8
Chromosomes X and Y are the sex chromosomes. Each man carries an
X chromosome and a Y chromosome. Every woman carries two X chromosomes.
As there are actually few genes on the Y chromosome, men and women each have
one active X chromosome that codes most of the information. More than 99 % of
the euchromatic sequence of the X chromosome has been determined. A dispropor-
tionately high number of mendelian diseases are documented for the X chromo-
some; of >1,000 genes, 168 mutations in 113 X-linked genes have been explained,
which in many cases were characterized with the aid of the DNA sequence.
Examples are defects in the gene responsible for Duchenne muscular dystrophy to
and fragile X mental retardation. As men have only one copy of the X chromosome,
it is easier to fi nd mutated genes on that one piece of DNA.
Genes
A gene is a sequence of chromosomal DNA that is required for the production of a
functional product: a polypeptide or a functional RNA molecule. Genes range in
size from small (1.5 kb for globin gene) to large (approximately 2,000 kb for
Duchenne muscular dystrophy gene). A gene includes not only the actual coding
sequences but also adjacent nucleotide sequences required for the proper expres-
sion of genes − that is, for the production of a normal mRNA molecule. Mature
mRNA is about one-tenth the size of the gene from which it is transcribed. The
same DNA strand of a gene is always translated into mRNA so that only one kind
of mRNA is made for each gene. Transcription is gene in action. Genes are often
described as blueprints of life and transmit inherited traits from one generation
to another.
The Genetic Code
The sequence of nucleotide bases of the “genetic code” in a particular gene is
refl ected in the specifi c sequence of amino acids in the polypeptide produced
through the protein synthesis mechanism. The co-linearity between the DNA mol-
ecule and the protein sequence is achieved by means of the genetic code. At any
position there are four possibilities (A, T, C, and G). Thus, for three bases, there are
43 or 64 possible triplet combinations. These 64 codons constitute the genetic code.
Gene Expression
The activity of a gene, so called gene “expression” means that its DNA is used as a
blueprint to produce a specifi c protein. Only a small number of these genes, about
15,000, are expressed in a typical human cell, but the expressed genes vary from one
1 Basic Aspects