1 Review List the chemical components of DNA Relate Cause and Effect Why are hydrogen bonds so

essential to the structure of DNA2 Review Describe the discoveries that led to the

modeling of DNA Infer Why did scientists have to use tools other than

microscopes to solve the structure of DNA3 Review Describe Watson and Crick’s model of the

DNA model

CH 12 DNA12.2 The Structure of DNA

Nucleic Acids and Nucleotides

DNA nucleotides are made up of Deoxyribose Phosphate Nitrogen base

DNA has four kinds of nitrogenous bases Adenine (A) Thymine (T) Guanine (G) Cytosine (C).

Nucleic acids are a chain of nucleotides Nucleotides can be joined in any order.

Solving the Structure of DNA

Chargaff’s Rules

Percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA

Same with guanine [G] and cytosine [C]

A=T C=G.

Franklin’s X-Rays

X-ray diffraction X-shaped pattern showing

that the strands in DNA are twisted

Angle of the X-shaped pattern suggested two strands

Width of X suggested width.

Watson and Crick

Used clues from Franklin’s X-ray

Used Chargaff’s rules Proposed structure for

double helix.

The Double-Helix Model

Antiparallel Strands Hydrogen Bonding Base Pairing.

Antiparallel Strands

Two strands of DNA run in opposite directions

A-T-C-G-C-G-A-T T-A-G-C-G-C-T-A

Hydrogen Bonding

Relatively weak chemical bonds

Allows for two strands to separate.

Base Pairing

A binds with T C binds with G.

1. Interpret Tables Which organism has the highest percentage of adenine

2. Calculate If a species has 35% adenine in its DNA what is the percentage of the other three bases

3. What did the fact that A and T, and C and G, occurred in equal amounts suggest about the relationship among these bases