) DNA Synthesis Nucleoside triphosphate + base form a complementary base pair for the template’s nucleotide Nucleotides DNA contains 4 nucleotides; nucleotides consist of 5-C sugar, P group & nitrogen base.  Nitrogenous bases: Adenine (A), Guanine (G), Thymine (T), Cytosine (C) o Purines: nitrogenous bases built from fused rings of C and N atoms  Adenine & Guanine o Pyrimidines: nitrogenous bases built from 1 C ring  Thymine & Cytosine o Chargaff’s Rules: # Purines = # Pyrimidines  # of Adenine = # of Thymine; # of Cytosine = # of Guanine  Ability to form polynucleotide chains o Sugar-phosphate backbone: deoxyribose linked by phosphate groups o Each phosphate is a bridge b/w 3’C of 1 sugar and 5’C of next sugar o Phosphodiester bond: entire polynucleotide chain + phosphate group o Has polarity and directionality (antiparallel)  Phosphate group bound to 5’ end  Hydroxyl group is bound to 3’ end 3 Deoxyadenosine 5 Deoxynucleoside Triphosphates (dNTP) Deoxyguanosine Nucleotide = Nucleoside + Phosphate Deoxythymidine  Nucleotide: Sugar, Base, Phosphate Deoxyuridine  Nucleoside: Sugar + Base Deoxycytidine Left and Right Handed Helices Jennifer Chiu, 3 Biology 1202B Chapter 12: DNA Structure & Organization  If you move along a helix in the direction of your right hand's thumb, the helix turns in the direction of your right hand's fingers, then it's a right-handed helix  If you move along a helix in the direction of your left hand’s thumb, the helix turns in the direction of your left hand’s fingers, then it’s a left-handed helix Differences between RNA and DNA Ribonucleic Acid (RNA) Deoxyribonucleic Acid (DNA) 1. Ribose Sugar 1. Deoxyribose Sugar 2. Uracil Nucleic Acid 2. Tyrosine Nucleic Acid 3. Single Stranded 3. Double Stranded Histones Histones are positively charged proteins that are compacted into DNA in eukaryote’s chromosomes.  5 types of histones: H1, H2A, H2B, H3, H4 (highly similar among eukaryotes)  Function: pack DNA molecules into narrow confines of a cell nucleus Chromatin Fibre  H1 brings about chromatin packing; 30 nm chromatin fibre or solenoid with 6 nucleosomes/turn o Binds to the nucleosomes and linker DNA  Protects from chemical and mechanical damage  DNA wound into nucleosomes and chromatin is more resistant to degradation by deoxyribonuclease Euchromatin  Euchromatin: true regions (transcription) and heterochromatin: different regions of DNA (no transcription) o Chromatin fibres fold and pack into thick, rodlike chromosomes and are visible during mitotic divisions  Large blocks of genes have been turned off and placed in compact storage form Jennifer Chiu, 4 Biology 1202B Chapter 12: DNA Structure & Organization  As 2 X chromosomes are inactive, it packs down into heterochromatin (Barr body); large enough under light microscope o Support histones play a role in regulating gene activity How do histones pack DNA? Nucleosomes! Nucleosomes are 2 molecules forming a nucleosome core particle (winds around DNA in 2 turns)  Linker: extends between 1 nucleosome and the next; 10 nm chromatin fibre  Nucleosome and linker includes 200 base pairs of DNA  147 base pairs of DNA are wrapped around an octamer (a few monomers) Non-histones: loosely defined as proteins that aren’t histones  Vary widely in structure  Negative, neutral and positively charged proteins  Function: help control expression of individual genes o Affect gene accessibility by modifying histones changing association with chromatin  Tightening or loosening DNA chromatin  i.e. Histone acetylases – loosen histone binding  i.e. Histone deacetylases – tightened histone binding o Activate or repress expressions of a gene  SRY Factor: Y chromosome o Defective SRY causes a male to acquire female characteristics; have the XY chromosome! Eukaryote DNA Organization Prokaryote DNA Organization Chromatin builds chromosomes and consists of Circular (majority) and linear chromosomes. 37.5% histones and 37.5% non-histones (positively Not organized into nucleosomes  nucleoid with charged chromosomal proteins). DNA organized into loops providing compaction. 15% DNA and 10% RNA (negatively charged). Nucleoid is suspended directly in the cytoplasm. RECALL FROM EARLIER* ^ Watson & Crick – DNA Structure and Semiconservative Replication Meselson & Stahl – Semiconservative Replication 15 14  Used non-radioactive “heavy
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