DNA

DNA is a nucleic acid• A nucleic acid is a polymer of

nucleotides• A nucleotide is made from a phosphate,

sugar and base.• www.chemsoc.org

Phosphate

• From phosphoric acid• http://bio.winona.edu/berg/ChemStructures/Phospha2.gif

Sugar

• Deoxyribose in DNA (Ribose in RNA)• http://www.mun.ca/biology/scarr/Deoxyribose_vs_Ribose.gif

Nucleotide• Condensation between the phosphate

and sugar (at C5) and between the sugar (at C1) and base

• http://distancelearning.ksi.edu/demo/bio378/DNA_files/image002.gif

Dinucleotide• Two nucelotides join by condensation• http://pages.towson.edu/jdrawn/Chapter_1_Intro_to_Bioinfo_files/image005.jpg

• Condensation gives phosphodiester bridge

Polynucleotide• Up to 5 x 106 nucleotides long• http://seqcore.brcf.med.umich.edu/doc/educ/dnapr/oligo.gif

• 5’ end

• 3’ end

DNA double helix

• The bases of the DNA nucleic acid form H bonds to bases of a second molecule of DNA

• A bonds to T with two hydrogen bonds• G bonds to C with three hydrogen

bonds.

• http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/BasePairing.gif

• A is the ~same amount as T• G is the ~same amount as C• Amount of purines equals the

amount of pyrimidines

• A always bonds to T• G always bonds to C

• Known as Chargaff’s Rule

Table from Biological Science – Green, Stout, Taylor

• The two nucleic acids wind into a double helix, with H bonds between polar groups of sugars keeping the shape of the helix.

• (diagram from Bioligical Sciences, green, Stout & Taylor)

• The two strands are ANTIPARALLEL• i.e. they are parallel, but run in

opposite directions.

http://www.accessexcellence.org/RC/VL/GG/images/dna2.gif

Arrangement of DNA in chromosomes

http://www.biology.emory.edu/research/Lucchesi/multimedia/images/insert2.jpg

Histones in nucleosome• DNA is wrapped around a group of 8

histone proteins and held together by another histone molecule

• http://employees.csbsju.edu/hjakubowski/classes/ch331/dna/nucleosome.gif

• Nucleosomes:• Help in supercoiling of DNA• Help to regulate transcription

DNA from a single skin cell, if straightened out, would be about six feet long but invisible. Half a gram of DNA,

uncoiled, would stretch to the sun. Again, you couldn't see it.