The four bases in DNA are adenine, cytosine, guanine, and thymine.
These four are divided into two groups. The pyrimidines, the single-ring nitrogenous bases, is where thymine and cytosine is included. The purines, the double-ring nitrogenous bases, includes adenine and guanine.
Cytosine will only bond with guanine; they form three hydrogen bonds. Adenine will only bond with thymine forming two hydrogen bonds.
Crossing over, independent assortment and fertilization are methods of genetic recombination. Genetic recombination is the production of offspring which possess combinations of traits that differ from those found in either of the parents. Crossover. This is the first way that genes are shuffled in sexual reproduction to produce genetic diversity. Chromosomes line up side by side and break off pieces of themselves, then trade those pieces with each other. Independent assortment. This is the process in which each homologous chromosome in a pair goes into a separate gamete, thus each gamete has only one copy of each gene and that gene may be a different version of the same gene that is in another gamete. Fertilization. External fertilization, common in organisms that live underwater such as fish and frogs, increases the number of eggs that sperm encounter. Not only does this result in a greater number of offspring, but it generates more genetic diversity. Internal fertilization e.g. in mice also has the advantage of producing four to eight pups after mating thus increasing diversity.
