The nervous system is built from two broad categories of cells, neurones and glial cells. The glial cells outnumber the neurones and the two cell types occupy a comparable amount of space in nervous tissue.The main glial cell types are, in the central nervous system, astrocytes and oligodendrocytes and, in the peripheral nervous system, Schwann cells, enteric glial cells and satellite cells.lia also maintain appropriate concentrations of ions and neurotransmitters in the neuronal environment. Increasing body of evidence indicates that glial cells are essential regulators of the formation, maintenance and function of synapses, the key functional unit of the nervous system.
Any of the pairs of nucleotides connecting the complementary strands of a molecule of DNA or RNA and consisting of a purine linked to a pyrimidine by hydrogen bonds. The base pairs are adenine-thymine and guanine-cytosine in DNA, and adenine-uracil and guanine-cytosine in RNA or in hybrid DNA-RNA pairing.
The nitrogenous bases point inward on the ladder and form pairs with bases on the other side, like rungs. Each base pair is formed from two complementary nucleotides (purine with pyrimidine) bound together by hydrogen bonds. The base pairs in DNA are adenine with thymine and cytosine with guanine. DNA has a spiral staircase-like structure. The steps are formed by the nitrogen bases of the nucleotides where adenine pairs with thymine and cytosine with guanine.