Our sense organs 45

DNA
– information storage
technology way beyond
computers
The body’s most precious substance is stored
deep inside the cells, in the tiny nucleus, namely
the genetic information, known as the genome.
If this information were to be written down
using the alphabet, it would fill one thousand
books, each having 1000 pages with 3,000 letters
to the page. The human genome (inherited material)
consists of three thousand million genetic
“letters”. If all these letters were typed in one line,
it would extend from the North Pole to the equator.
A good typist working 220 eight-hour days
per year, at a typing speed of 300 letters a
minute, would require 95 years for this task –
much longer than her entire working life!
Taking into account the time required for planning
and testing up to the implementation of the
final system, a scientific programmer produces
40 symbols of program code per day on average.
It would thus require a team of 8,000 programmers,
devoting their entire career to this project,
to program the human genome. But no human
programmer knows how to structure this program
and fit it into a DNA fibre measuring only
one metre if stretched out. 1
The DNA molecule. Schematic representation of the
three-dimensional double helical structure of DNA
(DNA = desoxyribonucleic acid). Two polynucleotide
strands are spirally wound round each other, thus forming
a double helix. The base pairs, coupled by means of
hydrogen bridges, are arranged in a plane perpendicular
to the helical axis. Base pairs comprise either adenine
with thymine, or cytosine with guanine. It follows that
the molar quantity ratios are A:T = C:G = 1:1. The distance
from one spiral loop to the next is 3.4 nanometres
(nm), the diameter is 2 nm, and the distance between the
stacked base pairs is 0.34 nm (1 nm = 10 -9 m = one thousand
millionth of a metre or 1 millionth of a millimetre).
Storage density: The storage medium of genetic
information is the double stranded DNA (chemical
name: de(s)oxyribonucleic acid; see diagram
on page 74). The volume 2 of human DNA is
extremely small, only three thousand millionths
of a cubic millimetre (3 x 10 -9 mm 3 ). Its storage
density is enormous, many magnitudes higher
than the latest computer chips. In fact, it is the
highest known. Let us try to visualise it:
If one could stretch out the head of a pin measuring
2 mm in diameter until it became a thread
having the same thickness as a DNA molecule, it
would be 33 times as long as the equator! Could
you have imagined that?
If the genome information were in printed form,
it would require 12,000 paperbacks of 160 pages
each. Compared to the current 16 Megabit computer
chips, a human DNA strand stores an
amazing 1,400 times as much information.
To get a further idea of the almost unimaginable
density of information in the DNA molecule,
imagine you had just enough DNA to fit into a
pinhead. Now imagine the information contained
in an ordinary pocket paperback of 160 pages.
How many such lots of information could you
store in this tiny amount of DNA? The answer is
15 x 10 12 (15 million million). If you actually had
that many of these books and put them on top of
one another in a pile, this would be 500 times as
high as the distance (384,000 km) from the earth
to the moon! To put it another way – if you distributed
these books equally among the roughly
6 billion people on Earth, each person would get
2,500 copies!
1
Total length of the DNA strand (germ cell):
L is equal to (the number of letters) x (the distance
between letters)
= 3 x 10 9 x 0.34 x 10 -9 m = 1.02 m
2
Volume of the DNA strand (germ cell):
V = (π/4) x (2 x 10 -9 m) 2 x 1.02 m = 3.2 x 10 -18 m 3
= 3.2 x 10 -9 mm 3 = 3.2 µm 3
1 m 3 (cubic metre) = 10 9 mm 3 (cubic millimetres)
1 mm 3 = 10 9 µm 3 (cubic micrometres)
75