In the 1950's, James Watson and Francis Crick suspected that DNA was a helix. They made models from scraps of metal connected by suitably angled “bonds” of wire. Later, Rosalind Franklin made the first clear x-ray diffraction image of DNA as it occurs in cells. She calculated that DNA is very long and identified a repeating pattern.

In 1950, Erwin Chargaff made two important discoveries about DNA. Chargaff ’s first rule is that the amounts of thymine and adenine are identical, as are the amounts of cytosine and guanine (A = T and G = C). Chargaff ’s second rule is that DNA of different species differs in its proportions of adenine and guanine.

Hydrogen bonds between the internally Positioned bases hold the two strands together. Only two kinds of base pairings form (supports Chargaff ’s first rule): • A to T • G to C.

Chromosome: structure that consists of DNA and associated proteins. Carries part or all of a cell’s genetic information.

Karyotype: an image of an individual’s diploid set of chromosomes.

In preparation for division, a cell copies its chromosomes so that it contains two sets. The process by which a cell copies its DNA is called DNA replication. Before DNA replication, a chromosome Consists of one molecule of DNA (one double helix) • As replication begins, enzymes break the hydrogen bonds that hold the double helix Together. The two DNA strands unwind and separate.

Ionizing radiation from x-rays, most UV light, and gamma rays may cause DNA damage:– Breaks DNA – Causes covalent bonds to form between bases on opposite strands – Fatally alters nucleotide bases – Causes adjacent nucleotide dimers to form.