1. THINK ABOUT IT
The DNA molecule must somehow specify how to assemble proteins, which are needed to regulate the various functions of each cell.
What kind of structure could serve this purpose without varying from cell to cell?
Understanding the structure of DNA has been the key to understanding how genes work.

2. The Components of DNA What are the chemical components of DNA?
DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds.

3. Nucleic Acids and Nucleotides
Nucleic acids are long, slightly acidic molecules originally identified in cell nuclei.
Nucleic acids are made up of nucleotides, linked together to form long chains.
The nucleotides that make up DNA are shown.

4. Nucleic Acids and Nucleotides
DNA’s nucleotides are made up of three basic components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.
The nucleotides in a strand of DNA are joined by covalent bonds formed between their sugar and phosphate groups.

5. Nucleic Acids and Nucleotides
The C of the sugar in the nucleotide are numbered #1-#5
Nitrogenous bases attach at C1
Phosphates attach at C5
To add a nucleotide, the next phosphate is attached at C3 (the end with the hydroxyl)

6. Nucleic Acids and Nucleotides
The strands are said to run 5’ to 3’ in each direction
5’ to 3’down; 5’ – 3’ up
Prime # refers to the C on the sugar

7. Nitrogenous Bases and Covalent Bonds
DNA has four kinds of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
The nitrogenous bases stick out sideways from the nucleotide chain.

8. Purines and Pyrimidines
There are 2 groups of DNA nucleotides – purines and pyrimidines
Pyrimidines – T and C; composed of 1 carbon ring
Purines – A and G; composed of 2 carbon rings
Purines complement pyrimidines (A – T; G – C)

9. Nitrogenous Bases and Covalent Bonds
The nucleotides can be joined together in any order, meaning that any sequence of bases is possible.

10. Nitrogenous Bases and Covalent Bonds
Nucleotides form double strand bonded by weak H bonds
A – T by 2 H bonds
G – C by 3 H bonds

11. Nitrogenous Bases and Covalent Bonds
The bases make up the rungs
The sugars and phosphates make up the sides
It is twisted to form a double helix or twisted ladder

12. Antiparallel Strands
In the double-helix model, the two strands of DNA are “antiparallel”—they run in opposite directions.
This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule.
It also allows each strand of the double helix to carry a sequence of nucleotides, arranged almost like letters in a four-letter alphabet.