FUNDAMENTAL BUILDING BLOCKS 279
were preparing to divide. In 1900,
Gregor Mendel’s experiments
with heredity in pea plants were
rediscovered—Mendel had been
the first to suggest that there were
units of heredity that came in pairs
(which would later be called genes).
At about the same time as Mendel
was being rediscovered, breeding
experiments by American physician
Walter Sutton and, independently,
by German biologist Theodor Boveri
revealed that sets of chromosomes
(the rod-shaped structures that
carry genes) pass from a dividing
cell to each of its daughter cells.
The ensuing Sutton–Boveri theory
proposed that chromosomes are
the carriers of genetic material.
Soon, more scientists were
investigating these mysterious
X-shaped bodies. In 1915, American
See also: Charles Darwin 142–49 ■ Gregor Mendel 166–71 ■ Thomas Hunt Morgan 224–25 ■ Barbara McClintock 271 ■
Linus Pauling 254–59 ■ Craig Venter 324–25
biologist Thomas Hunt Morgan
showed that chromosomes were
indeed the carriers of hereditary
information. The next step was to
look at the constituent molecules
of chromosomes—molecules that
might be candidates for genes.New pairs of genes
In the 1920s, two types of candidate
molecules were discovered: proteins
called histones, and nucleic acids,
which had been described
chemically in 1869 as nuclein by
Swiss biologist Friedrich Miescher.
The Russian-American biochemist
Phoebus Levene and others
gradually identified the main
ingredients of DNA in increasing
detail as nucleotide units, each
made up of a deoxyribose sugar, a
phosphate, and one of four subunitscalled bases. By the end of the
1940s, the basic formula of DNA as
a giant polymer—a huge molecule
consisting of repeating units, or
monomers—was clear. By 1952,
experiments with bacteria had
shown that DNA itself, and not its
rival candidates, the proteins inside
chromosomes, was the physical
embodiment of genetic information.Tricky research tools
The competing researchers were
using several advanced research
tools, including X-ray diffraction
crystallography, in which X-rays
were passed through a substance’s
crystals. A crystal’s unique
geometry in terms of its atomic
content made the X-ray beams
diffract, or bend, as they passed
through. The resulting diffraction
patterns of spots, lines, and blurs
were captured on photographic
film. Working backward from those
patterns, it was possible to figure
out the structural details within the
crystal. This was not an easy task.
X-ray crystallography has been ❯❯The structure of DNA is a double helix.
DNA carries genetic
information and must be
able to replicate.It encodes genetic
information in a series of
bases along its structure.X-ray images of the
structure show that it has
a helix shape.A double helix could
both carry genetic
information and provide a
way to replicate.It is one of the more striking
generalizations of
biochemistry...that the
twenty amino acids and the
four bases, are, with minor
reservations, the same
throughout Nature.
Francis Crick