DNA structure

1. DNA structure By Dr. NAGLAA FATHY Ass. Prof. of Biochemistry & Molecular Biology Faculty of Medicine Benha University E-mail : naglaa_fathy722000@yahoo.com nagla.alhusseini@fmed.bu.edu.eg

2. DNA Transcription Ribosome mRNA Translation Polypeptide (protein) IntroductionThe Central Dogma of Molecular Biology Cell

3. OH Phosphate NH2 - P HO O Base H+ H O N N N N H CH2 5’ O 1’ 4’ Sugar 3’ 2’ OH H OH A NucleotideAdenosine Mono Phosphate (AMP) Nucleotide Nucleoside

4. NH2 Thymine (DNA) Uracil (RNA) CH3 Adenine N N N N N O N N NH N O O O N O O NH NH N N N O NH2 NH2 Guanine Cytosine NH2 N N Purines Pyrimidines

5. Cytosine N O O N N N N Guanine H H O N H N N N H H Base PairingGuanine And Cytosine - + + - - +

6. H H + N - Adenine Thymine CH3 H - + N N N N N N O O N Base PairingAdenine And Thymine

7. + H H + N - Adenine - - Cytosine N O N N N O N H H N N N Base PairingAdenine And Cytosine

8. - + - Thymine CH3 H + + N N N N N O N O N Guanine O H N H H Base PairingGuanine And Thymine

9. 5’Phosphate group 3’Hydroxyl group CH3 OH O NH2 H OH P HO O HN N N N O O N N CH2 O O CH2 O NH2 B A S E S HO P O H O N H2N O H O H P HO O N O N N NH O SUGAR-PHOSPHATE BACKBONE N N O H2O H2O NH2 N O O CH2 N O CH2 N O HN N O HO P H2N H O O H H P HO O 5’Phosphate group O CH2 O O CH2 O 3’Hydroxyl group O HO P OH H HO DNA

10. - - AT - - CG - - GC - - TA - - 3.4 nm 1 nm - - - - GC TA - - CG - - AT - - - - AT - - CG - - GC - - 0.34 nm TA - - The Watson - Crick Model Of DNA Minor groove Major groove

11. Forms of the Double Helix B DNA Z DNA A DNA CG GC AT AT 3.9 nm 2.8 nm CG CG CG 1 nm GC GC 0.9 nm GC T A A T TA Minor groove GC 1.2 nm Minor groove CG GC 6.8 nm TA CG GC A T TA Major groove CG CG AT GC AT CG 0.57 nm GC CG Major groove TA GC GC AT GC CG GC 0.26 nm 0.34 nm 12 Bp/turn 11 Bp/turn 10.4 Bp/turn TA +34.6o Rotation/Bp -30.0o Rotation/Bp +34.7o Rotation/Bp

12. Even More Forms Of DNA B-DNA appears to be the most common form in vivo. However, under some circumstances, alternative forms of DNA may play a biologically significant role. • C-DNA: • Exists only under high dehydration conditions • 9.3 bp/turn, 0.19 nm diameter and tilted bases • D-DNA: • Occurs in helices lacking guanine • 8 bp/turn • E-DNA: • Like D-DNA lack guanine • 7.5 bp/turn • P-DNA: • Artificially stretched DNA with phosphate groups found inside the long thin molecule and bases closer to the outside surface of the helix • 2.62 bp/turn

13. Denaturation and Renaturation Denatured DNA Renaturation Denaturation HEAT ATGAGCTGTACGATCGTG ATGAGCTGTACGATCGTG ATGAGCTGTACGATCGTG TACTCGACATGCTAGCAC TACTCGACATGCTAGCAC TACTCGACATGCTAGCAC Double stranded DNA Double stranded DNA Single stranded DNA • Heating double stranded DNA can overcome the hydrogen bonds holding it together and cause the strands to separate resulting in denaturation of the DNA • When cooled relatively weak hydrogen bonds between bases can reform and the DNA renatures

14. Denaturation and Renaturation ACGAGCTGCACGAGC ATGATCTGTAAGATC TGCTCGACGTGCTCG TACTAGACATTCTAG 67 % GC content - 33 % GC content - ATGAGCTGTCCGATC TACTCGACAGGCTAG 50 % GC content - • DNA with a high guanine and cytosine content has relatively more hydrogen bonds between strands • This is because for every GC base pair 3 hydrogen bonds are made while for AT base pairs only 2 bonds are made • Thus higher GC content is reflected in higher melting or denaturation temperature High melting temperature Low melting temperature Intermediate melting temperature

15. Determination of GC Content Comparison of melting temperatures can be used to determine the GC content of an organisms genome To do this it is necessary to be able to detect whether DNA is melted or not Absorbance at 260 nm of DNA in solution provides a means of determining how much is single stranded Single stranded DNA absorbs 260 nm ultraviolet light more strongly than double stranded DNA does although both absorb at this wavelength Thus, increasing absorbance at 260 nm during heating indicates increasing concentration of single stranded DNA

16. The End