1. The Structure of DNA Reproduction occurs as a series of cell divisions that start within the nuclei of cells. Chromosomes, which can be seen with a microscope, are involved. Chromosomes contain DNA (deoxyribonucleic acid), a nucleic acid, which is divided into small units called genes. Genes determine the hereditary characteristics of a cell or organism.

2. Nucleic acids: DNA deoxyribonucleic acid RNA ribonucleic acid Nucleic Acids are polymers containing hundreds of millions of monomer units, called nucleotides.

3. Nucleotides: Each nucleotide consists of three parts: a phosphoric acid a five carbon sugar (ribose in RNA; deoxyribose in DNA) an organic base

4. Organic bases: AdenineA ……] CytosineC ……] in both DNA and RNA GuanineG ……] ThymineT ……] in DNA only UracilU ……] in RNA only

5. A nucleotide: The three components bond together as a result of condensation reactions in which water molecules are eliminated.

6. A nucleic acid chain: These diagrams are schematic and show only the primary structure of a nucleic acid chain.

7. Three important aspects to the DNA molecule: 1.DNA consists of a double chain, not a single one. The two chains wind around each other in the shape of a helix (double helix). 2.The DNA molecule can reproduce itself. 3.The sequence of bases in the chains embodies a pattern, a code, for the synthesis of proteins.

8. The Double Helix: This model was proposed in 1953 by James D. Watson and Frances H.C. Crick. The two chains of the helix are held together by Hydrogen bonding between pairs of bases. In DNA, the only pairs that bond to each other are:

9. Double-stranded DNA: A schematic representation of base-pairing in double-stranded DNA. Note that the two chains of DNA are not identical but they complement each other in a “fixed relationship of base pairing”.

10. The genetic code is embodied in the sequence of bases in the DNA molecule: 3-D views of DNA