DNA Chapter 12

What is dna? A long molecule found in the nucleus of the cell A type of nucleic acid is made up of smaller subunits called nucleotides joined into long strands by covalent bonds Nucleotides are made up of: 5-carbon sugar (deoxyribose) a phosphate group A nitrogenous base (adenine, guanine, cytosine, thymine)

The nucleotides are joined together by covalent bonds formed between the sugar of one nucleotide and the phosphate group of the next. The nucleotides can be joined in any order

Erwin Chargaff’s rule Percentages of: Adenine (A) = thymine (T) Guanine (g) = Cytosine (C) This rule was obeyed In all organisms.

Rosalind Franklin’s X-rays Used a technique called x-ray diffraction showed strands of DNA twisted around each other (in a helix) Showed Dna is made up of 2 strands Gave clues the nitrogenous bases are near the center of the DNA molecule

Watson and Crick: Double Helix Model Double helix = twisted ladder or spiral staircase Confirms Chargaff’s rule and Franklin’s X pattern Explains that the two strands are held together by weak hydrogen bonds between nitrogenous bases Antiparallel strands run in opposite directions Nitrogenous bases only formed bonds with specific nitrogenous bonds (A-T and G-C). This is known as Base pairing