degeneration. deterioration; change from a higher to a lower form, especially change of tissue to a lower or less functionally active form. When there is chemical change of the tissue itself it is true degeneration; when the change consists in the deposit of abnormal matter in the tissues, it is infiltration.apetoretinal degeneration degeneration of the pigmented layer of the retina. transneuronal degeneration atrophy of certain neurons after interruption of afferent axons or death of other neurons to which they send their efferent output.
There are four different DNA nucleotides, each defined by a specific nitrogenous base: adenine (often abbreviated A in science writing), thymine (abbreviated T), guanine (abbreviated G), and cytosine (abbreviated C) (Figure 2).
Some of the most fundamental properties of DNA emerge from the characteristics of its four basic building blocks, called nucleotides. Knowing the composition of nucleotides and the differences between the four nucleotides that make up DNA is central to understanding DNAâs role in living systems. DNA is a nucleotide polymer, or polynucleotide. 1  Each nucleotide contains three components: 2  A five carbon sugar. 3  A phosphate molecule. 4  A nitrogen-containing base.