Evolution__3rd_Edition

..
(a) Structure of single strand (b) Structure of double strand
5' end of chain
O
2.0 nm
5'
O
O
O
O
O
O
O
O
O
O
O
3'
O
3' end of chain
T
C
G
A
G · C
T · · · A
C · · · · · G
A · · · · · T
· ·
0.34 nm
3.4 nm
CHAPTER 2 / Molecular and Mendelian Genetics 23
Figure 2.2
The structure of DNA. (a) Each strand of DNA is made up of a
sequence of nucleotide units. Each nucleotide consists of a
phosphate (P), a sugar, and a base (of which there are four types,
here called G, C, T, and A). (b) The full DNA molecule has two
complementary strands, arranged in a double helix.
The DNA molecule consists of a sequence of units; each unit, called a nucleotide, consists
of a phosphate and a sugar group with a base attached. The alternating sugar and
phosphate groups of successive nucleotides form the backbone of the DNA molecule.
The full DNA molecule consists of two paired, complementary strands, each made up
of sequences of nucleotides. The nucleotides of opposite strands are chemically bonded
together. The two strands exist as a double helix (Figure 2.2).
2.2 DNA structurally encodes information used to build
the body’s proteins
How does the DNA encode the information to build a body? The DNA in an individual
human cell contains about 3 × 10 9 nucleotide units. This total length can be divided
into genes and various kinds of non-coding DNA. We will consider genes first. Some
genes lie immediately next to neighboring genes; others are separated by more or less
lengthy regions of non-coding DNA. Genes contain the information that codes for
proteins.
A crude but workable way to describe protein biology is to say that bodies are built
from proteins and are regulated, maintained, and defended by proteins. Different parts
of the body have their distinct characteristics because of the kinds of proteins they are
made of. Skin, for example, is mainly made of a protein called keratin; oxygen is carried
in red blood cells by a protein called hemoglobin; eyes are sensitive to light as a result of