Evolution And History

34 CHAPTER 2 | Genetics and Evolution

of deoxyribonucleic acid (DNA)—long strands of which
form chromosomes. DNA is a complex molecule with an
unusual shape, rather like two strands of a rope twisted
around each other with ladderlike steps between the two
strands. X-ray crystallographic photographs of the DNA
molecule created by British scientist Rosalind Franklin
contributed significantly to deciphering the molecule’s
structure. Alternating sugar and phosphate molecules
form the backbone of these strands connected to each
other by four base pairs: adenine, thymine, guanine, and
cytosine (usually written as A, T, G, and C). Connections
between the strands occur between so-called complemen-
tary pairs of bases (A to T, G to C; Figure 2.1). Sequences
of three complementary bases specify the sequence of
amino acids in protein synthesis. This arrangement al-
lows genes to replicate or make exact copies of themselves.
The term chromatid refers to one half of the “X” shape of
chromosomes visible once replication is complete. Sister
chromatids are exact copies of each other.
How is the DNA recipe converted into a protein?
Through a series of intervening steps, each three-base se-
quence of a gene, called a codon, specifies production of a
particular amino acid, strings of which build proteins. Be-
cause DNA cannot leave the cell’s nucleus (Figures 2.2), the

Cell

membrane Mitochondria

Nuclear membrane

DNA Nucleus

Endoplasmic

reticulum with

ribosomes

Cytoplasm

S

P

P P

P

S GCS

ATS

SATS

P

SCGS

P

P—Phosphate

S—Sugar

A—Adenine

T—Thymine

G—Guanine

C—Cytosine

Figure 2.1 This diagrammatic representation of a portion of

deoxyribonucleic acid (DNA) illustrates its twisted ladderlike

structure. Alternating sugar and phosphate groups form the

structural sides of the ladder. The connecting “rungs” are

formed by pairings between complementary bases—adenine

with thymine and cytosine with guanine.

Figure 2.2 Structure of a
generalized eukaryotic, or
nucleated, cell, illustrating the
cell’s three-dimensional nature.
DNA is located in the nucleus.
Because DNA cannot leave the
nucleus, genes must first be
transcribed into RNA, which
carries genetic information to
the ribosomes, where protein
synthesis occurs. Note also the
mitochondria, which contain
their own circular chromosomes
and mitochondrial DNA.

DNA Deoxyribonucleic acid. The genetic material consisting

of a complex molecule whose base structure directs the synthe-

sis of proteins.

chromatid One half of the “X” shape of chromosomes visible

once replication is complete. Sister chromatids are exact copies

of each other.

codon Three-base sequence of a gene that specifies a particu-

lar amino acid for inclusion in a protein.