Chapter 2 – Orientation for the Bio-Curious 37
Therefore, the whole human haploid genome contains (0.25 × 3 × 109)/(1.1 × 109)
GB = 0.68 GB. However, a normal human genome is diploid; thus, the infor
mation stored is 2 × 0.68 GB ≈ 1.4 GB.
One Blue-ray disk thus can hold 50/1.4 = 3.6 genomes or 3 complete genomes.
The “average” family consists of 2.58 people, and so the storage capacity of 1
Blue-ray disk is sufficient.
c
One complete “storage unit” is a turn of a single double-helical pitch of B-DNA
involving 10.5 base pairs enclosed within a cylinder of diameter 2.0 nm and
length 3.4 nm.
The number of bytes of information in 1 turn ≈ 10.5 × 0.25 = 2.62 B.
The volume of the cylinder enclosing one double-helical turn = π × (2.0 × 10−9 ×
0.5)2 × (3.4 × 10−9) = 1.1 × 10−26 m3.
Thus, the storage capacity density of B-DNA sD = 2.62/(1.1 × 10−26) = 2.5 × 1026 B m−3.
For a 2 TB USB hard drive, 2 TB = 2 × 210 GB = 241 B = 2.2 × 1012 B.
The volume of the hard drive = 12 × 8 × 2 cm3 = 192 cm3 = 1.92 × 10−4 m3.
Thus, storage capacity density of the hard drive sH = (2.2 × 1012)/(1.92 × 10−4) = 1.1
× 1016 B m−3.
The percentage value of more data on the DNA device = sD/sH = 100 × (2.5 × 1026)/
(1.1 × 1016) = 2.2 × 1012%
d
The volume of the nucleus Vn = 4 × π × (10 × 10−6 × 0.5)3/3 = 1.3 × 10−16 m3.
The volume of the DNA “cylinder” per base pair = (1.1 × 10−26)/10.5 = 1.0 × 10−25 m3.
Thus, the volume of the whole diploid genome Vg = 2 × (3 × 109) × (1.0 × 10−25) = 6.0
× 10−16 m3.
Therefore, the number of complete diploid genomes that could be stored in the
nucleus = Vn/Vg = (1.3 × 10−16)/(6.0 × 10−16) = 0.22. Since this value is <1, this
suggests that either DNA is present as a more compact form than B-DNA or
(more likely) DNA inside the nucleus is compacted to a greater extent than
the simple cylinder-based storage unit suggests (e.g., using histone proteins).
2.4 CELL PROCESSES
Cells can regulate their behavior, or phenotype, by ultimately controlling the number of
protein molecules of different types that are present inside the cell at any one time. This is
important since cells inside an organism may all have the same ultimate set of genes made
from the DNA inside each of their cell nuclei but may need to perform very diverse roles in
the organism. For example, in the human body there are roughly 200 different types of cells,
as classified by biological experts, cells that will have different sizes and shapes and have
catered biochemical and mechanical properties to be specialized in specific parts of the body,
such as in the nerves, bones, muscles, skin, and blood.
KEY POINT 2.12
Cells in an organism in general have the same set of genes but may be many different
cell types that are specialized for particular biological roles.
Most of the very smallest cells belong to the archaea domain in a genus subdivision also
called Mycoplasma, found commonly in soil, which are roughly 200 nm in diameter, very
close to the theoretical minimum size predicted on the basis of estimating the length of DNA
genetic code in principle required to generate the very barest essential components neces
sary for a cell to replicate itself and thus be “alive” and using the polymer physics proper
ties of DNA to predict its typical end-to-end distance. Mycoplasma “ghost” cell membranes