The Structure of DNA DNA carries genetic informationHow

The Structure of DNA

• DNA carries genetic information…How? • DNA specifies how to assemble proteins, how genes can be replicated, and inherited.

Components of DNA • DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds.

Nucleic Acids and Nucleotides • Nucleic acids are macromolecules (polymers) assembled from monomers called nucleotides. • Nucleotides consist of 3 parts: – 5 -carbon sugar – Phosphate group – Nitrogenous base

Nitrogenous Bases • Nitrogenous bases are bases that contain N • DNA has four kinds of nitrogenous bases – Adenine (A) – Guanine (G) – Cytosine (C) – Thymine (T)

Nitrogenous Bases • The nucleotides in a strand of DNA are joined by covalent bonds formed between the sugar of one nucleotide and the phosphate group of the next. • The nucleotides stick out sideways from the nucleotide chain. • The nucleotides can be joined together in any order- any sequence of bases is possible.

Chargaff’s Rule • Erwin Chargaff – Austrian-American biochemist • Discovered the percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. • Same is true for guanine [G] and cytosine [C]. • [A] = [T] and [G] = [C] became known as “Chargaff’s Rule”

• Which organism has the highest percentage of Adenine? Yeast • If a species has 35% adenine in its DNA, what is the % of the other three bases? T=35%, G=15% C=15% • What did the fact that A and T, and G and C, occurred in equal amounts suggest about the relationship among these bases?

Rosalind Franklin • British scientist • X-ray diffraction of DNA • Showed that strands of DNA were twisted around each other • Angle suggests that there are two stands in the structure • Other clues suggest the nitrogenous bases are near the center

Watson and Crick • James Watson- American biologist, Francis Crick- British physicist • The clues in Franklin’s x-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA • Discovery: DNA Double Helix

Double Helix Model • Twisted ladder- spiral staircase • Explains Chargaff’s Rule of base pairing and how the two strands of DNA are held together

Antiparallel Strands • The two strands of DNA run in opposite directions…”antiparallel” • Allows nitrogenous bases on both strands to come into contact at center of the molecule. • Allows each strand to carry a sequence of nucleotides.

Hydrogen Bonding • What forces held the 2 strands of DNA’s double helix together? • Watson and Crick discovered that hydrogen bonds could form between certain nitrogenous bases, providing just enough force to hold the two strands together

Base Pairing • Hydrogen bonds could create a nearly perfect fit between nitrogenous bases along the center of the molecule.

• Amoeba Sisters: Structure and Function of DNA