DNA STRUCTURE
Unit 4 – Part 1
DNA Facts
DNA DeoxyriboNucleic Acid
Basis for all living things Foundation for all diversity & unity on
Earth Every living thing has DNA. That means that
you have something in common with a zebra, a tree, a mushroom and a beetle!!!!
DNA is too small to see, but under an electron microscope it looks like a twisted up ladder!
History of DNA
Frederick Griffith Discovers that a factor in diseased
bacteria can transform harmless bacteria into deadly bacteria. (1928)
Rosalind Franklin X-ray photo of DNA. (1952)
Watson and Crick described the DNA molecule from
Franklin’s X-ray.(1953)
Structure of DNA
Nucleotide Small units that
link together to form DNA
Made of 3 parts Phosphate Group 5-C sugar
molecule deoxyribose
Nitrogen Base
Structure of DNA cont’d.
Nitrogen (N) base N base makes nucleotides different 4 bases
▪ Adenine (A)▪ Guanine (G)▪ Thymine (T)▪ Cytosine (C)
2 main groups of bases▪ Purines
▪ Double ring of C & N atoms▪ Includes adenine and guanine
▪ Pyrimidines▪ Single ring of C & N atoms▪ Includes thymine and cytosine
Base pairing rules Purines bind to pyrimidines Adenine binds to thymine Guanine binds to cytosine
Structure of DNA cont’d.
Double Helix Model created by
James Watson & Francis Crick
Structure of DNA “spiral staircase”
made of 2 strands of nucleotides twisting around a central axis.
Also known as a “twisted ladder”
▪ Sides of ladder are made of alternating sugar and phosphate units held together with strong covalent bonds.
▪ Rungs are made up of a purine and a pyrimidine held together by weak Hydrogen (H) bonds.
▪ Base pairing is specific and follows Chargoff’s rule,
A ~ T and G ~ C a.k.a. the Complementary base pairing rule.