1. One reason is extinctions happen because of some kind of change in the environment where a life form lives. While some species can adapt to the changes, others die, and if enough die, the species becomes extinct. These are some of the biggest factors in the extinctions of particular species. 2. Contaminants in the environment, such as pesticides.
A bond, called the glycosidic bond, holds the base to the sugar and the 3′-5′ ("three prime-five prime") phosphodiester bond holds the individual nucleotides together. Nucleotides are joined from the 3′ carbon of the sugar in one nucleotide to the 5′ carbon of the sugar of the adjacent nucleotide. The 3′ and the 5′ ends are chemically very distinct and have different reactive properties. During DNA replication, new nucleotides are added only to the 3′ OH end of a DNA strand. This fact has important implications for replication.
<span>Models are very helpful, but they also have limitations.Details - Models cannot include all the details of the objects that they represent. For example, maps cannot include all the details of the features of the earth such as mountains, valleys, etc.Approximations - Most models include some approximations as a convenient way to describe something that happens in nature. These approximations are not exact, so predictions based on them tend to be a little bit different from what you actually observe. Models do not behave exactly like the things they represent.Accuracy - In order to make models simplistic enough to communicate ideas, some accuracy is lost. For example, ball and stick models of atoms do not show all the details that scientists know about the structure of the atom.</span>
