Chargaff ’s first rule: 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: DNA of different species differs in its proportions of adenine and guanine

James Watson and Francis Crick suspect that DNA is a helix in the 1950s and built models from metal scraps with suitable angled bonds of wire

Hydrogen bonds between the bases holding the 2 strands together. Base pairs are one of the pairs A-T or C-G. They always consist of purine and pyrimidine.

A chromosome is a structure that consists of DNA and all associated proteins. It carries part or all of a cells genetic information.

A karyotype is an image of a person's diploid set of chromosomes.

As replication begins, enzymes break the hydrogen bonds that hold the double helix together. The two DNA strands unwind and separate. Another enzyme constructs primers: short, single strands of nucleotides. Primers serve as attachment points for DNA polymerase, the enzyme that assembles new strands of DNA. A primer base-pairs with a complementary strand of DNA. The establishment of base-pairing between two strands of DNA is called nucleic acid hybridization. DNA polymerases attach to the hybridized primers and begin DNA synthesis? Each nucleotide provides energy for its own attachment to the end of a growing strand of DNA. The enzyme DNA ligase seals any gaps, so the new DNA strands are continuous. Both of the two strands of the parent molecule are copied at the same time. As each new DNA strand lengthens, it winds up with its template strand into a double helix. Semiconservative replication produces two copies of a DNA molecule: one strand of each copy is new, and the other is parental

Ionizing radiation from x-rays, most UV light, and gamma rays may cause DNA damage. Such damage includes the breaking of DNA, covalent bonds from between bases of opposite strands, alters nucleotide bases fatally and causes adjacent nucleotide dimers to form.