Its name derived from the Latin word for "nut", the nucleus
of the cell is spherical, centrally located, and about 5-10 (10^-6) m across. It
is contained within a double membrane called the nuclear envelope. The
nuclear envelope is perforated by pores about 0.1 (10^-6) m across.
Within the nucleus are chromosomes which contain the genetic material DNA
(deoxyribonucleic acid). Your DNA was passed on to you
from your parents and it contains the blueprint for your entire body. The
DNA in each of your neurons is the same, and it is the same as the DNA
in the cells of your liver and kidney and other organs. What distinguishes
a neuron from a liver cell are the specific parts of the DNA that are used
to assemble the cell. These segments of DNA are called genes .
Each chromosome contains an uninterrupted double-strand braid of
DNA, 2 nm wide. If the DNA from the 46 human chromosomes were laid
out straight, end to end, it would measure more than 2 m in length. If we
were to compare this total length of DNA to the total string of letters that
make up this book, the genes would be analogous to the individual words.
Genes are from 0.1 to several micrometers in length.