FoundationalConceptsNeuroscience

is called the haploid chromosome number and forty-six the diploid
chromosome number. The haploid number is often used in reference
to distinct (counting only the genes from one parent) genetic material.
The twenty-three chromosomes of the haploid human genome
contain approximately three billion (3 x 109) nucleotide base pairs:
adenines, thymines, guanines, and cytosines. The entire human
genome has now been sequenced; every single one of the three billion
nucleotide base pairs is known, as well as the precise order in which
they occur. (This is also the case for the complete genomes of numer-
ous other organisms.) Of course, there are differences in nucleotide
sequence between individuals, and as the technology to sequence
DNA becomes more refined, it will become increasingly easier and
less expensive to obtain complete DNA sequences for anyone. It is
presently estimated that these three billion bases contain genes cod-
ing for approximately twenty-one thousand proteins.
Though every cell in a person’s body contains the same genetic
information, there are many different types of cells doing many differ-
ent kinds of things. This is because the information in the genome is
transcribed and translated differently in different cells. Very early in
organismal development, only hours (if not sooner) after fertilization
of the egg, cell differentiation begins to take place. This is regulated
via transcription factors—proteins that bind to regions of the DNA
and regulate the readout of genes (Fig. 10.1).