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Evolution__3rd_Edition

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..<br />

(a) Structure of single strand (b) Structure of double strand<br />

5' end of chain<br />

O<br />

2.0 nm<br />

5'<br />

O<br />

O<br />

O<br />

O<br />

O<br />

O<br />

O<br />

O<br />

O<br />

O<br />

O<br />

3'<br />

O<br />

3' end of chain<br />

T<br />

C<br />

G<br />

A<br />

G · C<br />

T · · · A<br />

C · · · · · G<br />

A · · · · · T<br />

· ·<br />

0.34 nm<br />

3.4 nm<br />

CHAPTER 2 / Molecular and Mendelian Genetics 23<br />

Figure 2.2<br />

The structure of DNA. (a) Each strand of DNA is made up of a<br />

sequence of nucleotide units. Each nucleotide consists of a<br />

phosphate (P), a sugar, and a base (of which there are four types,<br />

here called G, C, T, and A). (b) The full DNA molecule has two<br />

complementary strands, arranged in a double helix.<br />

The DNA molecule consists of a sequence of units; each unit, called a nucleotide, consists<br />

of a phosphate and a sugar group with a base attached. The alternating sugar and<br />

phosphate groups of successive nucleotides form the backbone of the DNA molecule.<br />

The full DNA molecule consists of two paired, complementary strands, each made up<br />

of sequences of nucleotides. The nucleotides of opposite strands are chemically bonded<br />

together. The two strands exist as a double helix (Figure 2.2).<br />

2.2 DNA structurally encodes information used to build<br />

the body’s proteins<br />

How does the DNA encode the information to build a body? The DNA in an individual<br />

human cell contains about 3 × 10 9 nucleotide units. This total length can be divided<br />

into genes and various kinds of non-coding DNA. We will consider genes first. Some<br />

genes lie immediately next to neighboring genes; others are separated by more or less<br />

lengthy regions of non-coding DNA. Genes contain the information that codes for<br />

proteins.<br />

A crude but workable way to describe protein biology is to say that bodies are built<br />

from proteins and are regulated, maintained, and defended by proteins. Different parts<br />

of the body have their distinct characteristics because of the kinds of proteins they are<br />

made of. Skin, for example, is mainly made of a protein called keratin; oxygen is carried<br />

in red blood cells by a protein called hemoglobin; eyes are sensitive to light as a result of

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