Our sense organs 45

Structure: The total amount of genetic information
can be compared to a library, where single
books represent chromosomes, and their chapters
are the genes. Genes are like entries in a
gigantic encyclopedia. There are 23 pairs of chromosomes
in the nuclei of our somatic (body)
cells, making up the diploid (Greek diplóos =
double) number of 46 chromosomes. Single chromosomes
can be distinguished according to their
total length, the length of the chromosomal
arms, and the position of the centromere, the
point at which they are constricted. With the
exception of the sex chromosomes, the chromosomes
from each parent correspond to those
from the other parent in regard to the type and
sequence of the hereditary characteristics.
Women have two equal sized sex chromosomes
(XX), but men have a larger and a smaller sex
chromosome (XY).
The 23 human chromosome pairs comprise
a double complement of approximately 100,000
inherited characteristics or genes. Every gene
occurs twice, one derived from the mother, and
the other from the father; they are thus known
as diploid chromosomes. In contrast to the body
cells, the germ cells (egg and sperm cells) have
a single complement of chromosomes, called haploid
(Greek haplous = single). Since the 30,000
genes are shared among 23 chromosomes, each
chromosome is made up of about 1,300 genes.
The DNA molecules of bacteria, when stretched
out, are around one millimetre long. This corresponds
to around 3 x 10 6 nucleotide pairs. The
well known bacterium E. Coli has 7.3 x 10 6
nucleotide pairs. In human body cells the total
length of the DNA is around 2 metres, about
6 x 10 9 nucleotide pairs.
We must distinguish between the gametes or sex
cells (which carry the information of heredity to
the next generation) and the somatic or body
cells. In the gametes (sperm and egg cells) the
total length of the DNA threads is around 1 m,
divided into 23 chromosomes. That represents
3x 10 9 nucleotide pairs. These can constitute 10 9
words (triplets, each of three chemical letters).
The nucleotides are the four chemical letters of
76
the genetic alphabet, called Adenine, Guanine,
Cytosine and Thymine. Human body cells carry
a dual batch of hereditary information – one
from the father and one from the mother. So
they have 2 x 23 = 46 chromosomes, corresponding
to a DNA length of 2 metres (6 x 10 9
nucleotide pairs).
The number of possible genes can be estimated.
We start with an average sized gene product
(protein), and look at the number of DNA building
blocks (nucleotides) needed to code for that
number of amino acids. For example, human
hemoglobin, the pigment in red blood cells. The
alpha chain has 141, the beta chain 146 amino
acids. Each amino acid needs three nucleotides to
code for it, so that means for both chains we
need 3 x (141+146) = 861 nucleotide pairs.
Therefore our DNA should theoretically be able to
code for 3 x 10 9 /861 genes coding for proteins
the size of hemoglobin. In reality, however, the
majority of the DNA consists of sequences which
do not code for proteins, and their function is
still unclear today (though some hints may be
gradually emerging). Only 50,000 to 100,000
genes actually code for proteins. To put it another
way: Only about three percent of the
genome actually codes for proteins such as
insulin or hemoglobin. Such program codes are
identical for all people. Remarkably, in most
instances, more than one gene codes for a given
characteristic (e. g. eye colour).
It seems necessary to assume that in addition to
its protein coding portions, DNA contains countless
additional levels of structure and function.
Such stored information concepts are just as
much required to code for the development of
the smallest organelles such as the mitochondria
and ribosomes, as for building the large organs
(e. g. heart, kidneys, brain) and the overall integrated
organism. As yet, no one has been able to
decode this incredibly complex system. Perhaps
some light will be shed on this by research over
the next few years.
If the total paternal contribution to heredity is
contained in a sperm cell, and the maternal in an
egg, then this would have to not just involve the