Cellular Metabolism and Reproduction: Mitosis and Meiosis
1962 after publishing their results. Rosalind -Franklin,
meanwhile, had tragically died of cancer prior to this event.
Today, however, these three are given credit for
discovering the structure of DNA, the molecule that
contains all the hereditary information of an individual. An
interesting account of the discovery of the nature of the
molecule was published in 1968 by James Watson in his
book The Double Helix. This discovery opened up whole
new fields of research for the 20th century: recombinant
DNA, the Human Genome Project, and genetic
engineering.
The Anatomy of Smooth Muscle.
DNA (deoxyribonucleic acid) is the hereditary material
of the cell. It not only determines the traits an organism
exhibits, but it is exactly duplicated during reproduction so
that offspring exhibit their parents’ basic character-istics.
An organism’s characteristics are due to chemi-cal
reactions occurring inside its cells. DNA governs these
chemical- reactions by the chemical mechanism of
-controlling what proteins are made.
Every DNA molecule is a double helical chain of
nucleotides (NOO-klee-oh-tides; Figure 4-7). A nucleo-
tide consists of a phosphate group (PO 4 ), a five-carbon
sugar (deoxyribose), and an organic nitrogen-contain-ing
base, either a purine or a pyrimidine. There are two
purines (PYOO-reenz), adenine (ADD-eh-neen) and
guanine (GWAHN-een), and two pyrimidines-
(pih-RIM-ih-deenz), thymine (THYE-meen) and
cytosine- (SYE-toh-seen). Adenine always pairs up with
thymine and guanine always pairs up with cytosine. Bonds
form between the phosphate group of one nu-cleotide and
the sugar of the next nucleotide in a chain. The organic
nitrogen base extends out from the sugar of the nucleotide.
It is easier to visualize the double heli-cal nature of the
DNA if we think of it as a spiral stair-case. The handrails of
the staircase are composed of the phosphate-sugar chain,
and the stairs of the staircase are the nitrogen base pairs.
If we look at Figure 4-8, we see that a pyrimidine
always pairs with a purine. A pyrimidine is a single ring of
six atoms (thymine and cytosine); a purine is a fused
double ring of nine atoms (adenine and gua-nine). These
organic nitrogen bases are a complex ring structure of
carbon and nitrogen atoms. Because we know how the
bases pair up in the double chain of nu-cleotides, if we only
know one side of the helix, we can figure out the second
side by matching bases. The two chains of the helix are
held together by weak hydrogen bonds between the base
pairs. There are two hydrogen
71
A diagram of a short chain of DNA and its double helical structureS = Deoxyribose, P = Phosphate, C = Cytosine,
G = Guanine, A = Adenine, T = ThymineS TA S (^)
P P (^)
S AT S (^)
P P (^)
S TA S (^)
P P (^)
S GC S (^)
P P (^)
S CG S (^)
P P (^)
S GC S
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P P
Learning^S AT S
Cengage^
©^Figure 4- 7 The double helical chain of
nucleotides of a DNA molecule (a very short
section).bonds between the pyrimidine thymine and the purine
adenine, whereas there are three hydrogen bonds be-tween
the pyrimidine cytosine and the purine guanine. Because of
the specific pairing of bases, the sequence of bases in one
chain determines the sequence of bases in the other. We
therefore refer to the two chains as com-plements of each
other. A gene is a sequence of organic nitrogen base pairs
that codes for a polypeptide or a protein.A major project of the 20th century that developed
from Watson, Crick, and Franklin’s discovery of DNA
structure was the Human Genome Project. The objec-tive
of this project was to identify all the genes on all 46
-chromosomes (DNA molecules). We know now that there
are approximately 3 billion organic base pairs that code
over 30,000 genes. We can think of the bases -adenine (A),
thymine (T), cytosine (C), and guanine (G) as the four
-letters of the alphabet of life. These base pairs determine
all the characteristics of all the life we know on our
planet—the basic structure of the DNA molecule is the
same for all living organisms.